Interfacing Methods and Circuits

Interfacing Methods and Circuits
Chapter 11

Introduction A sensor/actuator can rarely operate on its own.
Exceptions exist (bimetal sensors) Often a circuit of some sort is involved. can be as simple as adding a power source or a transformer can involve amplification, impedance matching, signal conditioning and other such functions. often, a digital output is required or desirable so that an A/D may be needed The same considerations apply to actuators

Introduction The considerations of interfacing should be part of the process of selecting a device for a particular application since this can simplify the process considerably. Example: if a digital device exists it would be wasteful to select an equivalent analog device and add the required circuitry to convert its output to a digital format. The likely outcome is a more cumbersome, expensive system which may take more time to produce. Alternative sensing strategies and alternative sensors should always be considered before settling on a particular solution

Introduction Many types of sensors and actuators based on very different principles There are commonalities between them in terms of interfacing requirements Most sensors’ outputs are electric (voltage, current, resistance) These can be measured directly after proper signal conditioning and, perhaps, amplification. If the output is a capacitance or an inductance - require additional circuitry such as oscillators

Introduction There is a large range of signal levels in sensors.
A thermocouple’s output is of the order of microvolts An LVDT may easily produce 5V AC. A piezoelectric actuator may require a few hundred volts to operate (very little current) A solenoid valve operates at perhaps 12-24V with currents that may exceed a few amperes. How does one measure these signals?

Introduction The circuitry required to drive and to interface them to, say a microprocessor are vastly different Require special attention on the part of the engineer. Must consider such issues as response (electrical and mechanical), spans and power dissipation as well as power quality and availability. Example: Systems connected to the grid and cordless systems have different requirements and considerations in terms of operation and safety.

Purpose Discuss general issues associated with interfacing
Outline general interfacing circuits the engineer is likely to be exposed to. No general discussion however can prepare one for all eventualities It should be recognized that there are both exceptions to the rules and extensions to the methods discussed here.

Purpose Example: an A/D is a simple – if not inexpensive – method of digitizing a signal for the purpose of interfacing This approach however may not be necessary and too expensive in some cases. Suppose the hall element senses the teeth on a gear. The signal from the hall element is an ac voltage - only the peaks are necessary to sense the teeth. In this case a simple peak detector may be adequate. An A/D converted will not provide any additional benefit and is a much more complex and expensive solution. On the other hand, if a microprocessor is used and an A/D is available it may be acceptable to use it for this purpose

Content Operational amplifiers and power amplifiers
A/D and D/A conversion circuits Bridge circuits Data transmission Excitation circuits Noise and interference

Amplifiers An amplifier is a device that amplifies a signal – almost always a voltage The low voltage output of a sensor, say of a thermocouple, may be amplified to a level required by a controller or a display. Amplification may be quite large – sometimes of the order of 106 or it may be quite small or even smaller than one, depending on the need of the sensor.

Amplifiers Amplifiers can also be used for impedance matching purposes even when no amplification is needed May be used for the sole purpose of signal conditioning, signal translation or for isolation Power amplifiers, which usually connect to actuators, serve similar purposes beyond providing the power necessary to drive the actuator. Amplifiers can be very simple – a transistor with its associated biasing network or may involve many amplification stages of varying complexity. Amplifiers are sometimes incorporated in the sensor

Amplifiers We will use the operational amplifier as the basic building block for amplification. Operational amplifiers are basic devices and may be viewed as components. An engineer, especially when interfacing sensor is not likely to dwell into the design of electronic circuits below the level of operational amplifiers. Although there are instances where this may be done to great advantage, op-amps are almost always a better, less expensive and higher performance choice. Same idea for power amplifiers

Operational Amplifiers
Operational amplifier is a fairly complex electronic circuit but: It is based on the idea of the differential voltage amplifier shown in Figure 11.1. Based on simple transistors, The output is a function of the difference between the two inputs. Assuming the output to be zero when both inputs are at zero potential, the operation is as follows:

Differential amplifier

Operational amplifier
When the voltage on the base of Q1 increases, its bias increases while that on Q2 decreases because of the common emitter resistance. Q1 conducts more than Q2 and the output is positive with respect to ground. If the sequence is inverted, the opposite occurs. If, both inputs increase or decrease equally, there will be no change in output.

An operational amplifier is much more complex than this but operates on the same principle. It contains additional circuitry (such as temperature and drift compensation, output amplifiers, etc.) These are of no interest to us other than the fact that they affect the specifications of the op-amp. There are also various modifications to op-amps that allow them to operate under certain conditions or to perform specific functions.

Some are “low noise” devices Others can operate from a single polarity source. If the input transistors are replaced with FETs, the input impedance increases considerably requiring even lower input currents from sensors These are important but are variations of the basic circuit. We will consider it as a simple block shown in Figure 11.2 and discuss its general properties based on this diagram

The operational amplifier

Op-Amps - properties Differential voltage gain: the amplification of the op-amp of the difference between the two inputs: Also called the open loop gain in a good amplifier it should be as high as possible. Gains of 106 or higher are common. An ideal amplifier is said to have infinite gain.

Op-Amps - properties Common-mode voltage gain.
By virtue of the differential nature of the amplifier, this gain should be zero. Practical amplifiers may have a small common mode gain because of the mismatch between the two channels but this should be small. Common mode voltage gain is indicated as Acm. The concept is shown in Figure 11.3.

Common mode signal an output

Op-amps - properties More common to specify the term Common Mode Rejection Ratio (CMRR) CMRR is the ratio between Ad and Acm: In an ideal amplifier this is infinite. A good amplifier will have a CMRR that is very high

Op-amps - properties Bandwidth: the range of frequencies that can be amplified. Usually the amplifier operates down to dc and has a flat response up to a maximum frequency at which output power is down by 3dB. An ideal amplifier will have an infinite bandwidth. The open gain bandwidth of a practical amplifier is fairly low A more important quantity is the bandwidth at the actual gain.

Op-amps - properties This may be seen in Figure 11.4
The lower the gain, the higher the bandwidth. Data sheets therefore cite what is called the gain-bandwidth product. This indicates the frequency at which the gain drops to one and is also called the unity gain frequency. In Figure 11.4: BW (open loop) is 2.5 kHz Unity Gain Frequency is 5 MHz

Bandwidth of op-amp

Op-amps - properties Slew Rate: the rate of change of the output in response to a change in input, given in V/s. If a signal at the input changes faster than the slew rate, the output will lag behind it and a distorted signal will be obtained. This limits the usable frequency range of the amplifier. For example, an ideal square wave will have a rising and dropping slope at the output defined by the slew rate.

