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**Circuit Design and Examples**

Design Guidelines Objectives Sensor Signal Conditioning Example Instrumentation Amplifiers AD620 OpAmps Op07 (modern) LM741 (ancient) 1. Hello again. In this presentation I will discuss design guidelines for transducer circuits with analog signal conditioning. If you follow these guidelines, you will plan your process prior to building the project and will generally be successful. First you must determine the objective of your measurement, then select the sensor to use and finally design your signal conditioning so the output meets your requirements whether it is 0 to 5 volts for an ADC or 4-20 mA for a current loop. Then, I will discuss an example of a problem from the text and how to design a circuit and how to produce a graph showing a sweep in the value of a resistor in PSpice. The specifications and uses of instrumentation amplifiers and a specific example, the AD620 will be discussed next and finally, a modern, low-cost, general purpose opamps specs will be compared with those of the venerable 741. Transition: First will be a more detailed discussion of design guidelines.

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**Design Guidelines-1 Define the measurement objective**

Parameter: What do you need to measure: pressure, temperature, flow, level, etc Range: What is the range of the measurements? F, psi, volts Accuracy: What accuracy is desired and what specification of accuracy will be used? 5% of Full Scale or 2% of the reading. Linearity: Must the measurement be linear? Noise: How much noise is allowed? 2 First you must define the measurement objective. The parameter to be measured must be well understood. For instance if you must measure the level of a chemical in a large tank, you must know if it is corrosive, if the tank is open or closed. You must physically understand the parameter and the conditions that you will measure. Next, you must understand the full range of measurements desired. For instance, if the temperature range can vary from -20 to 200 degrees Fahrenheit but you only need to measure from 32 to 100 degrees F, you must understand this to choose the correct sensor. The accuracy is important. For instance you may need to measure weight within ±1 pound or measure temperature within ±1 degree Celsius. The author often specifies the accuracy in terms of full scale of the measurement, but it can be an absolute accuracy like the two examples just discussed. Linearity may be important in the measurement while many sensors are nonlinear. For instance, the thermistor is highly nonlinear as is the light sensitive resistor. You can linearize measurements by using a table in a microcontroller or microprocessor or using analog techniques. All measurements have noise. This is due to the heating of the wires and resistors and IC’s or to interference at low frequencies (for instance from harmonics of the power line) or from pickup from other electrical devices. You must be aware of the noise and ensure that the measurement is accurate even taking the noise into account. Transition: Next is a look an selection of the sensor/transducer.

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**Design Guidelines-2 Select the sensor/transducer**

Sensor Definition in Engineering: the component of an instrument that converts an input signal into a quantity that is measured by another part of the instrument and changed into a useful signal for an information-gathering system. A transducer is an electronic device that converts energy from one form to another. Common examples include microphones, loudspeakers, thermometers, position and pressure sensors, and antenna. Although not generally thought of as transducers, photocells, LEDs (light-emitting diodes), and even common light bulbs are transducers. Design Guidelines-2 Select the sensor/transducer Parameters: What is the input and output of the transducer? E.g. pressure in resistance out, temperature in voltage out, light in current out Transfer Function: Output/input relationship? Time Response: 1st order, 2nd order? Range: What is the possible range of sensor parameters? C, 3-15 psi, etc Power: What is the power specification of the sensor? 3 The definitions of Sensors and Transducers is shown at the top of the slide. Many times the words sensor and transducer are used interchangably and you can see why from the definitions. There are many of types of sensors to measure most parameters. You will see some of the many types and their characteristics in later presentations, but to select the sensor/transducer you must know the characteristics of the measurement to be made. In addition to knowing which physical quantity to measure such as temperature, you must know the desired output like a current loop or a 0 to 5 volt voltage to an ADC. The Input-Output relationship, which is called the transfer function must also be known. You have seen transfer functions before and I used the output divided by the input to calculate mathematic transfer functions for filters. Sometimes a mathematical equation is not possible, but there is still a relationship defined by the maximum input and output values. There are always time constants involved in every measurement. However, there can be first order sensors with a single time constant or 2nd (or higher) order sensors with 2 or more time constants and or a damped sinusoidal response as was discussed earlier. The range and power are also important and you must know this before designing a circuit. Transition: Next is a look at several principles of designing the analog signal conditioning for a sensor system.

