ELECTRONIC ANALOG INSTRUMENTS

1. ELECTRONIC ANALOG INSTRUMENTS INSTRUMENTATION ENGENIERING Lecture 03.

2. OBJECTIVE • To analyze several methods to design simple basic electronic circuits to measure AC and DC voltage, IC current and Resistance.

3. L3:ELECTRONIC ANALOG INSTRUMENTS. Class Outline: • Introduction • General block diagram of a electronic instrument. • JFET based electronic voltmeter. • OPAMP based Schematics to design electronic instrument. DC and AC voltmeter, DC ammeter and Ohmeter.

4. Drawbacks of Electric Instruments: • Loading effects. DC and AC voltmeters have input resistance that is finite and it depends on the designed range and It provokes a systematic error. • Low voltages ranges have high loading effects because range design is controlled by the Multiplier Resistor. • Loading effects. Ammeters also have loading effects because input resistance is not zero. • AC measurementsdon’t show the true rms value. Measurements are only valid for 100% sinosoidal signal and they are based on rectification . • Low AC ranges are not accurate, because the voltage drop in diodes during rectification. • Resistance Scales are not linear and are inverted.

5. Introduction • Instrument Design are linked to Technology Advances in order to increase performance. • 1st Generation: Mechanical Instruments. • 2th Generation. Electric Instruments. • 3th Generation. Electronic Instruments. • 4th Generation. Smart Instruments ( Microprocessor based).Data processing and communication) • 5th Generation. Virtual Instruments ( PC Based). Advanced data processing and communication. • Electronic Sensor Advances: many physical non electrical variables can be measured by electronic means.

6. Electronic Instrument. Block Diagram. Physical Variable Electrical Signal M S Indicating Element Electronic Processor SENSOR Analog meter.DigitalDisplay(numeric, alphanumeric, graphic) Electronic Circuit analog or digital IC.(Opamps, glue logic,microprocessor), PC

7. JFET based electronic voltmeter • First electronic instruments were vacuum valve based instrument that were developed mainly at the first half of 20 Century. • They were replaced by JFET based voltmeters after transistor invention. • Small signal model of JFET are similar to vacuum valve. • The key parameter is the high input impedance Gate/Source

8. JFET The IDS current is modulated by VGS. GS Junction is reverse biased, it limits the conduction channel between DS.

9. JFET Topology

10. JFET Circuit bias

11. Output Input Characteristics of JFET

12. JFET IV Output Characteristics • 3 operating zones: linear, saturation and breakdown. • Linear zone: JFET works as variable resistance that is controlled by VGS. • Linear zone is used in JFET to build a voltmeter. • We have a very high input resistance because GS junction is reverse biased.

13. Small signal JFET Model (simplified) G D D G Ids rds S S gm- transconductance Ids= gm Vgs

14. Differential JFET Voltmeter. Basic Schematic y x

15. Differential JFET Voltmeter. Basic Schematic • Wheastone Bridge: 2 resistors RD and 2 JFET. • If all the components are equal and Vgs=0 so the Wheastone Bridge is in balance condition Vxy=0. • If some Vgs is applied, the resistance of JFET is changed, so the bridge is unbalanced and Vxy0. • We can put a electrical voltmeter in XY and detect Vxy by I on the meter.

16. Y V1 RD rds RD gmV1 Small signal analysis. JFET volt-meter • To calculate Vy (Vx), we replace each JFET by its model. Y

17. Thevenin Equivalent XY • Vth = Vy because Vx=0. • Thevenin resistance: y x

18. Measurement of Vxy with an electrical PMMC ammeter If we use a PMMC with Im, rm to detect Vxy . It shows a I given by: Ixy

19. Measurement of Vxy with an electrical PMMC ammeter • Ixy is a linear function of Vxy=V1. • We can calibrate I scale in voltage units. • Loading effect decreases because the high input resistance in GS Junction. • Input resistance is independent of range. • We have solved to main drawbacks of the electrical voltmeter !!!

