9.2 Typical Application

Figure 9-1 shows the basic connections required for operation of the INA12x. Applications with noisy or high

impedance power supplies may require decoupling capacitors close to the device pins as shown. The output is

referred to the output reference (REF) pin, which is normally grounded. This connection must be low-impedance

to provide good common-mode rejection. A resistance of 8 Ω in series with the REF pin causes a typical device

to degrade to approximately 80 dB CMR (G = 1).

Also drawn in simplified form:

INA128

Ref

G = 1 +

50 k

R

G

G = 1 +

49.4 k

R

G

INA128:

INA129:

DESIRED

R

NEAREST

R

NEAREST

G

G

GAIN (V/V)

( )

1% RG ( )

( )

1% RG ( )

1

NC

NC

NC

NC

2

50.00k

49.9k

49.4k

49.9k

5

12.50k

12.4k

12.35k

12.4k

10

5.556k

5.62k

5489

5.49k

20

2.632k

2.61k

2600

2.61k

50

1.02k

1.02k

1008

1k

100

505.1

511

499

499

200

251.3

249

248

249

500

100.2

100

99

100

1000

50.05

49.9

49.5

49.9

2000

25.01

24.9

24.7

24.9

5000

10.00

10

9.88

9.76

10000

5.001

4.99

4.94

4.87

INA128

INA129

NC: No Connection

A

1

A2

A3

6

7

4

3

8

1

2

RG

V+

INA128, INA129

5

Overvoltage

Protection

Protection

Load

Ref

(1) INA129: 24.7 k .

0.1 µF

0.1µF

V

25 k

(1)

25 k

(1)

40 k

40 k

40 k

40 k

Overvoltage

VO = G • (VIN+ – VIN–

+

VO

VO

VIN–

VIN+

VIN–

VIN+

VREF

) + VREF

RG

Figure 9-1. Basic Connections

9.2.1 Design Requirements

The devices are configured to monitor the input differential voltage when the input signal gain is set by the

external resistor, RG. The output signal is developed with respect to the voltage on the reference pin, REF.

The most common application is where the output is referenced to ground when no input signal is present by

connecting the REF pin to ground, as Figure 9-1 shows. In single-supply operation, offsetting the output signal

to a precise midsupply level is useful (for example, 2.5 V in a 5-V supply environment). To accomplish this level

shift, a voltage source must be connected to the REF pin to level shift the output so that the device can drive a

single-supply ADC.

Voltage reference devices are an excellent option for providing a low-impedance voltage source for the reference

pin. However, if a resistor voltage divider is used to generate a reference voltage, the voltage must be buffered

by an op amp to avoid CMRR degradation.

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INA128, INA129

SBOS051F – OCTOBER 1995 – REVISED MAY 2022

Copyright © 2022 Texas Instruments Incorporated

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