Download Instrumentation Amp

Lab #5 r01
EE 419 Electrical Laboratory
EE 452 Instrumentation
Instrumentation Amplifier
OBJECT - To investigate the properties of an instrumentation amplifier and to use
it to reject interference.
REFERENCES - EE 452 text, lab appendix and TL 081 and TL082 data sheets.
INSTRUCTIONS
1. Construct the instrumentation amplifier circuit shown below using TL081 and
TL082 op amps. Measure the values of all resistors when constructing the circuit.
Use ±15 volt supply.
Use THE TL081 op amp with offset null capability for the second stage.
Ground the inputs Vin+ and Vin– and use a nulling potentiometer on the TL081 to
obtain a zero output.
5
Vin 6
+
TL082
_
7
R1
10 kΩ *
R4
1 kΩ *
10 kΩ *
R3
2
Vin +
3
10 kΩ *
2.2 kΩ
3
1 kΩ *
R5
R2
_
TL082
+
* Precision Resistor
R6
2
1
9.56 k Ω * =
220 k Ω //10 k Ω *
≅ 440 Ω
decade
resistor unit
_
TL081
+
1
R7a
R7b
Fig. 1 Instrumentation Amplifier Schematic
6
5
Vout
100 k Ω
1.5 kΩ
2
Lab #5 r 01
2. Measure differential gain, G d, (Vout/Vd) of the amplifier by using the centertapped audio transformer shown in Fig. 2a to input a small (Vd ≈ 100mVp-p)
differential signal. The secondary center tap is grounded thus making the
common-mode input equal to zero. It is convenient for the oscilloscope to
measure Vd/2 (instead of Vd). Measure the differential gain over the frequency
range 10 Hz to 1 MHz. It is suggested that first measurements be made at 1000
Hz. Measure the amplifier itself i.e. be sure to exclude the transformer since it
has poor performance below 100 Hz and above 10 kHz.
20 kΩ : 2 kΩ ct
V
Vd
I nstrumentation Amp.
+
Hammond
145 R
+15 V
Fig. 2 Differential Gain Measurement
-
I nstrumentation Amp.
Vout
+
-15 V
+15 V
Vcm
Vout
-15 V
Fig. 3 Common-Mode Gain Measurement
1. Measure common mode gain, Gcm, by setting the differential input to zero (i.e.
connect the amplifier inputs together) as shown in Fig. 3. Input a large commonmode signal (approx. 10 Vpeak at 1000 Hz). Adjust R7 to give the minimum
output and record its value (compare this value with what would be expected).
Measure the optimum common mode gain and also measure for R7 opt ±30 Ω,
R7 opt ±100 Ω, and R7 opt ±200 Ω. Also verify that the differential gain is
essentially unchanged for these small changes in R7.
Return R7 to its optimum value and measure the common-mode gain over the
frequency range 10 Hz to 1 MHz. Plot common-mode rejection ratio in dB
(CMRR = Gd / Gcm) as a function of frequency using the data from parts 2 and 3.
+15 V
20 kΩ : 2 kΩ ct
+
Vd /2
Vd
V
Vcm
+
Instrumentation Amp.
Vd /2
+
Hammond
145 R
Vcm
Fig. 4 Interference Rejection Measurement
-15 V
Vout
3
Lab #5 r 01
4. Interference rejection is the main advantage of the instrumentation amplifier
and this be predicted by the above measured CMRR. This prediction assumes a
linear system that can be analyzed using superposition.
Verify this prediction by setting up the circuit of Fig. 4 and applying
simultaneous 1 kHz differential signal and 60 common mode interference. It is
suggested that the common-mode input voltage be 100 times greater than the
differential input voltage.
Hint: A convenient way to measure CMRR is to measure SNR (signal to noise
- or interference - ratio) at both the input and the output. CMRR = SNRout /
SNRin . SNR can be measured with both a spectrum analyzer and a scope –
consult your instructor.
5. Before dismantling your circuit, discuss electrocardiogram (EKG) measurement
with your lab instructor.
1. Question - Source impedance imbalance can convert common mode
interference to differential mode interference and substantially decrease the
CMRR of an instrumentation amplifier. Thus cabling from a signal source to a
signal amplifier should also have balanced impedance. With capacitance in the
amplifier input and connecting leads, a difference in source impedance leads to
unequal common-mode voltage drops and a differential voltage at the amplifier
input. For the circuit of Fig. 5 and with both Rs+ = 1kΩ and Rs+ = 10 kΩ
determine the common-mode signal gain at 100 Hz, 1, 10 and 100 kHz.
Hammond
145 R
+15 V
1 kΩ*
Rs100 pF
Instrumentation Amp.
Vout
Rs+
+
-15 V
100 pF
Vcm
Fig. 5 Unbalanced Source Impedance
7. Question - What would be the effect on the performance of the instrumentation
amplifier if the gain of the input stage is 100 and that of the output stage is 1?
Hint: consider frequency response and CMRR and include justification for your
answer.
8. Question – To reject 60 Hz interference in measurement or communication
systems, a notch filter could be used instead of an instrumentation amplifier.
What is the disadvantage of the notch filter.
1. Question - What is the turns ratio of the 20kΩ: 2kΩ transformer?