ELE3IES5FIE.pdf

8/16/2019 ELE3IES5FIE.pdf

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ELE3IES/ELE5FIE Instrumentation Electronics & Sensors 2016 Practical Session 3

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4 Preparing the Prototype Board

1. If coloured 4mm connector posts are available, install them in the three large holes at the end of

the board.

2. If coloured 4mm connector posts are not available: (a) Cut three insulated wires (red, green and black), about 1 m each, from the reels, and

(b) Attach them to the board in locations shown in figure 1.

3. If available, insert 0.1uF and 10uF capacitors between the power rails and ground. Remember the

10uF capacitors are polarised. Check correct polarity before soldering.

4. Solder the small jumper wires into the board.

Figure 1. Board layout for power routing.

5 Power

Supply

1. Turn the Tektronix DC power supply on and check that both A and B supplies are set to 12V

2. Always check the voltage before attaching your circuit to the power supply!

3. Turn the power off.

4. Attach wires as shown in figure 2.

5. Turn the power on. Using DMM, confirm the board is receiving ±12V.

6. ALWAYS TURN THE POWER OFF BEFORE SOLDERING.

Figure 2. Power supply connection diagram


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6 Differential Signal Generator

We will build a slightly‐modified version of the differential signal generator simulated in Lab 1. Note

that we have added a voltage divider to the op‐amp input to attenuate the input signal.

6.1 Build

1. Start building the circuit in Figure 3 by placing the 14‐pin IC socket in the board at the end of the

board furthest from the three large holes (ask the demonstrator staff before commencing soldering).

. Do not insert the op amp into the socket until you have finished building the whole circuit.

2. The recommended board layout has been given as a guide in Figure 5, except, as mentioned

above, the components should be mounted at the end of the board furthest from the three large

holes

3. The op‐amp requires power to operate. (+12V on pin 4 and ‐12V on pin 11)

4. A 110k Ω resistor for R3 can be made by putting a 100KΩ resistor in series with a 10KΩ.

5. Finish building the circuit neatly. Do not turn the power on just yet.

Figure 3. Differential signal generator schematic

Figure 4. LM348 quad op‐amp IC pin out (from NationaLSemiconductor Datasheet).


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Figure 5. Recommended layout for differential signal generator (BUT SEE NOTES IN TEXT)

6.2 Unused Op‐Amps Within A Package

Before we test the differential signal generator it is wise to consider the two unused op‐amps in the

LM348 IC package. The best result is achieved when the unused op‐amps are consuming minimal

power. This is achieved by setting the op‐amp output to 0V when using a dual‐supply op‐amp.

Theoretically, both circuits shown below should provide an output of 0V.

Figure 6. Alternative circuits to zero the op‐amp output.

1. Simulate the two circuits shown in Figure 6 using LTspice.

2. Which circuit produces a 0V output? Explain why the other one does not.

3. Explain the importance of neutralising unused op‐amps.

4. What are the typical and maximum input offset voltages of the LM348? (Refer to datasheet)

5. Neutralise the unused op‐amps on your board. Use 10KΩ resistor for the feed‐back loop of the

first op‐amp and a 100KΩ resistor for the second. This will allow you to easily expand your circuit

later on (refer to feedback resistors in figure 7).


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6.3 CRO Setup

If using a Tektronix TDS 2002B digital storage CRO in this lab, take the time to familiarise yourself with

its User Manual if available. You can make very accurate measurements using the measurement and

cursor functions of the CRO. You must however setup the CRO properly.

1. Attach a CRO probe to Channel 1. Plug the BNC socket in and then turn it to lock it in place.

2. Place the CRO probe on the PROBE COMP (compensation) output of the CRO.

(Image from Tektronix User Manual)

3. Press the AUTOSET button. A 5Vpp 1kHz square wave should appear on the screen.

4. Make sure probe attenuation is set to x10 if you do not get 5Vpp.

5. Finally press the Probe Check button. A message saying the probe has passed will appear on the

screen, otherwise replace the probe or check CRO settings.

6. Connect another probe to Channel 2 and repeat above steps.

6.4 Test

We can test the circuit you built by connecting a signal generator into the input and measuring the

outputs using a CRO.

1. Turn the power supply on and make sure the op‐amp is receiving power.

2. Confirm your unused op‐amps output 0V using DMM.

3. Connect the signal generator to the input of the circuit.

4. Apply a 1 kHz sine wave with 1.1V amplitude, use CRO to measure voltage.

5. Confirm the circuit is functioning by looking at the outputs A and B using a CRO.

6. By what factor is the input voltage reduced when it travels through the voltage divider R4 &

R5? Use Ohm’s Law to confirm.