20. Practical JFET voltmeter Schematic • Zero adjust potentiometer to take account differences between JFETs.. • Resistor Attenuator to multiple range applications. • Adjust potentiometer to get FSD.

21. OPAMP BASED ELECTRONIC INSTRUMENT. • OPERATIONAL AMPLIFIERS are the work-horses of analog processing. • Main parameters: high input impedance, low output impedance, high open loop gain, DC bandwidth. • Many possible configurations are useful to instrumentation application: Buffer, non inverter, inverter, differential .

22. OPAMP based DC Voltmeter • Vx provokes a current through the ammeter. • High input resistance, independent of Vx • Rcal is set up to get FSD with Vmax. Vx Im, rm

23. Example 1. • Design a Voltmeter of 10VDC, with a OPAMP and a PMMC meter with 100uA y 2000 Ohms.

24. Voltmeter Limitation • Voltage range is limited by IC technology. No more than 18V-36V • To get high voltage range we used voltage divider ( Resistive attenuators). • So we can get multiple range voltmeters.

25. Example 2. • Design a CD Voltmeter from Example 1 but with ranges of 10 V, 20 V, 50 V y 100V. Solution: add a 4 stage resistive divider. To measure 100 V, the OPAMP input is connected to Rd to get maximum attenuation and reduce from 100V to 10 V.

26. Example 2…. • We assume that the total resistance of divider is around the input impedance of OPAMP typically 10 Mega ohms. • To measure at the range of 50V, we switch to Rc so:

27. Example 2… • We procede in similar form to get 20 V • The low range of 10 V is taken directly . It defines Ra

28. 5 M 3M 1M 1M

29. OPAMP AC Voltmeter . • It is also based in a rectifier circuit. • But there is no diode cutting voltage effects. Ideal rectifier • Output is taken by mean point of R2-D2. • D1 is OFF to Vin<0, and on to Vi>0. • D2 is oppositive to D1 • For V<0 (D2 on) we get a negative gain of R2/R1 and for V>0 the output is zero. Half wave Rectifier. • Also it is based on rectification, we don’t get true rms. • A second OPAMP circuit is required to detect mean voltage of rectifier signal.

30. OPAMP AC Voltmeter

31. Example 3. • Design a AC voltmeter of 10 Vrms with a PMMC meter of 100uA, 2000 ohms. We assume sinusoidal signal , so 10 Vrms implies a peak voltage of: But this signal when is rectified with half wave we obtain a mean voltage of:

32. Example 3. • Finally we design the second stage with a DC voltmeter of 4.5 V to FSD

33. Example 3. Full design

34. DC Microammeter • Photodiode devices are current based. The reverse current is a function of light intensity. • We use a OPAMP Schematics to process low currents. • The output voltage is R*Iphoto. • This circuit have a very low input resistance. Its behavoir is like as ideal ammeter. • Measured Current I flows through OPAMP so is limited to a few microamperes. • Finally we need a second stage to measure V output. • Low value current is limited by bias current of opamp. • Input resistance is very low because V+ ~ V- in OPAMPS.

35. Electronic Ohm-meter • Two Schematics: • First Method: Current injection. • A Current source is used to applied some standard Io current value through some unknown resistance Rx. • The voltage drop in Rx is linear to Rx and proportional to Io. • We get a linear ohm-meter if we measure Vrx !!!

36. Current Source Design Io

37. Current Source Design. • PNP Transistor performs the feedback connection of OPAMP. The OPAMP supplies the necessary base current. • PNP works as current source because it is in common base connection so collector current Ic~ emitter current Ie • Emitter current is impose by OPAMP connection, R3 and the voltage in divider R1, R2. • There is no limitation for Rx when Rx is low, but when Rx increase VCE drops until VCE saturation limit (0.1 V)

38. Electronic Ohm-meter 2. • Second Method. Comparison. • We use an inverter configuration and RX is in feed back position. • Vo is linear with Rx