Datasheet

7
®
INA122
APPLICATION INFORMATION
Figure 1 shows the basic connections required for operation
of the INA122. Applications with noisy or high impedance
power supplies may require decoupling capacitors close to
the device pins.
The output is referred to the output reference (Ref) terminal
which is normally grounded. This must be a low-impedance
connection to ensure good common-mode rejection. A resis-
tance of 10Ω in series with the Ref pin will cause a typical
device to degrade to approximately 80dB CMR.
SETTING THE GAIN
Gain of the INA122 is set by connecting a single external
resistor, R
G
, as shown:
(1)
Commonly used gains and R
G
resistor values are shown in
Figure 1.
The 200kΩ term in equation 1 comes from the internal metal
film resistors which are laser trimmed to accurate absolute
values. The accuracy and temperature coefficient of these
resistors are included in the gain accuracy and drift specifi-
cations of the INA122.
The stability and temperature drift of the external gain
setting resistor, R
G
, also affects gain. R
G
’s contribution to
gain accuracy and drift can be directly inferred from the gain
equation (1).
OFFSET TRIMMING
The INA122 is laser trimmed for low offset voltage and
offset voltage drift. Most applications require no external
offset adjustment. Figure 2 shows an optional circuit for
trimming the output offset voltage. The voltage applied to
the Ref terminal is added to the output signal. An op amp
buffer is used to provide low impedance at the Ref terminal
to preserve good common-mode rejection.
G
k
R
G
=+5
200 Ω
FIGURE 1. Basic Connections.
100kΩ
25kΩ
25kΩ
100kΩ
INA122
5
4
2
3
6
5
8
1
2
1
8
3
7
6
R
G
R
G
V
IN
A
2
A
1
–
V
IN
+
V
IN
–
V
IN
+
V+
V–
INA122
0.22µF
0.1µF
V
O
V
O
Ref
Ref
Load
+
–
Also drawn in simplified form:
V
O
= (V
IN
– V
IN
) G
–+
200kΩ
R
G
G = 5 +
Dual Supply
Single Supply
DESIRED GAIN R
G
NEAREST 1%
(V/V) (Ω)R
G
VALUE
5NCNC
10 40k 40.2k
20 13.33k 13.3k
50 4444 4420
100 2105 2100
200 1026 1020
500 404 402
1000 201 200
2000 100.3 100
5000 40 40.2
10000 20 20
NC: No Connection.
INPUT BIAS CURRENT RETURN PATH
The input impedance of the INA122 is extremely high—
approximately 10
10
Ω. However, a path must be provided for
the input bias current of both inputs. This input bias current
is approximately –10nA (current flows out of the input
terminals). High input impedance means that this input bias
current changes very little with varying input voltage.
FIGURE 2. Optional Trimming of Output Offset Voltage.
10kΩ
OPA336
±10mV
Adjustment Range
100Ω
100Ω
100µA
1/2 REF200
100µA
1/2 REF200
V+
V–
R
G
INA122
Ref
V
O
V
IN
–
V
IN
+