INA322

10

SBOS174

SETTING THE GAIN

and 6, determines the gain of the INA322. With an internally

set gain of 5, the INA322 can be programmed for gains

greater than 5 according to the following equation:

The INA322 is designed to provide accurate gain, with gain

error guaranteed to be less than 0.4%. Setting gain with

matching TC resistors will minimize gain drift. Errors from

external resistors will add directly to the guaranteed error,

and may become dominant error sources.

INPUT COMMON-MODE 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

reference voltage according to the following equation:

(See Typical Performance Curves for Input Common-Mode

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 equa-

tion:

The reference pin requires a low-impedance connection.

Any resistance in series with the reference pin will degrade

the CMRR. The reference pin may be used to compensate

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 INA322 is ideal

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

of less than 10pA makes the INA322 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,

FIGURE 3. Providing an Input Common-Mode Path.

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

mode range and the input amplifier will saturate. Figure 3

shows how bias current path can be provided in the cases of

microphone applications, thermistor applications, ground

returns, 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.

Center-tap

provides bias

current return

Bridge resistance

provides bias

current return

Transformer

Bridge

Amplifier

Bridge

Sensor

V

B

(1)

V

EX

NOTE: (1) V

common-mode range, dependent

on REF.

INA322

5

3

2

8

7

6

4

1

V+

Shutdown

V

V–

V

REF

INA322

5

3

2

8

7

6

4

1

V+

Shutdown

V

V–

V

REF

47k

Ω

Microphone,

Hydrophone,

etc.

V

B

(1)

INA322

5

3

2

8

7

6

4

1

V+

Shutdown

V

V–

V

REF

V

OUT

RG

V

OUT

RG

V

OUT

RG