廬
APPLICATION BULLETIN
Mailing Address: PO Box 11400 鈥?Tucson, AZ 85734 鈥?Street Address: 6730 S. Tucson Blvd. 鈥?Tucson, AZ 85706
Tel: (520) 746-1111 鈥?Twx: 910-952-111 鈥?Telex: 066-6491 鈥?FAX (520) 889-1510 鈥?Immediate Product Info: (800) 548-6132
INCREASING INA117 DIFFERENTIAL INPUT RANGE
By R. Mark Stitt (520) 746-7445
1
The INA117 is a monolithic difference amplifier with the
unique ability to accept up to
鹵200V
common-mode input
signals while operating on standard
鹵15V
power supplies.
Because the gain of the INA117 is set at 1V/V, and because
the output would saturate into the rails at about
鹵12V,
the
maximum specified differential input range is
鹵10V.
Since the common-mode input range is
鹵200V,
it makes
sense that some designers would also like to use the part for
differential inputs greater than
鹵10V.
Figure 1 shows the
recommended circuit. Adding resistors to the input may
seem simpler, but there are some problems with that ap-
proach.
ently of CMR by adjusting the inverter resistors, R
6
, R
7
. Gain
drift is preserved so long as R
6
and R
7
track with temperature.
Furthermore, noise at the output is improved by the gain
reduction factor whereas it is unchanged with the other
approach.
To understand how the circuit works, consider the INA117 to
be a four-input summing amplifier as shown in Figure 2.
2
4
R
1
380k鈩?/div>
V
2
2
R
3
380k鈩?/div>
V
3
3
R
5
21.1k鈩?/div>
R
4
20.0k鈩?/div>
INA117
1
V
1
5
V
5
*V
OUT
= V
3
鈥?V + 19 V 鈥?18 V
2
5
1
鈥?/div>
+
R
2
380k鈩?/div>
5
6
*V
OUT
R
1
380k鈩?/div>
V
2
2
R
3
380k鈩?/div>
V
3
3
R
5
21.1k鈩?/div>
8
1
5
鈥?/div>
+
R
2
380k鈩?/div>
6
V
OUT
=
V
3
鈥?V
2
19 R
7
R
6
INA117
R
7
R
6
6
7
8
9
10
11
12
13
R
6
(k
鈩?/div>
)
20.0
20.0
20.0
8
1+
R
4
20.0k鈩?/div>
FIGURE 2. INA117 Shown as a Four-Input Summing Am-
plifier.
CMR is preserved and the gain is reduced if a small portion
of the output signal is inverted and fed back to pin 5 with V
1
set to zero (V
1
grounded).
Where: V
OUT
= V
3
鈥?V
2
+ 19 鈥?V
5
鈥?18 鈥?V
1
If, V
5
= 鈥揤
OUT
鈥?R
7
/R
6
, then
V
OUT
=
V
3
鈥?V
2
1+
19 鈥?R
7
R
6
鈥?/div>
OPA27
+
FIGURE 1. INA117 with Increased Differential Input Range.
The performance of the INA117 depends on extremely
precise resistor matching (0.005% for 86dB CMR). Resistors
added to the input must be adjusted to at least this accuracy
to maintain high performance. Both gain error and CMR
must be adjusted. Maintaining 86dB CMR over temperature
requires 1ppm/擄C resistor TCR tracking. Significant resis-
tance added external to the INA117 would require the same
performance.
By using the circuit shown in Figure 1, internal resistor
matching is preserved, and the INA117 CMR and CMR drift
with temperature are maintained. Gain can be set independ-
SELECTED-GAIN EXAMPLES
GAIN
(1)
(V/V)
1/2
1/4
1/5
R
7
(k
鈩?/div>
)
1.05
3.16
4.22
14
15
NOTE: (1) INA117 is not stable in Gain < 1/5.
漏
1990 Burr-Brown Corporation
AB-001B
Printed in U.S.A. June, 1995
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