Op-amps - properties Input impedance: the impedance seen by the sensor when connected to the op-amp. Typically this impedance is high (ideally infinite) It varies with frequency. Typical impedances for conventional amplifiers is at least 1 M but it can be of the order of hundreds of M for FET input amplifiers. This impedance defines the current needed to drive the amplifier and hence the load it represents to the sensor.

Op-amps - properties Output impedance: the impedance seen by the load.
Ideally this should be zero since then the output voltage of the amplifier does not vary with the load In practice it is finite and depends on gain. Usually, output impedance is given for open loop whereas at lower gains the impedance is lower. A good amplifier will have an output resistance lower than 1.

Op-amps - properties Temperature and noise refer to variations of output with temperature and noise characteristics of the device respectively. These are provided by the data sheet for the op-amp and are usually very small. For low signals, noise can be important while temperature drift, if unacceptable must be compensated for through external circuits.

Op-amps - properties Power requirements. The classical op-amp is built so that its output can swing between ±Vcc Dual supply operation is common in op-amps The limits can be as low as ±3V (or lower) and as high as ±35V (sometimes higher). Many op-amps are designed for single supply operation of less than 3V and some can be used in single supply or dual supply modes.

Op-amps - properties Current through the amplifier is an important consideration, especially the quiescent current (no load) Gives a good indication of power needed to drive it. Particularly important in battery operated devices. The current under load will depend on the application but it is usually fairly small – a few mA. In selection of a power supply for op-amps, care should be taken with the noise that the power supply can inject into the amplifier. The effect of the power supply on the amplifier is specified through the power supply rejection ratio (PSRR) of the specific amplifier.

Op-amps - data sheets The 741 op-amp is an older, general purpose amplifier. It is a fairly low performance device but is characteristic of the low-end amplifiers. Very common and quite suitable for many applications. LM741.PDF

Op-amps - data sheets The TLC27L2C is a dual, low power op-amp, suited for battery operated devices Part of a series of amplifiers using FETs as input transistors TLC27L2C

Inverting and noninverting amplifiers
Performance of the amplifier depends on how it is used and, in particular on the gain of the amplifier. In practical circuits, the open loop gain is not useful and a specific gain must be established. For example, we might have a 50mV output (maximum) from a sensor and require this output to be amplified, say by 100 to obtain 5V (maximum) for connection to an A/D. This can be done with one of the two basic circuits shown in Figure 11.5, establish a means of negative feedback to reduce the gain

Inverting op-amp

Non-inverting op-amp

Inverting op-amp The output is inverted with respect to the input (180 out of phase). The feedback resistor, Rf, feeds back some of this output to the input, effectively reducing the gain. The gain of the amplifier is now given as: In the case shown here this is exactly –10

Inverting op-amp The input impedance of the amplifier is given as
Here it is equal to 1 k. If a higher resistance is needed, larger resistances might be needed Or, perhaps, a different amplifier will be needed (noninverting amplifier)

Inverting op-amp The output impedance of the amplifier is given as
AOL is the open loop gain as listed on the data sheet Open loop gain is the open loop gain at the frequency at which the device is operated

Inverting op-amp Example, for the LM741 amplifier, the open loop output impedance is 75W and the open loop gain at 1 kHz is This gives an output impedance of: The bandwidth is also influenced by the feedback:

Non-inverting amplifier
The non-inverting amplifier gain is: For the circuit shown, this is 11 The gain is slightly larger than for the noninverting amplifier for the same values of R. The main difference however is in input impedance.

Input impedance is: Rop is the input impedance of the op-amp as given in the spec sheet Aol is the open loop gain of the amplifier. Assuming an open loop impedance of 1 M (modest value) and an open loop gain of 106, we get an input impedance of . (almost ideal)

The output impedance and bandwidth are the same as for the inverting amplifier. The main reason to use a noninverting amplifier is that its input impedance is very large making it almost ideal for many sensors. There are other properties that need to be considered for proper design such as output current and load resistance but these will be omitted here for the sake of brevity.

The voltage follower The feedback resistor in the noninverting amplifier is set to zero The circuit in Figure 11.6 is obtained. The gain is one. This circuit does not amplify. Why use it?

The voltage follower

Voltage follower The input impedance now is very large and equal to:
The output impedance is very small and equal to:

Voltage follower The value of the voltage follower is to serve in impedance matching. One can use this circuit to connect, say, a capacitive sensor or, an electret microphone. If amplification is necessary, the voltage follower may be followed by an inverting or noninverting amplifier

Instrumentation amplifier
The instrumentation amplifier is a modified op-amp Its gain is finite and both inputs are available to signals. These amplifiers are available as single devices To understand how they operate, one should view them as being made of three op-amps (it is possible to make them with two op-amps or even with a single op-amp), as shown in Figure 11.7.

The gain of an amplifier of this type is: In a commercial instrumentation amplifier all resistances are internal and produce a gain usually around 100. Ra is external and can be set by the user to obtain the gain required.

The output of the instrumentation amplifier is The main use of this amplifier is to obtain an output proportional to difference between inputs. Important in differential sensors, especially when one sensor is used to sense the stimulus and an identical sensor is used for reference (such as when temperature compensation is needed)

Each of the inputs has the high impedance of the amplifier used The output impedance is low (inverting amp.) The main problem in a circuit of this type is that the CMRR depends on the matching of the resistances (R, R2 and R3) in each section of the circuit. These are internal and are adjusted during production to obtain the required CMRR.

Charge amplifier The so-called charge amplifier is shown in Figure 11.8. Charge cannot be amplified but the output voltage can be made proportional to charge as follows: The output of the inverting amplifier is: C0 is the capacitance connected across the inverting input.

Charge amplifier Assuming that a change in charge occurs on the capacitor, equal to Q = C0V, the output voltage may be written as In effect the charge generated at the input is amplified

Charge amplifier If C is small, a small change in charge at the input can generate a large voltage swing in the output. The main method of connecting capacitive sensors such as pyroelectric sensors whose output is low (piezoelectric sensors, on the other hand produce a higher voltage). It is necessary for the input impedance to be very high and care must be taken in connections (such as the use of very good capacitors). Commercial charge amplifiers use FET transistors to ensure the necessary high input impedance.

Charge amplifier

Current amplifier Another example of the use of an amplifier to a specific end is the current amplifier

Current amplifier The input voltage is Vi=ir.
Just like the inverting amplifier, the output now is Useful with very low impedance sensors. May be used with thermocouples whose impedance can be trivially low. They may be connected directly (r then represents the resistance of the thermocouple). The output is a direct function of the current the thermocouple produces which can be fairly large

The comparator An op-amp operated in open loop mode
Because its gain is so high, a very small signal at the input will saturate the output. For practically any input, the output will be either +Vcc or –Vcc. Consider Figure The negative input is set at a voltage V and V+=0. Therefore the output is AolV=Vcc. Suppose we increase V+. Output is (V+V)Aol. As long as V+<V-, the output remains –Vcc. If V+>V, the output changes to + Vcc.