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**Design Guidelines-3 Analog Signal Conditioning**

Parameter of output? Voltage, current, pressure, frequency Range? 0-5V, 3-15psi, 4-20mA, Hz Input impedance of the signal conditioning circuit? Many sensors require a specific impedance input or a range of allowable inputs Output impedance to the next stage? 4 Most of the items here were covered earlier, but you must know the input and output impedance of the signal conditioning circuit that you build. Sensors generally specify their output. For instance a transducer that you will use has a current output, but the maximum output current is only a few mA. You must ensure that your signal conditioning circuit does not try to draw to much current from the sensor or it will either burn out or provide incorrect results. Also, as an example, if you are feeding an power device with your signal conditioning circuit, you must ensure that your output impedance matches the input impedance of the next stage for maximum power transfer. Transition: Next is the first example of the design of an analog signal conditioning circuit.

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**Example: Problem 2.33 in text My solution (check the solution manual for the author’s solution)**

2.33: A bridge circuit has R1=R2=R3=120 ohms and V=10.0 volts. Design a signal conditioning system that provides an output of 0 to 5 volts as R4 varies from 120 to 140 ohms. Plot Vout vs R4. Evaluate the linearity Desired output is VA-B 5 The problem is provided on the left of the slide. The bridge circuit drawn using schematics is also shown. This is a standard method of placing a sensor in a Bridge circuit so that when the resistance of the sensor varies, the output voltage between nodes A and B is measured and the result input to the analog signal conditioning system. Transition: Next is a look at how to create a graph of the voltage difference between nodes A and B, in PSpice, by varying the resistance of R4. A A B

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**How to create a graph with a varying resistance**

Place a normal resistor as R4. Double click on the value of the resistance Enter {Rvar} [Yes the braces are required] Go to “Get New Part” in the Draw Menu Place the part name “PARAM” on the schematic page Double click on Parameters Enter Rvar as Name1 And Value1=100 How to create a graph with a varying resistance Go to Analysis/Setup menu Check and open the DC Sweep box Check Global Parameter and Linear Enter the Name of the variable Which is Rvar here, the start and end values of the sweep and the increment for the sweep 12. Simulate 6 The circuit is shown in the lower right of the slide and the steps to create a graph of the voltage between node A and node B is shown on the left. Of course, you must select the correct graph to plot in Probe for this problem Transition: Next is a plot of the voltage between node A and node B for a large range of values of R4. A A B 10

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A B 7 Notice that I selected sweep values from 10 ohms to 1000 ohms to show you the nonlinearity of the graph. Also notice that I had Probe do the math for the difference between the voltages between nodes A and B. The equation for the difference in voltages is also shown on the slide and you can see that it is simply the difference between the two voltage dividers multiplied by the input voltage of 10. Notice in the blue Probe cursor box that at R4 = 120 ohms the difference is approximately 0 (m means millivolts here). You can also see that at R4=240 ohms, the voltage difference is volts which you can verify using the equation. You can see that the line is not linear, but if only look at a small part of the line, it can look linear Transition: Next is a look at the voltage difference for a variation of only 120 to 140 ohms as required in the problem. VA-VB

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**VA-VB 0 to .3846 to 0 to 5 Requires a gain of 5/.3846=13.0 A B 8**

Here is a plot of the variation in VA-VB for a variation of 120 to 140 ohms for R4. You can see by the Probe cursor measurements that the output voltage is 0 for R4=120 ohms and is volts for 140 ohms. The graph appears linear because we are only using a small portion of the larger curve on the previous graph. So, what is needed is an output of 0 to 5 volts after signal conditioning with an input of 0 to before the signal conditioning. This seems pretty straightforward and we just need a gain of 5/.3846, which turns out to be 13.0. Transition: Next is look at a circuit that can provide a differential gain of 13.0.

