8

®

INA115

APPLICATION INFORMATION

Figure 1 shows the basic connections required for operation of

the INA115. 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 5

Ω in series with the Ref pin will cause a typical

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

The INA115 has a separate output sense feedback connection

(pin 12). Pin 12 must be connected (normally to the output

terminal, pin 11) for proper operation. The output sense

connection can be used to sense the output voltage directly at

the load for best accuracy.

SETTING THE GAIN

Gain of the INA115 is set by connecting a single external

resistor, R

G:

FIGURE 1. Basic Connections.

DESIRED

R

G

NEAREST 1% R

G

GAIN

(

Ω)(Ω)

1

No Connection

No Connection

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

G

= 1 + 50 kΩ

R

G

(1)

Commonly used gains and resistor values are shown in

Figure 1.

For G=1, no resistor is required, but connect pins 2-3 and

connect pins 14-15. Gain peaking in G=1 can be reduced by

shorting the internal 25k

Ω feedback resistors (see typical

performance curve Gain vs Frequency). To do this, connect

pins 1-2-3 and connect pins 8-14-15.

The 50k

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

internal feedback resistors. These are on-chip 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 specifications of the

INA115.

The stability and temperature drift of the external gain setting

resistor, R

G

, also affects gain. R

G

’s contribution to gain error

and drift can be directly inferred from the gain equation (1).

Low resistor values required for high gain can make wiring

resistance important. The “force and sense” type connections

illustrated in Figure 1 help reduce the effect of interconnection

resistance.

A

1

A

2

A

3

12

11

10

25k

Ω

25k

Ω

25k

Ω

25k

Ω

13

7

5

4

V

IN

V

IN

R

G

V+

V–

INA115

–

+

Over-Voltage

Protection

25k

Ω

25k

Ω

Over-Voltage

Protection

15

14

3

2

8

V

O2

V

O1

1

Load

G = 1 +

50k

Ω

R

G

V

O = G • (VIN – VIN)

0.1µF

0.1µF

+

–

V

O

Also drawn in simplified form:

Ref

INA115

V

IN

V

IN

R

G

–

+

V

O

V

O2

V

O1