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Laboratory Exercises for Electronic Devices, Buchla, © 2001 Prentice-Hall, Inc.
Experiment 23Troubleshooting Problem
An OP1771 is an ultra precise op-amp that has a specified minimum CMRR’ of 130 dB. If you try to measure this with the circuit in Figure 23-4, you are likely to find a lower CMRR’. Why?
?
1Data sheet available at http://www.analogdevices.com
100 k
R B = 100
R A = 100
100 k
1 .0 µ F
1 .0 µ F
2
3
6
7
4+
-
-15 V
+ 15 V
O P 1 7 7
Laboratory Exercises for Electronic Devices, Buchla, © 2001 Prentice-Hall, Inc.
Experiment 23Troubleshooting Problem Solution
An OP1771 is an ultra precise op-amp that has a specified minimum CMRR’ of 130 dB. If you try to measure this with the circuit in Figure 23-4, you are likely to find a lower CMRR’. Why?
100 k
R B = 100
R A = 100
100 k
1 .0 µ F
1 .0 µ F
2
3
6
7
4+
-
-15 V
+ 15 V
O P 1 7 7
The CMRR’ you measure includes the effect of mismatched resistors in the external circuit. If the resistors are mismatched, there will be a small signal generated by the mismatch which appears as a differential signal to the op-amp, hence it is amplified.
Laboratory Exercises for Electronic Devices, Buchla, © 2001 Prentice-Hall, Inc.
Experiment 23Related Problem Assume an OP1771 is used in a circuit that has a differential gain of 100. If a 1.0 V common-mode signal is applied to it, what is the output signal? (assume CMRR’ = 130 dB)
Laboratory Exercises for Electronic Devices, Buchla, © 2001 Prentice-Hall, Inc.
Experiment 23Related Problem Solution Assume an OP1771 is used in a circuit that has a differential gain of 100. If a 1.0 V common-mode signal is applied to it, what is the output signal? (assume CMRR’ = 130 dB) The common mode gain is:
The output signal is:
66)cm(
)cm(
5.6
)cm(
)cm(
)d('
106.311016.3
100
10010
100log20dB 130
log20CMRR
v
v
v
v
v
A
A
A
A
A
V 1.0106.31 6
in)cm(out
in
out)cm(
VAV
V
VA
v
v
31.6 V
This is too small to be measured by typical laboratory instruments!