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**Need Differential Gain of 13**

9 A differential amplifier like that looked at earlier will provide the correct gain of 13.0 so that the output to the load is 0 to 5 volts. Two items to remember about this circuit are: 1. The differential amplifier gain is only as good as the values of the 4 resistors. The two 130k resistors must be exactly matched and the two 10k resistors must also be exactly matched to provide a gain of 13.0 with 3 significant figures. 2. The output to the load is also slightly nonlinear and would look exactly like the graph on the previous slide except it would go from 0 to 5 instead of 0 to So, this is a solution of this problem. A better solution would be to use an instrumentation amplifier. An instrumentation amplifier provides a specific differential gain that can be set using a single resistor. Transition: Next is a look at instrumentation amplifiers. Need Differential Gain of 13

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**Instrumentation Amplifiers**

Analog Devices Inc. is the largest supplier of instrumentation amplifiers in the world. The AD620 is a low cost, high accuracy instrumentation amplifier which requires only one external resistor to set gains of 1 to Furthermore, the AD620 offers lower power (only 1.3 mA max supply current), making it a good fit for battery powered, portable (or remote) applications. The AD620, with its high accuracy of 40 ppm maximum nonlinearity, low offset voltage of 50 µV max and offset drift of 0.6 µV/°C max, is ideal for use in precision data acquisition systems, such as weigh scales and transducer interfaces. The low noise, low input bias current, and low power of the AD620 also make it well suited for medical applications such as ECG and noninvasive blood pressure monitors. The low input bias current of 1.0 nA max is made possible with the use of Superbeta processing in the input stage. The AD620 works well as a preamplifier due to its low input voltage noise of 9 nV/Hz at 1 kHz, 0.28 µV p-p in the 0.1 Hz to 10 Hz band, 0.1 pA/µHz input current noise. The AD620 is also well suited for multiplexed applications with its settling time of 15 µs to 0.01% and its cost is low enough to enable designs with one in amp per channel. 10 Lets look at the first In-Amp on the Analog Devices instrumentation webpage. Analog devices has outstripped most of its competitors for analog semiconductor devices and also produces an excellent line of digital signal processing products. Some of the amplifier specifications are shown here, in writing, rather than a spec table. You will see some of these in a more traditional form later. Transition: Next is a look at the a some of the specifications for this In-Amp.

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AD620 Specifications common-mode rejection ratio (CMRR): The ratio of the common-mode interference voltage at the input of a circuit, to the corresponding interference voltage at the output. 11 The specs shown here are from the Analog Devices website shown on the slide. Notice that the gain of 1 to 1000 can be set with a single resistor rather than having to match resistors with the differential amplifier. The input offset voltage is quite small, only 50uV and the input offset drift due to temperature change is also in the microvolt range. The Common-Mode Rejection Ratio, called CMRR, of 100 dB minimum is important when a differential amplifier is used. This means that if, for instance, the same 1 volt, 60 Hz sine wave is at BOTH the + and – inputs, the output will only contain the 60 Hz sine wave with an amplitude that is down by a minimum of 100dB in voltage. 100 dB is a factor of 100,000 so the output would be less than 10 uvolts. There is often common noise or power supply voltages on both inputs to a differential amplifier. The differential amplifier that I designed for the example problem eliminates a lot of this, but the integrated circuit instrumentation amplifier has all of the resistors matched internally and if far more precise than one made from an opamp. This is a low noise amplifier. We are not covering this specification at this time. The Bandwidth is 120 kHz with a gain of 100 so the Gain-Bandwidth Product (GBWP) is 12 MHz, which is good. What this means is that if the instrumentation amp is set for a gain of 1, the 3 dB frequency will be at 12MHz. If the gain is set at 10, the 3dB frequency will be 1.2 MHz and for a gain of 1000, the 3dB frequency would be 12 KHz. Some of the applications are listed at the bottom. In-Amps are widely used in accurate instrumentation. Transition: Next is a look at the electrostatic warning for this device.

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**Electrostatic Warning for the AD620 In-Amp**

12 Those of you already working in any area of electronics already know the precautions taken so that ESD does not destroy integrated circuits during assembly and handling of circuit boards and devices. Shown here is the ESD warning for the AD-620 instrumentation amplifier. Remember that almost all Integrated circuits should be handled with care even though many circuit boards and individual IC’s have built in protection circuitry, you should still ground yourself prior to handling them. An example is replacing a circuit card in a Personal Computer. You should sit and touch a bare metal part of the computer frame prior to opening the new card or replacing the old card so that your body is at the same potential (voltage) as the computer frame. Transition: Next is a look at a typical error budget for an application.

