INA331, INA2331

SBOS215B

10

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SETTING THE GAIN

and 6, determines the gain of the INA331. With an internally

set gain of 5, the INA331 can be programmed for gains

greater than 5 according to the following equation:

The INA331 is designed to provide accurate gain, with gain

error less than 0.1%. Setting gain with matching TC resistors

will minimize gain drift. Errors from external resistors will add

directly to the error, and may become dominant error sources.

COMMON-MODE INPUT RANGE

The upper limit of the common-mode input range is set by the

common-mode input range of the second amplifier, A2, to

1.2V below positive supply. Under most conditions, the

amplifier operates beyond this point with reduced perfor-

mance. The lower limit of the input range is bounded by the

output swing of amplifier A1, and is a function of the refer-

ence voltage according to the following equation:

(See typical characteristics “Common-Mode Input Range vs

Reference Voltage”).

REFERENCE

The reference terminal defines the zero output voltage level.

In setting the reference voltage, the common mode input of

A3 should be considered according to the following equation:

The reference pin requires a low-impedance connection. As

little as 160

Ω in series with the reference pin will degrade the

CMRR to 80dB. The reference pin may be used to compen-

sate for the offset voltage (see Offset Trimming section). The

reference voltage level also influences the common-mode

input range (see Common-Mode Input Range section).

INPUT BIAS CURRENT RETURN

Ω, the INA331 is ideal for

use with high-impedance sources. The input bias current of

less than 10pA makes the INA331 nearly independent of

input impedance and ideal for low-power applications.

For proper operation, a path must be provided for input bias

currents for both inputs. Without input bias current paths, the

inputs will “float” to a potential that exceeds common-mode

range and the input amplifier will saturate. Figure 3 shows

FIGURE 3. Providing an Input Common-Mode Path.

how bias current path can be provided in the cases of

microphone applications, thermistor applications, ground re-

turns, and dc-coupled resistive bridge applications.

When differential source impedance is low, the bias current

return path can be connected to one input. With higher

source impedance, two equal resistors will provide a bal-

anced input. The advantages are lower input offset voltage

due to bias current flowing through the source impedance

and better high-frequency gain.

47k

Ω

Microphone,

Hydrophone,

etc.

Center-tap

provides bias

current return

Bridge resistance

provides bias

current return

Transformer

Bridge

Amplifier

Bridge

Sensor

V

B

(1)

V

B

(1)

V

EX

NOTE: (1) V

common-mode range, dependent

on REF.

INA331

5

3

2

8

7

6

4

1

V+

Shutdown

V

V–

V

REF

INA331

5

3

2

8

7

6

4

1

V+

Shutdown

V

V–

V

REF

INA331

5

3

2

8

7

6

4

1

V+

Shutdown

V

V–

V

REF

V

OUT

RG

V

OUT

RG

V

OUT

RG