10

®

INA155

SETTING THE GAIN

Gain of 10 is achieved simply by leaving the two gain pins

(1 and 8) open. Gain of 50 is achieved by connecting the

gain pins together directly. In the G = 10 configuration, the

gain error is less than 0.1%. In the G = 50 configuration, the

gain error is less than 0.25%.

Gain can be set to any value between 10 and 50 by connect-

following equation:

10 + 400k

Ω/(10kΩ + R

(1)

This is demonstrated in Figure 1 and is shown with the com-

absolute value of internal resistors is not guaranteed, using the

INA155 in this configuration will increase the gain error and

gain error drift with temperature, as shown in Figure 3.

FIGURE 3. Typical Gain Error and Gain Error Drift with

External Resistor.

OFFSET TRIMMING

The INA155 is laser trimmed for low offset voltage. In most

applications, no external offset adjustment is required. How-

ever, if necessary, the offset can be adjusted by applying a

correction voltage to the reference terminal. Figure 4 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.

INPUT BIAS CURRENT RETURN

The input impedance of the INA155 is extremely high—

Ω, making it ideal for use with high-imped-

ance sources. However, a path must be provided for the input

bias current of both inputs. This input bias current is less than

10pA and is virtually independent of the input voltage.

Input circuitry must provide a path for this input bias current

for proper operation. Figure 5 shows various provisions for

an input bias current path. Without a bias current path, the

inputs will float to a potential that exceeds the common-

mode range and the input amplifier will saturate.

If the differential source resistance is low, the bias current

return path can be connected to one input (see the thermo-

couple in Figure 5). With higher source impedance, using

two equal resistors provides a balanced input with advan-

tages of lower input offset voltage due to bias current and

better high-frequency common-mode rejection.

FIGURE 4. Optional Trimming of Output Offset Voltage.

FIGURE 5. Providing an Input Common-Mode Current Path.

Gain (V/V)

10

15

20

30

25

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

400

360

320

280

250

200

160

120

80

40

0

35

40

50

45

Gain Error Drift

Gain Error

OPA336

INA155

Ref

(1)

3

6

2

5

V

O

Adjustable

Voltage

V

IN

V

IN

NOTES: (1) V

level. The value of V

range. (2) For best performance, common-mode input voltage

should be less than (V+) – 1.8V or greater than (V+) – 0.8V.

1

8

INA155

3

6

6

6

6

25

V

REF

47k

Ω

Microphone,

Hydrophone, etc.

INA155

3

2

3

2

5

V

REF

Center-tap

provides bias

current return

Low-resistance

thermocouple

provides bias

current return.

Bridge resistance

provides bias

current return

INA155

3

2

10k

Ω

5

V

REF

V

B

(1)

Thermocouple

INA155

5

V

REF

Bridge

Sensor

1

8

1

8

1

8

1

8

V

B

(1)

V

B

(1)

V

EX

NOTE: (1) V

common-mode range, dependent

on V