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**AD620 vs opamp Make vs. Buy: A Typical Bridge Application Error Budget**

The AD620 offers improved performance over “homebrew” three op amp IA designs, along with smaller size, fewer components and lower supply current. In the typical application, a gain of 100 is required to amplify a bridge output of 20 mV full scale over the industrial temperature range of –40°C to +85°C. Regardless of the system in which it is being used, the AD620 provides greater accuracy, and at low power and price. Note that for the homebrew circuit, the OP07 specifications for input voltage offset and noise have been multiplied by 2, because a three op amp type in-amp has two op amps at its inputs. AD620 vs opamp 13 This slide shows a page from the AD620 data sheet that compares the use of the AD620A instrumentation amplifier with the use of three OP07 modern opamps to make a homebrew instrumentation amplifier to amplify the differential output of a precision bridge transducer. The gain for both is set at 100. Transition: Next is a look at the advantages of an Instrumentation amplifier over the homebrew type in this application, according to Analog Devices.

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14 This table from the Analog Devices AD620A data sheet shows that it is superior to the 3 opamp instrumentation amplifier that you could build using Op07 opamps. The Op07 is a modern, low-cost precision opamp that is used at ITT Aerospace-Communications system for opamp circuits placed aboard weather satellites. Notice that all of the errors are lower for the AD620. This is certainly understandable since it has all of the devices and resistors internal to the chip rather than having to match resistors, etc. Transition: Next is a look at a modern opamp, the Op07. Error Budget

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The OP-07 has very low input offset voltage (25µV max for OP-07A) which is obtained by trimming at the wafer stage. These low offset voltages generally eliminate any need for external nulling. The OP-07 also features low input bias current (±2nA for OP-07A) and high open-loop gain (300V/mV for the OP-07A). The low offsets and high open-loop gain make the OP-07 particularly useful for high-gain instrumentation applications. The wide input voltage range of ±13V minimum combined with the high CMRR of 110dB (OP-07A) and high input impedance provides high accuracy in the non-inverting circuit configuration. Excellent linearity and gain accuracy can be maintained even at high closed-loop gains. The OP-07 is available in five standard performance grades. 15 The Op07 is a modern opamp while the 741 is an old opamp that is still quite good for general purpose use. The LM709 was one of the very first opamps in the early 1970’s and the 741 replaced it. You will probably not see 741’s in industry except for very low cost, low precision circuits. Transition: Next is a comparison of some of the specifications of these two devices. The LM741 series are general purpose operational amplifiers which feature improved performance over industry standards like the LM709. They are direct, plug-in replacements for the 709C, LM201, MC1439 and 748 in most applications. Op07 vs LM741

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**Op07 vs 741 (Inexpensive versions of each) Op07 (Analog Devices)**

$1.25 for one Op07 25 for $25 $0.44 for one LM741 25 for $8 From Digikey (Inexpensive versions of each) Op07 (Analog Devices) LM741 (National Instruments) Input Offset Voltage 30 to 75 uV 6 to 7.5 mV Input Offset Current .4 to 2.8 nA 200 to 300 nA CMRR 110 dB Min 70 dB Min Closed Loop BW (gain = 1) .6 MHz .437 MHz Slew Rate .3 V/uSec .5 V/uSec 16 Some of the specs for the old LM 741 are compared with those of the newer Op07 design. You can see that the very low input offset voltage and current for the Op07 are highly desirable. The Common-mode rejection ratio of the Op07 is also much better than the 741, meaning that a common signal to both the + and = terminal will be reduced by a factor of 40 dB more than with a 741. This is a factor of 100. The closed loop bandwidth and the Slew rate for the Op07 are better than for the 741, but not a huge improvement. That is because the Op07 is sold as a device with low offset so that adjustments for offset voltage and current do not have to made for precision instrumentation like in the weather satellites. You can see the cost of an 8 pin dip for each device and for 25 of each from Digikey. This is why the 741’s are still used throughout the U.S in colleges and in low cost devices. Transition: Next is a summary of this presentation.

