EE 321 Lab 7 Fall 2003
EE321 – Lab 7
Strain Gauge — Using Instrumentation Amplifiers
In this lab we will experiment with differential amplifiers and use a so-called ‘instrumentation
amplifier’ to measure the output of a strain gauge. The instrumentation amplifier is a high-gain
high-input-impedance high-CMRR differential amplifier.
Differential Amplifiers
1. Construct the following differential amplifier (Figure 1) with R1between 4 and 6 kΩ and the
difference of Vaand Vbamplified by about 30. Build it in the center of your protoboard to
leave plenty of room for later additions.
Connect vBto ground and vAto the signal generator, and check that the gain is about 30.
With the difference signal set to zero (as shown in the common-mode test circuit, figure 2)
test that the circuit attenuates or rejects a common-mode input signal Vcm , and measure the
‘gain’ of the amplifier for a common mode signal. Use a common mode input Vcm = 10 V p-p
at 100 Hz. Why is the output not zero?
R2
−15V
3
27
6
4
+15V
vo
vB
vA
R
R
R2
1
1
LF411
Figure 1.
2. Improve the common mode rejection by replacing part of R2in the non-inverting leg with a
potentiometer (with the pot set to the middle of its range, the total resistance replacing R2
should be equal to R2). Adjust to maximize the common mode rejection; compute the new
common mode gain. Sketch circuit. Why has the common mode rejection been improved?
3. The ‘Common Mode Rejection Ratio’ (CMRR) is defined as the ratio of the signal gain to
the common mode gain. Compute the CMRR of the above circuit.
Instrumentation Amplifier
4. Convert your differential amplifier to the classical ‘instrumentation amplifier’ shown (Figure 3)
by adding a non-inverting amplifier to each input. Use 411 op amps for each of the non-
inverting stages. Lay the circuit out neatly. Select the resistance values to give the amplifier
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