8

®

INA121

APPLICATION INFORMATION

Figure 1 shows the basic connections required for operation

of the INA121. 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 resis-

tance of 8

Ω in series with the Ref pin will cause a typical

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

SETTING THE GAIN

Gain of the INA121 is set by connecting a single external

resistor, R

Commonly used gains and resistor values are shown in

Figure 1.

(1)

G

= 1 +

50k

Ω

R

G

The 50k

Ω term in Equation 1 comes from the sum of the two

internal feedback resistors of A

metal film resistors 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 INA121.

The stability and temperature drift of the external gain

gain accuracy and drift can be directly inferred from the gain

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 (possi-

bly an unstable gain error) in gains of approximately 100 or

greater.

DYNAMIC PERFORMANCE

The typical performance curve “Gain vs Frequency” shows

that, despite its low quiescent current, the INA121 achieves

wide bandwidth, even at high gain. This is due to the

current-feedback topology of the INA121. Settling time also

remains excellent at high gain.

A

1

A

2

A

3

6

40k

Ω

40k

Ω

40k

Ω

40k

Ω

7

4

3

8

1

2

V

IN

V

IN

R

G

V+

V–

INA121

G = 1 +

50k

Ω

R

G

–

+

5

Over-Voltage

Protection

25k

Ω

25k

Ω

Over-Voltage

Protection

Load

V

+

–

0.1µF

0.1µF

+

–

V

O

R

G

Also drawn in simplified form:

INA121

Ref

V

O

V

IN

–

V

IN

+

Ref

DESIRED

GAIN

(

Ω)(Ω)

1NC

NC

2

50.00k

49.9k

5

12.50k

12.4k

10

5.556k

5.62k

20

2.632k

2.61k

50

1.02k

1.02k

100

505.1

511

200

251.3

249

500

100.2

100

1000

50.05

49.9

2000

25.01

24.9

5000

10.00

10

10000

5.001

4.99

NC: No Connection.

FIGURE 1. Basic Connections.