The comparator

The comparator The function of this device is to compare the two inputs and to indicate which one is higher. The comparator is useful beyond simple comparison. It will be used extensively in A/D and D/A conversion of signals and in many other aspects of sensing and actuation

Power amplifiers A power amplifier is a device or circuit whose power output is the input power multiplied by a power gain: That is, the amplifier is capable of boosting the power level of a signal to match the needs of an actuator.

Power amplifiers The obvious use of power amplifiers is in driving actuators, (speakers, voice coil actuators and solenoid actuators and motors). The power amplifier is really either a voltage amplifier or a current amplifier (also called transconductance amplifier). In a voltage amplifier, the input signal is a voltage. This voltage is amplifier and in the final stage a sufficiently high current provided so that the required power is met.

Power amplifiers In a current amplifier the opposite occurs.
Power amplifiers are divided into linear and PWM (pulse width modulated) amplifiers. In a linear amplifier, the output (voltage) is a linear function of the input and can be anything between ±Vcc. In a PWM amplifier the output is either Vcc or zero and the power delivered is set by the time the output is on. The latter is controlled by the width of the pulse that controls the output.

Linear power amplifiers
First step is to amplify the signal to the required output. Can be done using any amplifier We shall assume an op-amp was used for this purpose. Then this voltage is applied to an “output stage” It does not need to amplify but, rather, supplies the necessary current. A simple example is shown in Figure

Linear power amplifier

Class A linear amplifier
This is the so called Class A power amplifier. Set for a gain of 101 (noninverting amplifier). The output then drives the transistor whose output will swing, at most between 0 and V Will supply a current which is V/RL Class A designation indicates amplifiers for which the output stage is always conducting as in the case above. Also assumes output does not saturate. The BJT can be replaced with a MOSFET for higher currents.

Class A linear amplifier
This type of amplifier is sometimes used to drive relatively small loads such as light indicators, small dc motors and some solenoid valves. In some cases the amplification is set high enough to saturate the amplifier in which case the amplifier operates as an on/off circuit rather than a class A amplifier Typically used to turn on/off relays, lights, motors, etc.

Class B amplifier A Class B or push-pull amplifier is shown in Figure It is usually a better choice. It operates exactly as in the previous case except that under no input, the output is zero and there is no conduction in the transistors (or MOSFETs). When the input is positive, the upper transistor conducts supplying the load and when the input is negative, the lower transistor supplies the load.

Class-B (push-pull) power amplifier

Class B amplifier The voltage in the load can swing between +Vcc and –Vcc The current is again defined by the load. The output stage is made of a pair of power transistors, one PNP and one NPN (or of a P and an N type MOSFET). There are many variations of the basic amplifiers. For example, feedback may be added and it is common to protect the output stage from short circuits as well as from spikes due to inductive and capacitive loads.

Class B amplifier In terms of performance, the obvious are the power output and the type and level of input. For example, an amplifier may be specified as supplying 100W for a 1V input. Next is the distortion level. Distortions are specified as a percentage of output. The most common specification is the THD (total harmonic distortions) as % of output. A good amplifier will have less than, say, 0.1% THD. Other specifications are temperature rise and output impedance of the amplifier (must match load).

Class B amplifier Power amplifiers of various power level exist either as integrated circuits or as discrete components circuits. Usually the discrete circuits can supply higher powers. An example of an integrated amplifier is the TDA2040 which can supply 20W and is designed for use as an audio amplifier. Nevertheless it can drive other loads such as light bulbs, small motors, etc.

PWM amplifiers The PWM approach is shown schematically in Figure The power transistors are driven on and off so that the voltage on the load can only be zero or Vcc. The time the power is on is controlled by the timing circuit. This defines the average power at the load.

The PWM principle

PWM amplifiers The pulse width modulator is an oscillator which generates a square wave whose duty cycle can be controlled based on the required power. For example, in Figure 11.14, the timing circuit defines for how long the input signal is connected to the transistor, hence for how long it conducts. The power in the load is a function of this timing. This circuit is not particularly useful but others are.

PWM driving of a load

PWM driving Figure shows an example often used to control speed and direction of small dc motors. It is called an H-bridge for obvious reasons. A pulse of constant amplitude but varying duty cycle connected to point A, will drive MOSFETs 1 and 4, turning the motor into one direction. The duty cycle defines the average current in the motor and hence its speed. Connecting to point B, turns on MOSFETs 2 and 3 reversing the process.

H-Bridge driven from a PWM source

H-bridge PWM driver Some precautions must be taken to ensure that only opposite transistors conduct This is one of the most common circuits used for bidirectional control of motors and other actuators. The controllers for these devices can be a small microprocessor Integrated PWM circuits and controllers are available commercially

A/D and D/A converters These are the means by which a signal can be converted from analog to digital or from digital to analog as necessary. The idea is obvious but implementation can be complex. There are certain types of D/A and A/D that are trivially simple. We will start with these and only then discuss some of the more complex schemes. In certain cases one of these simple methods is sufficient.

A/D and D/A converters Analog to digital and, to a lesser extent, digital to analog conversion are common in sensing systems since most sensors and actuators are analog devices and most controllers are digital. Most A/Ds required voltages much above the output of some sensors. Often the output from the sensor must be amplified first and only then converted. This leads to errors and noise and has resulted in the development of direct digitization methods based on oscillators (to be discussed below).

Threshold digitization
In some cases, an analog signal represents simple data such as the presence of something. For example, in chapter 5 we discussed the detection of teeth on a gear using a hall element. The signal obtained is quite small and looks more or less sinusoidal with the peaks representing the presence of the teeth. In such a case it is sufficient to use a threshold amplifier which will then produce a digital output. An example is shown in Figure 11.16a.

The output from the the hall element varies from 100mV to 150mV. This signal can be fed into a comparator as shown in Figure 11.17 The negative input is set by the resistors to 0.13V. Normally the output is zero until the voltage on the positive input rises above the threshold. When the input dips below 0.13V the output goes back to zero. The output in Figure 11.16b is obtained and now, each pulse represents a tooth on the gear.

Comparator threshold digitization

Counting the teeth in a given time can give the speed of rotation of the gear or other data. A missing tooth, the corresponding pulse will be represented by a missing pulse This method is very effective when voltages at the input change across the comparison point At the comparison point itself, the output of the comparator is not properly defined and the output can change states back and forth creating pulses which are spurious.

