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APPLICATION BULLETIN
Mailing Address: PO Box 11400 鈥?Tucson, AZ 85734 鈥?Street Address: 6730 S. Tucson Blvd. 鈥?Tucson, AZ 85706
Tel: (602) 746-1111 鈥?Twx: 910-952-111 鈥?Telex: 066-6491 鈥?FAX (602) 889-1510 鈥?Immediate Product Info: (800) 548-6132
PROGRAMMABLE-GAIN INSTRUMENTATION AMPLIFIERS
By David Jones (602) 746-7696 and R. Mark Stitt
The INA115 is a precision instrumentation amplifier (IA).
Its gain-sense and amplifier output connections are available
so that gains can be accurately set using external resistors.
By adding switches or a multiplexer, you can make a
precision programmable-gain IA (PGIA). Using the circuit
topology shown in this bulletin, the switches are in series
with op amp inputs so their resistance does not add error.
Burr-Brown offers fixed and programmable-gain IAs with
amplifiers similar to the INA115. If you need a fixed gain of
100V/V, use the INA131. If you need a digitally program-
mable IA with decade gains of 1, 10, 100, and 1000V/V, or
binary gains of 1, 2, 4, and 8V/V, use the PGA204 or
PGA205.
If you need an IA with more gain steps, you can cascade two
or more PGIAs. For example, if you cascade two PGA205s
you will get gains of 1, 2, 4, 8, 16, 32, and
64V/V.
For other gains or gain-steps, you may want to make your
own PGIA using an INA115. The circuits and equations
below make it easy to design a programmable-gain IA with
any gains and any number of gain steps.
MODEL
INA131
PGA204
PGA205
GAINS (V/V)
100
1, 10, 100, 1000
1, 2, 4, 8
TABLE I. Fixed and Programmable-Gain IAs.
R
0
1 V
O1
R
1
G
1
R
2
R
m
G
2
G
m
G
n
NC
3
12
V
IN
鈥?/div>
4
2
Amp
B
INA115
R
n
Amp
C
11
V
OUT
NC
R
m
G
n
G
m
G
2
G
1
R
1
V
IN
+
14
Ref
10
15
5
Amp
A
R
2
8 V
O2
R
0
FIGURE 1. Programmable-Gain Instrumentation Amplifier with n Gain Steps and Lowest Gain > 1V/V.
漏
1992 Burr-Brown Corporation
AB-042
Printed in U.S.A. September, 1992
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