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**Summary Design Guidelines Example Instrumentation Amplifiers OpAmps**

Objectives Sensor Signal Conditioning Example Instrumentation Amplifiers AD620 OpAmps Op07 (modern) from Texas Instruments, Linear Technology, or Maxim LM741 (old but useful) from National Semiconductor 17 General design guidelines were discussed including determination of the objectives of an analog signal conditioning circuit. Some factors to take into account in the selection of a sensor were also discussed as well as items to include when planning the actual circuitry. An example was discussed including a method producing a graph of a DC sweep with a variable resistance to determine the effect of varying the resistance on a circuit. Some factors in the design of instrumentation amplifiers were discussed along with the specifications of a modern In-Amp, the AD620 from Analog Devices. Finally, the specifications for a modern, low offset opamp, the Op07, were compared with those for the venerable LM741. Transition: So long for now.

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**Digital Signal Conditioning**

AC Bridges Number systems Boolean Algebra Example Tristate Buffers Comparators and Circuits Schmidt Trigger Window Detector 1 In the first part of this presentation I will discuss AC bridge circuits that are similar to the resistor circuits discussed previously, except that they use resistors, capacitors and inductors and are driven by an AC source. Then, I will discuss number systems and finish this with an example of an assembly line circuit that uses simple Boolean algebra expression to determine an outcome. Several on/off type circuits are then discussed including buffers, comparators, the Schmidt trigger, and the window detector. Transition: We already covered Wheatstone bridge type resistive circuits, so first lets look at some AC bridge circuits.

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**AC Bridge Circuits Balanced when: Z1Zx = Z2Z3 Generalized AC Bridge A**

Resistor bridge circuits were discussed earlier. AC bridges are also useful and all take the general form shown on this slide. Instead of all resistors, impedances can be placed in each of the legs of the bridge and an AC signal used to drive the bridge. The condition for the bridge to be balanced is the same as with resistors except the impedances must exactly match for the voltage from A to B to equal zero. Transition: Next is a look at some useful AC bridge circuits. Balanced when: Z1Zx = Z2Z3

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**Condition for Balanced Bridge**

Rx 3 The AC bridge shown here is commonly known as a Wien bridge and is used frequently for oscillator circuits. The complex number representation of the condition required for a balanced bridge is shown at the top of the slide. Please work this out for yourself and ensure that you understand how the equation is derived. Of course, this equation must be in standard rectangular or polar format to ensure that the real and imaginary portions of each side of the equation are exactly the same. This can be worked out in this general form, but is easier to do when numbers are substituted. Transition: Next is an example of an active oscillator created using an Wien bridge. R3 Cx Wien Bridge

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**Wien Bridge Oscillator Circuit http://niuhep. physics. niu**

.001 uF 4 A Wien bridge oscillator circuit taken from the web site shown at the top of the page is shown here. The Physics lab at the website would make an excellent lab and you may substitute this for any of your labs, but ensure that your report includes information on Wien bridge oscillators. Note that the two resistive legs of the Wien bridge are the upper and lower portions of the potentiometer. Transition: There are many other types of AC bridges used widely in industry and a few of these are shown on the next slide. 10KΩ 10KΩ .001 uF Expected Sine Wave Frequency=15.9 KHz Adjust the 50K resistor for a sine wave output

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AC Bridges 5 Here are some other AC bridges that are useful, including the Wheatstone bridge that was discussed earlier. More information on these bridges can be found at the Website shown on the slide. Transition: Next is a look at some of the concepts and practice of using number systems.

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**Number Systems http://www.ibilce.unesp.br/courseware/datas/data1.htm**

BITS A bit is the smallest element of information used by a computer. A bit holds ONE of TWO possible values: 0 meaning Off/False/NotSet and 1 meaning On/True/Set Boolean Values Boolean algebra recognizes True and False. So a single bit can represent a Boolean variable. NIBBLE A nibble is a group of FOUR bits. This gives a maximum number of 16 possible different values ^ 4 = 16 LSB and MSB: The Least Significant Bit (LSB) is always drawn at the extreme right and has the least value and the Most Significant Bit (MSB) is always shown on the extreme left, and is the bit with the greatest value. 6 The URL shown at the top of the slide contains information about aspects of number systems. The three definitions shown here come from this site and should already be familiar to you. Remember that a binary one usually means On, True, or Set while a binary 0 means that bit is Off, False, or Not Set. A single bit can be True or False and thus, can represent a Boolean variable. Transition: Next are more basic number system definitions.