To avoid this a hysteresis is added to the comparator so that the transition from low to high occurs say, at V0 and the transition from high to low occurs at V0-V. Hysteresis can be added to comparators through external components. Another approach is to use of a Schmitt trigger. The Schmitt trigger is essentially a digital comparator with a built in hysteresis as described above whose transition is around Vcc/2.

Threshold digitization is a very simple method of digitization and is sufficient for many applications. It is commonly used for the purpose above but also in flow meters in which a rotating paddle operates a hall element or another magnetic sensor It is also useful for optical sensors which use the idea of interruption of the beam. It is not however suitable for measuring the level of a signal such as voltage from a thermocouple.

Direct voltage to frequency conversion
In many sensors, the output is too small to use the method above or to be sent over normal lines for any distance. In such cases a voltage to frequency conversion can be performed at the location of the sensor and the digital signal then transferred over the line to the controller. The output now is not voltage but rather a frequency which is directly proportional to voltage (or current).

These voltage-to-frequency converters or voltage controlled oscillators are relatively simple and accurate circuits and have been used for other purposes. Their main advantage over the threshold method above is that lower levels of signals may be involved and the problems with noisy transitions around the comparison voltage are eliminated. A circuit of this type is shown in Figure 11.18, as used with a light sensor. The circuit is an op-amp integrator.

The voltage across the capacitor is the integral of the current in the noninverting leg of the amplifier. This current is proportional to the voltage across R2. As the voltage on the capacitor rises, a threshold circuit checks this voltage When the threshold has been reached, an electronic switch shorts the capacitor and discharges it. The switch then opens and allows the capacitor to recharge. The voltage on the capacitor is a triangular shape whose width (i.e. the integration time) depends on the voltage at the noninverting input.

If no light is present on the sensor, it has a dark resistance and the voltage at the noninverting input will have a certain value. The output of the amplifier changes at a frequency f1. If now light falls on the sensor, its resistance goes down and the total resistance at the noninverting input falls. This reduces the input voltage and hence the integration time until the capacitor reaches the threshold increases. The result is that the amplifier changes state slower and the output is a lower frequency f2. Since small changes in frequency can be easily detected, this is a very sensitive method of digitization for small signal sensors.

Voltage to frequency conversion
Other V/F converters require a much higher voltage and they are more suitable for A/D conversion after amplification of the lower signals or for sensors whose output is high to begin with. There are two basic methods. One type is essentially a free running oscillator whose frequency can be controlled by the input voltage. The second is a modification of Figure and is called a charge-balance V/F converter.

Voltage to frequency conversion
A simple V/F method is shown in Figure It consists of a square wave oscillator (called a multivibrator) and a control circuit. The multivibrator operates by charging and discharging a capacitor. The on/off times of the waveform (hence frequency) are controlled by charging/discharging times of the capacitor. To control frequency voltage to be converted is amplified and fed as currents to the bases of the two transistors. The larger the base current, the larger the collector current and the faster the charge/discharge and hence the higher the frequency of the multivibrator.

Simple voltage to frequency conversion - multivibrator

V/F conversion A different approach is shown in Figure 11.20.
The amplifier acts as an integrator and the FET across the capacitor is the switch. The capacitor charges at a rate proportional to the current I=V0/R which is proportional to the voltage to be converted. When the output has reached the threshold voltage of the schmitt trigger it changes state, turning on and this turns on the FET switch. Discharging of the capacitor occurs and the output resets to restart the process. Again, as before, only relatively large level voltages can be converted.

Simple voltage to frequency conversion - integrator

Dual slope A/D converter
The simpler (and slower) of the true A/D converters Based on the following principle: a capacitor is charged from the voltage to be converted through a resistor, for a fixed, predetermined time T. The capacitor reaches a voltage VT which is:

At time T, Vin is disconnected A negative reference voltage of known magnitude is connected to the capacitor through the same resistor. This discharges the capacitor down to zero in a time T

Since these are equal in magnitude we have: In addition, a fixed frequency clock is turned on at the beginning of the discharge cycle and off at the end of the discharge cycle. Since T and T are known and the counter knows exactly how many pulses have been counted, this count is the digital representation of the input voltage. A schematic diagram of a dual slope converter based on these principles is shown in Figure

Dual slope A/D conversion

The method is rather slow with approximately 1/2T conversions per second. It is also limited in accuracy by the timing measurements, accuracy of the analog devices and, of course, by noise. High frequency noise is reduced by the integration process and low frequency noise is proportional to T (the smaller T the less low frequency noise).

The dual slope A/D is the method of choice for many sensing applications in spite of its rather slow response because it is simple and readily built from standard components. For most sensors its performance and noise characteristics is quite sufficient Because of the integration involved, it tends to smooth variations in the signal during the integration. The method is also used in digital voltmeters and other digital instruments.

Successive approximation A/D
This is the method of choice in A/D converter components and in many microprocessors. It is available in many off the shelf components with varying degrees of accuracy Depending on the number of bits of resolution it may resolve down to a few microvolt. The basic structure is shown in Figure It consists of a precision comparator, a shift register a digital to analog converter and a precision reference voltage Vref.

Successive approximation A/D conversion

The operation is as follows: First, all registers are cleared, which forces the comparator to HIGH. This forces a 1 into the MSB of the register. The D/A generates an analog voltage Va which for MSB=1 is half the full scale input. This is compared to Vin. If Vin is larger than Va, the output stays high and the clock shifts this into the next bit into the register.

The register now shows If it is smaller than Vin, the output goes low and the register shows Assuming that the input is still higher, the D/A generates a voltage Va=(1/2+1/4)Vfs. If this is higher than the input, the register will show but if it is lower, it will show and so on, until, after n steps the final result will be obtained. The data is read from the shift register and represents the voltage digitally. This digital value can now be shifted out and used by the controller.

A/D of this type exists with resolution of up to 14 bits with 8 and 10 bits being quite common. An 8 bit A/D has a resolution of: Vin/28=0.004Vin. For a 5V full scale, the resolution is 20 mV. This may not be sufficient for low level signals in which case a 10, 12 or 12 bit A/D may be used (a 14 bit A/D has a resolution of 0.3mV). There are also techniques of extending this resolution but it is almost always necessary to amplify signals from devices such as thermocouples if they must be digitized.

The advantage of the successive approximation A/D is that the conversion is done in n steps (fixed) It is much faster than other methods. On the other hand the accuracy of the device depends heavily on the comparator and the D/A converter. Commercial devices are fairly expensive, especially if more than 10 bits are needed. This type of A/D has been incorporated directly into microprocessors and can sometimes be used for sensing as part of the overall circuitry. Some microprocessors have multiple A/D channels.