7. Measure the gain of both op‐amps. Does this agree with the simulated results in Lab 1?

8. Increase the input sine wave amplitude until the output is just below saturation.

9. Measure slew rate by increasing the input frequency until the output no longer exhibits a

sinusoidal form.

10. At what frequency does this occur?

11. Slew rate is equal to the positive slope of the sawtooth wave seen. Measure the slope and record

it as the slew rate with units V/μs.

12. Compare with the slew rate given in the manufacturer’s datasheet. Explain why your result is

different.


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7 Two Op‐Amp Instrumentation Amplifier

Figure 7. Two op‐amp instrumentation amplifier schematic.

1. Build the circuit in figure 7 using the unused op‐amps which were neutralised previously. Note

that you would already have R7 and R9 installed.

2. Attach the outputs from the differential signal generator into the inputs of the instrumentation

amplifier. ( Aout and Bout go to Ain and Bin respectively)

3. Reduce the amplitude of the signal generator to 100mV, keeping the frequency at 1 kHz.

4. Observe the signal at the output of the instrumentation amplifier. Is it what you expected?

7.1 Differential Gain and Bandwidth

1. Measure the gain of the instrumentation amplifier v o / (v Ain ‐v Bin). Since it is receiving a differential

signal input, we call this the differential gain ( Ad ).

2. Compare the measured differential gain to the one calculated in the preliminary work.

3. Slowly reduce the input frequency and confirm the differential gain does not decrease even at

very low frequencies (i.e. the low break‐frequency, f L = 0Hz).

4. Now increase the input frequency until the gain begins to diminish.

5. The high break frequency is the point at which the mid‐band gain of the amplifier decreases by a

factor of 2.

6. Determine Ad / 2 ( Ad = differential gain measured in step 1) Note: 1/ 2 70%

7. Increase input frequency until a gain of Ad / 2 is found. Mark this frequency as the high break

frequency ( f H).

8. As a check, measure also the phase shift between the input and output waveforms (see Figure 8).

9. Determine the differential mode bandwidth of the amplifier: BW = f H – f L


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Figure 8. input and output phase difference: Phase = 360o x t/T

7.2 Common Mode Gain, Bandwidth and CMRR

Figure 9. Instrumentation amplifier with common‐mode input.

1. Disconnect the differential signal generator from the instrumentation amplifier ( Aout and

Bout must be disconnected from Ain and Bin respectively).

2. Connect Ain and Bin together so both op‐amps receive the same input (figure 9). Then connect

this point to the signal generator.

3. Apply a 1 kHz sine wave with 5V amplitude to the instrumentation amplifier.

4. Measure the common mode gain:

5. Compare the measured common mode gain with that calculated in the preliminary work.

Explain why it is different.

6. Measure the common mode bandwidth.

7. Determine the Common Mode Rejection Ratio: 8. Is it better to have an instrumentation amplifier with a high or low CMRR?


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7.3 Adjustable Gain

Figure 9. Modified instrumentation amplifier.

1. Modify the instrumentation amplifier by connecting R10 as shown in figure 9.

2. Determine the following:

a. Differential‐mode gain

b. Differential‐mode bandwidth

c. Common mode gain

d. Common mode bandwidth

e. CMRR

3. What effect does R10 have on the circuit?

4. Insert another 10KΩ resistor in parallel with R10, effectively making it 5KΩ.

5. Repeat measurements made in step 2.

8 Investigation

If time and resources permit, do the following as an investigative exercise:

1.

Use the frequency sweep function of the signal generator and the FFT function of the CRO to

confirm the bandwidth measurements made earlier

2.

Modify the differential signal generator to include an interference source (refer to lab 1). Use

another signal generator to provide common mode interference of 50Hz. Observe the outputs

of the differential signal generator and the instrumentation amplifier.

9 Report

You

must

write

a

report

for

this

lab.

It

is

to

be

completed

individually . You will have the same results

as your lab partner, but your explanations and answers to questions need to be your own. You must

answer all questions including those below:

1. What is the purpose of an instrumentation amp? What applications does it have?

2. You measured the gains and bandwidths of the instrumentation amp with three different

configurations: without R10, with R10 = 10KΩ, and R10 = 5KΩ. Present these results in a table.

3. What is the effect of increasing gain on CMRR and bandwidth? Do you think this should be an important design consideration?

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