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**BYTES Bytes are a grouping of 8 bits. This comprises TWO nibbles.**

Binary Coded Decimal [BCD] Binary code decimal digits (0-9) are represented using FOUR bits. The valid combinations of bits and their respective values are 0000 through 1001 with the binary combinations 1010 to 1111 not used. If the computer stores one BCD digit per byte, its called normal BCD. The unused nibble may be all 0's or all 1's. Packed BCD: If two BCD digits are stored per byte, its called Packed BCD. This occurs in data transmission where numbers are being transmitted over a communications link. Packed BCD reduces the amount of time spent transmitting the numbers, as each data byte transmitted results in the sending of two BCD digits. 7 Shown on the slide are 3 more fundamental definitions used with digital signals. BCD is a simple code to convert from binary to decimal and vice versa. It simple uses the binary numbers 0000 through 1001 to represent their decimal equivalents. So, one binary nibble can hold a BCD representations of a single decimal digit. Of course, two nibbles, or one byte can then be used to hold two decimal digits and this has been used to send digits. The problem with BCD is that it wastes the binary numbers from 1010 to 1111. Transition: Next is a look at the hexadecimal system that is a good method of representing bytes of information in a short alphanumeric format. Number Systems

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**Number Systems Hexadecimal**

Refers to the base-16 number system, which consists of 16 unique symbols: the numbers 0 to 9 and the letters A to F. e.g. decimal 15 is represented as F in hexadecimal. This is useful because it can represent a byte (8 bits) as two hexadecimal digits. It is easier to read hexadecimal numbers than binary numbers. To convert a value from hexadecimal to binary, translate each hexadecimal digit into its 4-bit binary equivalent. Hexadecimal numbers have either an 0x prefix or an h suffix. For example, the hexadecimal number 0x3F7A translates to the following binary number: 8 The hexadecimal number system is important because it provides a shorthand notation for the binary numbers that are understood by microprocessors and microcontrollers. You should already be familiar with this number system. Notice that in the slide show, each of the underlined definitions refers directly to its definition in the Webopedia website. The homepage of this important website is shown near the bottom of the slide. When I need to look up a computer definition, I usually end up at Webopedia. Transition: Next is a look at multiplication and division.

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**Multiplication/Division**

Multiplication by 10: Shifting left in decimal multiplies by 10. E.g Multiplication by 2: Shifting left in binary multiplies by 2. E.g which translates to Division works the same way in that shifting right divides by 10 in decimal, 2 in binary, 8 in octal, and 16 in hexadecimal. 9 This slide is also a simple review of multiplication and division in different number systems. Shifting one place to the left in any number system multiplies the previous number by the base of the number system as shown on the slide for decimal and binary. Shifting right in any number system divides by the base value of the number system. You all learned this in decimal long ago and this is just a reminder that the basic operations you learned in decimal apply to other number systems. Binary and hexadecimal should be the most familiar to you because of your previous work with computers and programming. Transition: Next is a look at an On/OFF circuit.

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**Push-On Push-Off Control Circuit http://www. oldradio**

N.C. 24 volt dc N.O. Relay Coil 10. The most basic control is On-Off control of a process or system. Here is a relatively simple push button On-Off control circuit that was the National First Prize Winner for Broadcast Management and Engineering Magazine's “Great Idea Contest" for It is still quite useable. Here is how it works from the website shown at the top of the slide: When power is first applied to the circuit, the parallel combination of C & R2 is charged to 1/2 VCC through the voltage divider R1/R2. When The Push-Button Switch is closed, The capacitor discharges through the relay coil, RL1. This pulls in the relay , and the wiper is held in by the current through RL1 and R1. Any charge remaining on the capacitor discharges through R2, so when PB1 is pressed again, the capacitor has zero voltage on it and acts like a short. This makes the relay drop out, returning the circuit to its original condition. So, the circuit provides a toggling 'Push-On / Push-Off' action using two resistors, a capacitor, and a relay. And because of the simplicity, it's virtually foolproof. Transition: Next is an example of the use of Boolean algebra in an on-off assembly line. Push-Button Switch