Digital to Analog Conversion
Digital to analog conversion is less often used with sensors but is sometimes used with actuators. This occurs when a digital device, such as a microprocessor must provide an analog output. This should be avoided if possible by use of digital actuators (such as brushless dc motors and stepper motors) but there will be cases in which D/A will be necessary. It is often a part of A/D conversion

Digital to Analog Conversion
There are different ways of accomplishing D/A conversion. The most common method used in simple converters is based on the ladder network shown in Figure It consists of a voltage follower. Its input impedance is high and the output of the follower equals the voltage at its noninverting input. The voltage is generated by the resistance network.

Ladder network D/A conversion

The ladder network is chosen so that the combination of series and parallel resistances represent the digital input as a unique voltage which is then passed to the output. The switches are digitally controlled analog switches (MOSFETs). Depending on the digital input, various switches will connect resistors in series or in parallel.

For example, suppose that the digital value 100 is to be converted. The switches will be as in Figure The voltage at the amplifier’s input is exactly 5V. The ladder can be extended as necessary for any number of bits. The accuracy and usefulness of a D/A depends on the quality and accuracy of the ladder network and the reference voltage used.

Bridge circuits Bridge circuits are some of the oldest circuits used in sensors as well as other applications. The bridge is known as the Wheatstone bridge (variations of the bridge exist with different names.) The basic Wheatstone bridge is shown in Figure It consists of 4 impedances Zi=Ri+jXi.

The impedance bridge

The impedance bridge The output voltage of the bridge is
The bridge is said to be balanced if Under this condition, the output voltage is zero.

The impedance bridge If, for example, Z1 represents the impedance of a sensor, by proper choice of the other impedances the output can be set to zero at a given value of Z1. Any change in Z1 will change the value of Vo indicating the change in stimulus. Of course, one can do much more than that and bridges can be used for signal translation and for temperature compensation among other things. One important property of bridges is their sensitivity to change in stimuli

The impedance bridge The sensitivity of the output voltage to change in any of the impedances can be calculated as: Summing up gives the bridge sensitivity

The impedance bridge This relation reveals that if Z1=Z2 and Z3=Z4 the bridge is balanced If the change, is such that dZ1=dZ2 and dZ3=dZ4, the change in output is zero. This is the basic idea used in compensating a sensor for temperature variation and any other common mode effects. For examples, suppose that a pressure sensor has impedance Z1=100  and a sensitivity to temperature dZ1= 0.5 /C.

The impedance bridge We use two identical sensors as Z1 and as Z2
Sensor Z2 is not exposed to pressure (only exposed to the same temperature as Z1). Z3 and Z4 are equal and are made of the same material – these are simple resistors. Under these conditions, there will be no output due to temperature changes The sensor is properly compensated for temperature variations. If however pressure changes, the output changes

The impedance bridge If all impedances in the bridge are fixed and only Z1 varies (this is the sensor), then dZ2=0, dZ3=0, dZ4=0 and the bridge sensitivity becomes Or:

The impedance bridge This bridge, especially with resistive branches is the common method of sensing with: strain gauges, piezoresistive sensors, hall elements, thermistors force sensors and many others. Use of bridges allows a convenient reference voltage (nulling), temperature compensation and other sources of common mode noise. It is very simple and it can be easily connected to amplifiers for further processing

Temperature compensation of bridges
Temperature compensation in sensors eliminates the errors due to temperature or any other common mode effect. It does not eliminate errors external to the sensors such as variations of Vi with temperature. These have to be compensated for in the construction of the bridge itself. There are many techniques by which this can be accomplished but this is beyond the scope of this course.

Bridge output The output from the bridge is likely to be relatively small. For example, suppose that the bridge is fed with a 5V source and a thermistor, Z4=500 (at 0C) is used to sense temperature. Assuming the bridge is balanced at 0C, the other three resistances are also 500. This gives an output voltage zero. Now, suppose that at 100C the resistance of the thermistor goes down to 400W.

Bridge output The output voltage now is:
Most sensors will produce a much smaller change in impedance Some sort of amplification will be necessary. The op-amp discussed above is ideal for this purpose. There are many ways this can be accomplished. Two methods are shown in Figure

Amplified bridge

Active bridge

Amplified bridge In Figure 11.25a, the bridge is connected directly between the inverting and noninverting inputs. If we assume that the resistance of the resistance of the sensor changes as Rx=R0(1+), the voltage output of the bridge is: This circuit provides an amplification of (1+n) but requires that the voltage on the bridge be floating

Active bridge Circuit in Figure 11.25b does not provide amplification but rather places the sensor in the feedback loop. This is called an active bridge and its output is: This circuit provides buffering (higher input impedance, lower output impedance).

Data transmission Transmission of data from a sensor to the controller may take many forms. If the sensor is passive, it already has an output in a usable form such as voltage or current. It would seem that it is sufficient to simply measure this output directly to obtain a reading. In other cases, such as with capacitive or inductive sensors, indirect measuring is often used. The sensor is often likely to be in a remote location.

Data transmission Neither direct measurement of voltage and current or using the sensor as part of the circuit (in an oscillator) may be an option in such a case In such cases, it is often necessary to process the sensor’s output locally and to transmit the result to the controller. The controller then interprets the data and places it in a suitable form.

Data transmission The ideal method of transmission is digital.
Often employed in “smart sensors” since they have the necessary processing power locally. In most cases a sensor of this type will have a local microprocessor supplied with power from the controller or have its own source of power The digital data may then be transmitted over regular lines or even through a wireless link. Since digital data is much less prone to corruption, the method is both obvious and very useful.

Data transmission Many sensors are analog and,
Their output may eventually be converted into digital form but: It is not always possible to incorporate the electronics locally. This may be because of cost or because of operating conditions such as elevated temperatures.

Data transmission Example, in a car there may be a half dozen sensors that control ignition, air intake and fuel, all of which are needed for control of the engine and are processed by a central processor. It is not practical to supply each sensor with power and electronics to digitize their data when the processor can do that for all of them. In other cases, such as, for example, the oxygen sensor, the sensor operates at elevated temperatures, beyond the temperature range of semiconductors making it impossible to incorporate electronics in them.

Data transmission In such cases the analog signal must be transferred to the controller. A number of methods have been developed for this purpose. Three of these methods, suitable for use with resistive sensors, or with passive sensors are discussed next

Four wire sensing In sensors that change their resistance, such as thermistors, and piezoresistive sensors, one must supply an external source and measure the voltage across the sensor. If done remotely, the current may vary with the resistance of the connecting wires and produce an erroneous reading. To avoid this the method in Figure may be used.

