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 10

Ω 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

(1)

Figure 1.

The 200k

Ω 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

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.

100k

Ω

25k

Ω

25k

Ω

100k

Ω

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

–

+

200k

Ω

R

G

G = 5 +

Dual Supply

Single Supply

DESIRED GAIN

NEAREST 1%

(V/V)

(

Ω)R

5NC

NC

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—

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.

10k

Ω

OPA336

±10mV

Adjustment Range

100

Ω

100

Ω

100µA

1/2 REF200

100µA

1/2 REF200

V+

V–

R

G

INA122

Ref

V

O

V

IN

–

V

IN

+