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**Example A B C D 1 Wt Sensor A IR Sensor B Robot Welder C Vision System**

1 Example Wt Sensor A IR Sensor B Robot Welder C Vision System D A + B AB A B Nand 11 Shown here is an assembly line example using simple boolean algebra and should be a review. Assume that an assembly line has carts moving to the right as shown at the top of the slide. There are four positions on the line where decisions can be made. The weight sensor at A, the Infrared Sensor and B, the Robot Welder and C and the Operator at D. At point A, a one denotes that the weight sensor has detected and weighed a cart at its position. At point B, a one denotes that the infrared sensor has measured the length of the cart as correct at its position. At point C, a one denotes that the robot welder has completed working on the cart. At point D, a one denotes that the vision system has inspected the results and it is ok. ABCD indicates that everything is ok and the line moves forward and is shown on the right of the logic diagram on the bottom right of the slide. One critical time is when sensor A or B is zero and then the system must remain paused since either the sensors are not activated, meaning a cart is not in place in front of them or there is a malfunction. Another critical time is when either C or D is zero, meaning that the Welder is not finished or the vision system has not finished inspecting the results. During both of these times, the system must pause until the sensors register correctly. These two conditions are shown at the output of the NAND gates in the diagram so that if either of these is True, a switch can be turned off to alert the supervisor that there is a problem. This is a simple example, but should provide some material for you to review. Transition Next is a look at Tri-state buffers. ABCD C D CD Nand C + D

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**Tri-state Buffers 1 1 Enable**

Enable 12 PLC’s, microcontrollers, and other computers use tri-state buffers to connect inputs and outputs. The buffer shown here is from the website shown at the top of the slide. The state of the buffer is determined by the digital line labeled “C”. If “C” is clear, or low, or “0”, the output of the buffer is high impedance. If “C” is set, or high, or “1”, the output of the buffer is the same as the input. Remember two things from this simple example and from the web page shown: 1. A buffer is generally used to protect an output. For those of you who took EET205 and used PIC microcontrollers, you know that Tristate buffers had to be set or cleared for each input/output pin on the chip. 2. Be careful of information on the Web. The website shown is interesting and well-done, but it is a homework assignment done by a student and the answer was wrong. I correct the “C” inputs as shown on the slide so that they were correct. Transition: Next is a look at the use of comparators. 1 1

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**National Semiconductor Fairchild Semiconductor**

Comparators Maxim National Semiconductor 13 Shown here are four web pages with additional information about comparators that these companies sell. On the left is the Maxim comparator page. You can see that there are a great variety of comparators available. You can also see that some of the uses and important specifications of comparators are shown. Transition Next is a look at comparator specifications. Texas Instruments Fairchild Fairchild Semiconductor

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**Article on the meaning of Rail-to-rail**

National Semiconductor LM111 Comparator: (LM311 is $0.52 each at Digikey) Open-drain outputs are outputs which at any given time are either actively sinking current (i.e., low voltage, typically considered logic 0) or are high impedance, but which never source current (high voltage, logic 1). Open-drain refers to the drain terminal of a MOS FET transistor. The equivalent concept on a bipolar device is called open-collector. 14 The LM111 is not inexpensive, but provides a table, as shown, that shows some of the important specifications for comparators. The response time, is 100 nanoseconds, which is sometimes called the delay. The supply voltages shows that it can operate over a wide range of voltages and the supply current shows that it is not a low power device since 5 mA times 30 volts gives 150 mW, which is far to much for battery operation. The input range is not R to R, meaning not Rail to Rail. The definition of Rail-to-Rail is that the opamp, or comparator can swing close to the power supply voltage. Since this device is not rail-to-rail, you must be sure that you do not count on going all the way to the supply voltage. An interesting article that discusses what rail-to-rail operation really means is The output is open drain. The meaning of an open-drain output is shown on the slide. Just above the open-drain definition is the website where the definition is located. An open drain refers to a field effect transistor, and the effect is the same as an open-collector configuration for a bipolar transistor. Not that this chip can sink 50 mA and only has a 3 mV voltage offset. The LM311 is the low cost version of this chip and has a smaller temperature operation range. The LM111 costs $4.63 each for the wider temperature range. Transition: Next is a look at a comparator circuit. Article on the meaning of Rail-to-rail