Four wire sensing

Four wire sensing The sensor is supplied from a current source, i0.
This current is constant since the internal impedance of a current source is very high. The voltage on the sensor is independent of the length of the wires and their impedance. A second pair of wires measures the voltage across the sensor Since a voltmeter has very high impedance there is no current (ideally) in this second pair of wires, producing accurate reading. This is a common method of data transmission when applicable.

Two wire sensing for passive sensors
Passive sensors produce a voltage. It is sometimes possible to measure the voltage remotely (no current is involved in the measurement). Especially true for dc outputs such as in thermocouples. In sensors with high impedance it is much more risky to do so because of the noise the lines can introduce. In most cases a twisted pair line is used because it reduces the noised picked up by the line.

Two wire transmission for active sensors
A common method of data transmission for sensors, and a method that has been standardized is the 4-20 mA current loop. The output of the sensor is modified to modulate the current in the loop 4 mA corresponds to minimum stimulus 20 mA corresponds to maximum stimulus The configuration is shown in Figure

4-20 mA current loop data transmission

The sensor’s output must be modified to conform to this industry standard and this may require additional components. Many sensors are made to conform to this standard so that the user only has to connect them to the two-wire line. The power supply depends on the load resistance and the transmitter’s resistance but it is between 12 and 48V.

Usually the sensor’s network allows for setting the range (minimum and maximum value of the stimulus) to the 4 mA and 20 mA range as shown. The current transmitted on the line is then independent of the length of the line and its resistance. The voltage measured across the load resistance is then processed at the controller to provide the necessary reading.

Other methods of transmission
There are other methods of transmission that may be incorporated. 6-wire transmission is used with bridge circuits in which the 4 wire method above is supplemented by two additional wires which measure the voltage on the bridge itself. A new 1-wire protocol has become very popular for many devices including sensors. In this protocol both power to the device and data to/from it are passed on a single pair of wires, An effective and economical method for sensing.

Transmission to actuators
There are only two ways the power can be transmitted to the actuator. One is to get the actuator close to the source that provides the power. This implies that lines must be very short. Possible in some cases (audio speakers, control motors in a printer, etc.). In some cases this is not practical and the controller and the actuator must be at considerable distance (robots on the factory floor, etc.).

Transmission to actuators
In such cases one of the methods above may be used to transfer data but the power must then be generated locally at the actuator site. The controller now issues commands as to power levels, timings, etc. and these are then executed locally to deliver the power necessary. Much of this is done digitally through use of microprocessors on both ends.

Excitation methods and circuits
Sensors and actuators must often be supplied with voltages or currents Either ac or dc. These are the excitation sources for the sensors and actuators. First and foremost is the power supply circuit. In many sensors the power is supplied by batteries Many others rely on line power through use of regulated or unregulated power supplies.

Excitation methods and circuits
Other sensors require current sources (for example - Hall elements) Still others require ac sources (LVDTs) These circuits affect the output of the sensor and its performance (accuracy, sensitivity, noise, etc.) Are an integral part of the overall sensor’s performance.

Power supplies There are two types of power supplies
Linear power supply Switching power supply. There are also so called dc to dc converters which are used to convert power from one level to another, sometimes as part of the circuit that uses the power.

Power supplies A linear power supply is shown in Fig. 11.28.
Consists of a source, (line voltage) and a means of reducing this voltage to the required level ( a transformer). The transformer is followed by a rectifier which produces dc voltage from the ac source. This voltage is filtered and then regulated to the final required dc voltage. A final filter is usually provided. This regulated power supply is very common in circuits especially where the power requirements are low. Some of the blocks may be eliminated depending on the application. If, for example the source is a battery the transformer and the rectifier are not needed and the filtering may be less important.

Linear regulated power supply

Linear power supply Consider the circuit in Figure 11.29.
This is a regulated power supply capable of supplying 5V at up to 1A. Transformer reduces the input voltage to 16V rms. This is rectified through the bridge rectifier and produces 22V (16x1.4) across C1, C2. These two capacitors serve as filters – the large capacitor reducing low frequency fluctuations on the line, the smaller capacitor is better suited for high frequency filtering.

Fixed voltage regulated power supply

Linear power supply The LM05 is a 5V regulator which essentially drops across itself 19V to keep the output constant. Does so for any input voltage down to about 8V. The capacitors at the output are again filters. The current is limited by the capacity of the regulator to dissipate power due to the current through it and the voltage across it. Other regulators are available that will dissipate more or less power.

Linear power supply These regulator exist at standard voltages, either positive or negative as well as adjustable variable voltage regulators. Discrete components regulators can be built for almost any voltage and current requirements. This circuit or similar circuits are the most common way of providing regulated dc power to most sensor and actuator circuits.

Linear power supply The advantage is that they are simple and inexpensive but they have serious drawbacks. The most obvious is that they are big and heavy, mostly because of the need for a transformer which must handle the output power. In addition, the power dissipated on the regulator is not only lost but it generates heat and this heat must be dissipated through heat exchangers.

Switching power supply
An alternative method of providing dc power is through use of a switching power supply. Switching power supplies rely on two basic principle to eliminate the drawbacks of the linear power supply. The principle is shown in Figure First, the transformer is eliminated and the line voltage is rectified. This high voltage dc is filtered as before.

Regulated switching power supply

The switching transistor is driven with a square wave It turns on for a time ton and off for a time toff When on, a current flows through the inductor charging the capacitor to a voltage which depends on ton When the switch is off, the current in L1 is discharged through the load supplying it with power for the off-time The voltage is stabilized by sampling the output and changing the duty cycle (ratio between ton and toff) to increase or decrease the output to its required value This change in duty cycle is done by use of a PWM (pulse width modulation) generator

In a practical power supply additional considerations must apply. First, it is necessary to separate or isolate the input (which is connected to the line) and output. In the linear PS this was accomplished by the transformer. Second, the switching, which must necessarily be done at relatively high frequencies, introduces noise into the system. This noise must be filtered for the PS to be usable

DC to DC converters Transformerless DC-DC converters also common
DC to DC converters are a different type of switching power supply. They take the dc source and convert it into an ac voltage This is then converted through a transformer to any required level and then rectified back to dc and regulated. The advantage of this approach is that now the transformer provides the isolation required for safety because the operation is at high frequencies, the transformer is much smaller than the power transformer in linear power supplies Transformerless DC-DC converters also common

Current sources The generation of constant current can take various levels of complexity. One can resort to something as simple as a large resistor in series with a power supply In this configuration the current is not constant but rather varies because the resistance of the sensor More accurate methods of current generation are needed for higher accuracy requirements.

