Additional information on Passive Probes (10x) Take note that the scope’s input capacitance, C in must falls within the probe’s compensation range For example, Agilent’s 8000 Series oscilloscope’s input characteristics show that at 1 M  input it exhibits 13 pF of typical input capacitance. And the probe you’re going to use with this scope has 6-15 pF of compensation range.

Active oscilloscope probes: For connecting fast rising and high frequency signal, active probe is used - range of frequency up to 2 GHz As indicated by the name, this type of scope probe has active components incorporated within the probe itself. This enables greater levels of functionality and higher levels of performance to be attained. However they are much more expensive and normally reserved for more exacting or specialist requirements.

FET acts as a source follower. The stage that follows the FET provides bipolar transistors wired as emitter follower. The FET provides a high-input resistance for low frequency signals and a low capacitance for high frequency signals to the circuit being probed, and the bipolar transistors produce a current gain. Normally active probe is used for i.high input signal impedance – normally around 1 M  ii.high frequencies signals iii.low signal level Block Diagram of an Active Probe Probe tip

1 M  High Impedance – no current flow + Vin - Circuit under test 10 M  + Vout - At low frequency 1 M  Low capacitance (2 pF) - small reactance + Vin - Circuit under test At high frequency 10 M  The common collector provides a current source with small output impedance to reduce the loading effect

MEASURANDTRANSDUCER SIGNAL CONDITIONING DISPLAY RECORD BASIC ARCHITECTURE OF ELECTRONICS INSTRUMENTATION MEASUREMENT SYSTEM

SENSORS AND TRANSDUCERS TRANSDUCERS a device that converts a primary form of energy into a corresponding signal take form of a sensor or an actuator SENSORS a device that detects/measures a signal ACTUATOR a device that generates a signal Example, a Heater is an actuator while a Thermometer is the sensor

Electronic sensors Generally electronic sensor consists of a primary transducer: changes “real world” parameter into electrical signal for example, heat, sound, etc for example, a microphone (input device) converts sound waves into electrical signals for the amplifier to amplify (a process), and a loudspeaker (output device) converts these electrical signals back into sound waves

Quantity being Measured Input Device (Sensor) Light Level Light Dependant Resistor (LDR) Photodiode Photo-transistor Solar Cell Temperature Thermocouple Thermistor Thermostat Resistive Temperature Detectors Force/Pressure Strain Gauge Pressure Switch Load Cells Position Potentiometer Encoders Reflective/Slotted Opto-switch LVDT Speed Tacho-generator Reflective/Slotted Opto-coupler Doppler Effect Sensors Sound Carbon Microphone Piezo-electric Crystal

Input type transducers or sensors, produce a voltage or signal output response which is proportional to the change in the quantity that they are measuring. The type or amount of the output signal depends upon the type of sensor being used. The types of sensors can be classed as two kinds, either Passive Sensors or Active Sensors.

Active Sensors Generally, active sensors require an external power supply to operate, called an excitation signal which is used by the sensor to produce the output signal. Active sensors are self-generating devices because their own properties change in response to an external effect producing for example, an output voltage of 1 to 10V DC or an output current such as 4 to 20mA DC.

EXAMPLE 1 – STRAIN GAUGE It does not generate an electrical signal itself, but by passing a current through it (excitation signal), its electrical resistance can be measured by detecting variations in the current and/or voltage across it. Force

The Wheatstone bridge provides a way to convert these changes in resistance to changes in voltage, which are easy to work with. R 2 in the diagram is set at a value equal to the strain gauge resistance with no force applied. R 1 and R 3 are set equal to each other. Thus, with no force applied to the strain gauge, the bridge will be symmetrically balanced and the voltmeter will indicate zero volts, representing zero force on the strain gauge.

where R 4 is the resistance of the strain gauge So let say R 1 = R 3 = R 2 = R Which means that before any force is applied R 4 also equals to R Hence, any changes in R 4 can be denoted as (R +  R)

No. of coins Output Voltage (mV) REF: instrumentation Consider that the excitation voltage applied is 10 V and the value of R is 120 . Answers: , 0.22 % i.What is the new resistance value of the strain gauge for V o = 5.32 mV? ii.Calculate the percentage of increment of the resistance

Relationship with Change of resistance,  R = R o G  Where R o = initial resistance when there is no applied stress, G = gauge factor and  is the strain unit deformation and  =  / E Where  = the mechanical stress (N/m 2 ) E = Young’s Modulus which is specific for each type of material Continue from previous example: Given G = 2.12, and the Young’s Modulus of aluminium is 72 GPa. Calculate the mechanical stress. Answers: 74.3 MPa

Derive the output voltage equation Answers: V o = (  R / 2R) V i

Passive Sensors Unlike an active sensor, a passive sensor does not need any additional energy source and directly generates an electric signal in response to an external stimulus. For example, a thermocouple or photo-diode. Passive sensors are direct sensors which change their physical properties, such as resistance, capacitance or inductance etc.

EXAMPLE 1 – THERMOCOUPLE Operation: a conductor generates a voltage when subjected to a temperature gradient. a second conductor material will also generates a different voltage under the same temperature gradient. The voltage difference generated by the two materials can then be measured and related to the corresponding temperature gradient. thermocouples can only measure temperature differences and need a known reference temperature to yield the absolute readings.

Where, S A and S B are referred to as Seebeck Coefficients (unit is V/K) and with the assumption that the coefficient remains constant through out the metal T ref T tip

Material Seebeck coefficient relative to platinum (μV/K) Selenium900 Tellurium500 Silicon440 Germanium330 Antimony47 Nichrome25 Iron19 Molybdenum10 Cadmium, tungsten7.5 Gold, silver, copper6.5 Rhodium6.0 Tantalum4.5 Lead4.0 Aluminium3.5 Carbon3.0 Mercury0.6 Platinum0 (definition) Sodium-2.0 Potassium-9.0 Nickel-15 Constantan-35 Bismuth-72

The thermocouple below is using Metal A as copper and Metal B as constantan. If the measured voltage in the following circuit is 3.53 mV, what is the temperature of the hot junction (in °C), if the cold junction is at 0°C? T ref T tip Answer: 85°C