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A B 15 Here is a photocell detector circuit using two comparators from the LM339 quad comparator IC. The website at the top of the slide can be consulted for more information. Essentially, there are two photocells, both labeled R2. Both resistors labeled R1 are potentiometers so you can adjust the trip point of the comparator. This circuit is similar to the first lab where you used a voltage divider to turn on a transistor, which turned on a buzzer. When the voltage at point A on the left of the circuit is less than ½ the power supply voltage, the left comparator output goes low and current is sinked into the comparator. Since it is an open collector/open drain circuit, the power supply voltage is connected to an appropriate resistor and LED, which lights when the output is low and there is a voltage difference between the power supply voltage and the output of the comparator. The right half of the circuit works in almost the same manner. Transition Next is a look at a zero-crossing detector circuit using a comparator. Comparator Circuit

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**Zero Crossing Detector**

LM339 is $0.52 at Digikey 16 The zero crossing detector shown on the slide uses the LM139, which is a higher cost version of the inexpensive LM339 that is commonly used. This is a high quality zero-crossing detector and can easily be used to make a square wave output from a sine wave input. Notice that a + and – supply is used here as well as a 5.1k resistor from the positive power supply to sink current for the open drain/collector output. Transition Comparators are very easy to use and valuable for many circuits. Next is a look at a method of providing a deadband in a circuit so that a comparator will not trip due to noise voltage. Zero Crossing Detector

34
**Schmidt Trigger Circuit**

100 Hz Sinusoid Sin(2π*100*t) 17 Shown is a Schmidt trigger circuit using the LM111 comparator that was drawn in Schematics. The schematic file is attached with the PPT file for you to download and run. Notice that the LM111 is an open collector output so the Load resistor, which is R1, is connected to the positive power supply. Also notice that the 10k resistor, R3, is connected between the positive input and the output, so it provides positive feedback. What this does is create a deadband, also called hysteresis, so that the sinusoidal input triggers at a different voltage in the positive direction than in the negative direction. Without the 10k feedback resistor, the circuit will trigger at exactly 2.5 volts, but noise voltage might affect it. Transition Next is a look at the output waveform for this circuit.

35
**Schmidt Trigger Switches Low Switches High 18**

The two probe measurements for this waveform are taken when the green Schmidt trigger output switches. The red waveform is the input which is a 2.5 volt peak sinusoid offset by 2.5 volts dc. Since this is a 100 Hz sinusoid, it passes through 2.5 volts at exactly 5 msec, 10 msec, 15, msec, 20 msec, etc. Without the 10k feedback resistor, it would switch at these times. Here, you can see that the output goes low at msec and goes high at msec. So, there is a deadband, or hysteresis, area where switching does not take place. This deadband is caused by the voltage divider effect of the 1k input resistor and the 10 k resistor. Download the schematic and run and then experiment with the circuit until you understand it. Transition Next is a look at a circuit known as a Window comparator. Schmidt Trigger

36
17 Many times you need a comparator circuit that only detects voltages within a particular range of voltages, that is, within a window of voltage. This is called a Window detector or Window comparator. This circuit, from the website shown on the slide detects voltages whose peak height lies within a narrow range. You can drive it with a sine wave and see the result. You should try a PSpice circuit and see how it operates. Transition Next is a summary of this presentation. Window Detector

37
**Summary AC Bridges Number systems Boolean Algebra Example**

Tristate Buffers Comparators and Circuits Schmidt Trigger Window Detector 20 After a discussion of AC bridges, I discussed the number systems that you will use the most and a simple example using boolean algebra. Several important digital circuits including the Tristate buffer, comparators, the Schmidt trigger, and the window detector were also discussed. Transition So long for now.

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