Current sources A simple constant current source can be built based on the properties of FETs Shown in Figure As long as the voltage across the FET is above its pinch-off voltage (Vp), the current is constant and equals (Vcc-Vp)/R Vp is constant for any given FET

FET constant current generator

Current sources Another simple way of supplying constant current to a load is shown in Figure The Zener diode voltage Vz produces a current in the load equal to (Vz-0.7)/R3 (the voltage across the base-emitter junction is fixed at 0.7V and the zener voltage is fixed to Vz).

Zener controlled constant current generator

Current sources A stable circuit is the so-called current mirror

Current sources A current iin is generated as V1/R1 and is kept constant. The collector current in the lower left transistor is virtually equal to iin. The voltage across the base of Q1 keeps the current through the load equal to iin, hence the name current mirror. As long as iin is constant, so will the current in the load.

Current sources The properties of the voltage follower based on an op-amp can be used to generate a constant current as shown in Figure The output of the voltage follower is V1 and the current is V1/R1. The transistor is necessary to provide currents larger than those possible with an op-amp

Voltage follower based constant current generator

Voltage references Many applications call for a constant voltage reference. A regulated power supply is a voltage reference but what is meant here is a constant voltage, usually of the order of 0.5-2V that supplies very little current, if any, and is used as reference to other circuits. These reference voltages must be constant under expected fluctuations in power supplies.

Voltage references The simplest voltage reference is the Zener diode
Reversed biased diode, biased at the breakdown voltage for the junction. The resistor limits this current so that the diode does not overheat. As long as the maximum current of the Zener diode is not exceeded the voltage across the diode is kept at the breakdown voltage. These diodes are very commonly used for voltage regulation and other purposes.

The Zener diode

Reference zener diode A Zener diode specifically designed for voltage reference (called reference Zener diode) The breakdown voltage is kept constant and Temperature compensated using two diodes in series, one forward and one reversed biased In the forward biased diode, an increase in temperature decreases the forward voltage (by V or about 2mV/C) In the reversed biased diode it decreases it by roughly the same amount.

The reference Zener diode

Reference zener diode The total voltage is constant (or nearly so).
Reference diodes are available in voltages down to about 3V. Another device that is used for this purpose is the band-gap reference. It is superior to Zener diodes and is available in voltages that go down to 1.2V. Reference diodes are available commercially in standard voltages from about 1.2V to over 100V.

Oscillators Many sensors and actuators require voltages or currents that are variable in time. Example: the LVDT requires a sinusoidal sources, often at a few kHz in frequency. Magnetic proximity sensors use ac currents of constant amplitude and frequency to produce an output voltage which is proportional to position. Transformer based sensors must use an ac source. Other sensors require special waveforms such as square waves.

Oscillators Some sensors/actuators use line power (60 or 50Hz),
All other sources must be generated at the correct frequency and at the required waveform. Often must be frequency stabilized and amplitude regulated to make useful sources. There are virtually hundreds of different ways of generating as signals but there are a few basic principles involved.

Oscillators An oscillator is an unstable amplifier.
Starting with an amplifier of some sort, one can provide a positive feedback to make it unstable and hence to set it into oscillation. The unstable circuit must be forced to oscillate at a specific frequency by means of: an LC tank circuit (or equivalent) or a delay in the feedback The circuit must be made to oscillate with a required waveform through use of these or additional components.

Crystal oscillators Based on a quartz crystal or other piezoelectric materials Cut and placed between two electrodes The equivalent circuit is an RLC circuit Can oscillate in one of two modes. One is a series oscillation mode, The other is parallel mode oscillation When connected in a circuit that can provide the proper positive feedback, it will oscillate at the resonant frequency of the crystal

Structure of a crystal

Equivalent circuit of a crystal

A 1 MHz crystal

Sinusoidal crystal oscillator
Simple sinusoidal oscillator The feedback from output to input (collector to base) is supplied by the crystal. The output is entirely defined by the crystal and is taken at the collector. The trimmer capacitor modifies the equivalent circuit.

Sinusoidal crystal oscillator

Square wave crystal oscillator
Based on two inverting gates Because the gate can only take two states, the output will swing between Vcc and ground. The positive feedback is delayed due to the delay of the gate and the frequency is controlled by the crystal. These oscillators can be used, for example, in mass humidity sensors in which the frequency will change with humidity (mass of the crystal).

TTl based square wave crystal oscillator

RC Oscillators Oscillators can easily be built from discrete as well as integrated components without a crystal. A simple square wave oscillators based on the delay of the feedback signal (RC) is shown next

RC oscillators The inverters are triggered when the input voltage rises above about Vcc/2. Resistor R and capacitor C form a charging circuit. Suppose left gate is on (zero input, Vcc output). The second gate must be off (its output is zero) Lower capacitor charges (time constant RC) and after a time t0 triggers left gate to change state. Now its output is zero and the capacitor discharges through R. The upper capacitor is only needed for stability of the circuit.

RC oscillators The following circuit is somewhat similar.

RC oscillator Positive feedback through R3 sets the level at which the amplifier changes state. R4 and C1 form the charging/discharging circuit. Suppose that Vout is high. The positive input will be set at a value that depends on R3, R2 and R1. C1 charges through R4. When the voltage at the negative input exceeds that at the positive input the output goes negative Now the capacitor discharges through R4, repeating the process.

LC oscillator Examples of sinusoidal oscillators
An LC circuit is provided which oscillates at the required frequency A feedback is provided from output to input The feedback is through the lower part of L1 or through the lower half of the LVDT coil (figures)

Sinusoidal LC oscillator

Noise and interference
Noise is understood as anything that is not part of the required signal. Many sources and many types of noise. We will distinguish between two broad types Inherent noise to the sensor (internal). Interference noise (external).

Inherent noise Noise must be reduced as much as possible
elimination is not an option since noise cannot be entirely eliminated More important is to properly consider it in the design and in the specification of the sensor. Example: a temperature sensor generates 10 V/C and a good microvolt meter is capable of reliably measuring 1 V. This, would imply a resolution of 0.1C.

Inherent noise Suppose noise (from all sources) is, say, 2 V
Only signals above the noise levels are useful Any signal below 2 V is useless. The resolution cannot be more than 0.2 C. In many cases, things are worse than this since the noise can only be estimated. When amplification occurs, noise is also amplified and the amplifier itself can add its own noise. Clearly then noise cannot be ignored even when it is small.

Inherent noise Inherent noise is due to many effects in the sensor
Some of the sources are avoidable, Some of the sources are intrinsic. One of the main sources in sensors is the thermal noise or Johnson noise in resistive devices. The noise power density is usually written as: k is the Boltzman constant (k=1.38x10-23 J/K), T is the temperature in K, R is the resistance in  f is the bandwidth in Hz.

