Datasheet

INA117
SBOS154A
6
APPLICATION INFORMATION
Figure 1 shows the basic connections required for operation.
Applications with noisy or high-impedance power-supply lines
may require decoupling capacitors close to the device pins.
The output voltage is equal to the differential input volt-
age between pins 2 and 3. The common mode input
voltage is rejected.
Internal circuitry connected to the compensation pin 8 can-
cels the parasitic distributed capacitance between the feed-
back resistor, R
2
, and the IC substrate. For specified dy-
namic performance, pin 8 should be grounded or connected
through a 0.1µF capacitor to an AC ground such as V+.
COMMON-MODE REJECTION
Common-mode rejection (CMR) of the INA117 is depend-
ent on the input resistor network, which is laser-trimmed for
accurate ratio matching. To maintain high CMR, it is impor-
tant to have low source impedances driving the two inputs.
A 75Ω resistance in series with pin 2 or 3 will decrease CMR
from 86dB to 72dB.
Resistance in series with the reference pins will also degrade
CMR. A 4Ω resistance in series with pin 1 or 5 will decrease
CMRR from 86dB to 72dB.
Most applications do not require trimming. Figures 2 and 3
show optional circuits that may be used for trimming offset
voltage and common-mode rejection.
TRANSFER FUNCTION
Most applications use the INA117 as a simple unity-gain
difference amplifier. The transfer function is:
V
O
= V
3
– V
2
V
3
and V
2
are the voltages at pins 3 and 2.
FIGURE 1. Basic Power and Signal Connections.
+
+
380kΩ 380kΩ
380kΩ
21.1kΩ
20kΩ
47
2
3
815
–15V +15V
1µF
Tantalum
1µF
Tantalum
–In = V
2
+In = V
3
V
O
= V
3
– V
2
6
R
1
R
2
R
4
R
5
R
3
380kΩ
380kΩ 380kΩ
21.1kΩ 20kΩ
47
2
3
815
V– V+
V
2
V
3
V
O
= V
3
– V
2
6
100kΩ
10Ω
50kΩ
±1.5mV
Range
–15V
+15V
380kΩ 380kΩ
380kΩ
21.1kΩ 20kΩ
47
2
3
815
V– V+
V
2
V
3
V = V – V
O 3 2
6
10kΩ
±10mV
100Ω
100Ω
V+
V–
100µA
1/2 REF200
100µA
1/2 REF200
OPA27
Offset adjustment is regulated—
insensitive to power supply variations.
(a)
(b)
FIGURE 2. Offset Voltage Trim Circuits.
Some applications, however, apply voltages to the reference
terminals (pins 1 and 5). A more complete transfer function
is:
V
O
= V
3
– V
2
+ 19 • V
5
– 18 • V
1
V
5
and V
1
are the voltages at pins 5 and 1.