G = 1 +

49.4 kW

¾

R

G

RG

Also drawn in simplified form:

Ref

VO

VIN

VIN

+

49.4kW

R

G

NC: No Connection

A

1

A

2

A

3

6

7

4

3

8

1

2

V

IN

V+

+

5

Over Voltage

Protection

Over Voltage

Protection

Load

+

O

V

Ref

0.1 F

m

0.1 F

V-

m

24.7kΩ

24.74kΩ

40kΩ

40kΩ

40kΩ

40kΩ

V

(V

- V )

·

IN

IN

+

-

-

-

-

G = 1 +

DESIRED

GAIN (V/V)

R

( )

G

W

NEAREST

1% R ( )

G

W

1

2

5

10

20

50

100

200

500

1000

2000

5000

10000

NC

49.4K

12.35K

5489

2600

1008

499

248

99

49.5

24.7

9.88

4.94

NC

49.9K

12.4K

5.49K

2.61K

1K

499

249

100

49.9

24.9

9.76

4.87

INA129-EP

SBOS508 – DECEMBER 2009

www.ti.com

APPLICATION INFORMATION

Figure 23 shows the basic connections required for operation of the INA129. 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) terminal which is normally grounded. This must be a

low-impedance connection to assure good common-mode rejection. A resistance of 8

Ω in series with the Ref pin

will cause a typical device to degrade to approximately 80 dB CMR (G = 1).

Setting the Gain

(1)

Commonly used gains and resistor values are shown in Figure 23.

The 49.9-k

Ω term in Equation 1 comes from the sum of the two internal feedback resistors of A1 and A2. These

on-chip metal film resistors are laser trimmed to accurate absolute values. The accuracy and temperature

coefficient of these internal resistors are included in the gain accuracy and drift specifications of the INA129.

gain accuracy and drift can be directly inferred from Equation 1. Low resistor values required for high gain can

make wiring resistance important. Sockets add to the wiring resistance which will contribute additional gain error

(possibly an unstable gain error) in gains of approximately 100 or greater.

Figure 23. Basic Connections

Dynamic Performance

Figure 1 shows that, despite its low quiescent current, the INA129 achieves wide bandwidth, even at high gain.

This is due to the current-feedback topology of the input stage circuitry. Settling time also remains excellent at

high gain.

10

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