Inherent noise This noise exists, in resistive sensors and in simple resistors Ff the resistance is high, the noise can be very high. The Johnson noise is fairly constant over a wide range of frequencies Hence it is called a white noise

Inherent noise Shot noise:
Produced in semiconductors when dc current flows by random collisions of electrons and atoms: Preference is for lower currents in as much as this noise is concerned.

Inherent noise Pink noise:
Unlike white noise has higher energy at low frequencies. A particular problem with sensors which tend to operate at low frequencies (slowly varying signals). The noise spectral density is 1/f and at low frequencies it may be larger than all other sources of noise.

Inherent noise Noise levels are very difficult to measure even when the noise is constant. Because it is not generally harmonic in nature, its rms or even peak to peak values are difficult to ascertain. The noise distribution is not constant (usually Gaussian) so that at best we can estimate the noise level. Usually maximum expected levels are indicated.

Interference By far the largest source of noise in a sensor or actuator Originates outside the sensor and is coupled to it. Sources of interference can be many: Best known perhaps are the electric sources: coupling of transients from power supplies, electrostatic discharges radio frequency noise from all electromagnetic radiative systems (transmitters, power lines, almost all devices and instruments that carry ac currents, lightning and even from extraterrestrial sources).

Interference Interference can be mechanical Thermal sources (
Vibrations gravitational forces acceleration and others, Thermal sources ( temperature variations Seebeck effect in conductors Also: ionization sources, errors due to changes in humidity and even chemical sources.

Interference Some errors are introduced in the layout of the sensors components or in the circuits connected to them through improper circuit design and improper use of materials. Electrical sources of noise are called electromagnetic sources (including static discharges and lightning) Are bundled together under the umbrella of electromagnetic interference or electromagnetic compatibility issues.

Interference In some cases, a noise is easily identifiable.
Example: a common noise in electrical system, especially those that contain long wires, is a 120Hz noise (100 Hz in 50Hz power systems) and is due power lines. This type of noise is also a good example of a time-periodic noise. Other sources, especially when transient or random are almost impossible to identify and hence to correct.

Interference Interference noise may affect different sensors differently. The simplest is an additive influence. That is, the noise is added to the signal. Additive noise is independent of the signal. Additive noise is more critical at low signal levels Example: drift due temperature variations depends on temperature but not on the signal level. This type of noise can be minimized by using a differential sensor

Interference A second type of noise is multiplicative.
That is, it grows with the signal and is due to a modulation effect of the noise on the signal. More pronounced at higher signal levels. The noise may be minimized by using two sensors as previously the output is divided by the reference sensors’ output. Example: a stimulus is measured (say, pressure) and a noise due to change in temperature T is present and multiplicative.

Interference Assume the transfer function is V=(1 + N)Vs
One sensor senses both the stimulus and the noise and produces an output V1 which is: The second sensor senses only the temperature and produces a voltage V2 V0 can be assumed constant (i.e. it is only dependent on temperature change)

Interference The ratio between the two is:
Since V0 is independent of the sensed stimulus, the ratio is also independent of the noise. This is called a ratiometric method and is most suitable for this type of noise

Interference Reduction of noise before it reaches the sensor.
Most important is electrical noise Electrical noise can reach the sensor in four ways through direct resistive coupling Through capacitive coupling Through inductive coupling By radiation from outside the sensor

Interference - Resistive coupling
Source of noise and the sensor share a common resistive path. May be the resistance between the connection of a sensor, through the sensor’s body. That is, the sensor is not electrically insulated from the source of noise. Solution: isolation of the sources of noise (usually current carrying conductors such as power lines) from the sensor. Often this will require that the sensor be floating.

Interference - capacitive coupling
Capacitance exists between any two conductors, Any two wires, any two connectors will produce a stray capacitance that can cause coupling. Capacitances are small - impedances are high. Capacitive coupling is a problem at higher frequencies. There are however sensors, especially capacitive sensors which use small capacitances Any capacitive coupling may be too high for accurate sensing.

Solution: the sensor must be electrostatically shielded from the sources that might couple noise. An electrostatic shield is usually a thin conducting sheet, sometimes a conducting mesh, which envelopes the protected area and is grounded (connected to the reference potential. In effect this shorts the noise source to ground. An example is shown in Figure

Electrostatic shielding

The coupling capacitance is shorted This also creates a new capacitance between the protected device and ground. But, the noise signal is zero. Cables leading to the sensor must also be shielded The shield must be at a constant potential. Example: shielding a cable and then grounding it at both ends, will immediately produce a loop which may itself generate noise.

Interference - inductive coupling
A particular problem between current carrying conductors Example: between power lines and sensors’ conductors and in particular the wires leading to the sensor. 120 Hz noise from power liner usually links to sensors through inductive coupling Actuators may induce currents in sensors Sensors may interfere with each other

Interference - inductive coupling
At high frequencies, a conducting shield just like the electrostatic shield should envelope the source. The use of coaxial cables is such an example. Based on the idea of skin depth (Chapter 9) and simply takes advantage of attenuation of high frequency fields in conductors. If the noise signal is very low in frequency, a magnetic shield is necessary. Usually a thick ferromagnetic shield (box) that envelopes the protected device to guide low frequency (or DC) fields away from the sensor. Proximity sensors often use this type of shield.

Interference Together, conduction, capacitance and inductance form a class of coupling called conductive coupling and is part of the common problem of conducted emission and conducted interference in electromagnetic compatibility.

Interference - radiated emission
Any conductor carrying an ac current is in effect a transmitting antenna. Any other conductor becomes a receiving antenna. If that conductor is part of a loop, a current will be induced in the loop. This noise is particularly large from sources of intentional emissions such as transmitters Can occur with any current, internal or external to the sensor.

Reduction of this source relies extensively on reduction of lengths of wires and on reduction of size (area) of loops. Shielding is very effective in reducing radiated interference. Other precautions: use of decoupling capacitors in circuits and power supplies Twisting of the two wires leading to a device together to reduce the area of the loop they form.

Coaxial cables can reduce or eliminate most radiated interference. One common cure for many ills is the introduction of a ground plane – a sheet of metal under the circuit (such as a conducting sheet under a printed circuit board). This helps in reducing the inductance of the circuit and hence will be effective in reducing both inductive coupling and radiated interference.

Mechanical noise Mechanical noise, especially from vibrations can often be eliminated or reduced through isolation Some sensors, such as piezoelectric sensors, any force (due to acceleration) will produce errors These errors can be compensated either through use of the differential or ratiometric methods Many other sources of noise

Other sources of noise Example: any junction between different metals becomes a thermocouple and introduces a signal in the path. This may affect the reading of the sensor and is called Seebeck noise. It may not be a big problem in most cases but it is when sensing temperature. The issue of noise is both difficult and ill-defined. Often finding the source of noise will depend on sleuthing work and on experimentation.

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