INA321

INA2321

SBOS168B − DECEMBER 2000 − REVISED JULY 2004

www.ti.com

11

SETTING THE GAIN

and 6, determines the gain of the INA321. With an

internally set gain of 5, the INA321 can be programmed for

gains greater than 5 according to the following equation:

The INA321 is designed to provide accurate gain, with gain

error guaranteed to be less than 0.1%. 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

performance. 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 Characteristics 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

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 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 INA321 is ideal

for use with high-impedance sources. The input bias

current of less than 10pA makes the INA321 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 how bias current path can be provided in

the

cases

of

microphone

applications,

thermistor

applications, ground returns, and dc-coupled resistive

bridge applications.

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.

INA321

5

3

2

8

7

6

4

1

V+

Shutdown

V

V

−

V

REF

INA321

5

3

2

8

7

6

4

1

V+

Shutdown

V

V

−

V

REF

INA321

5

3

2

8

7

6

4

1

V+

Shutdown

V

V

−

V

REF

V

OUT

RG

V

OUT

RG

V

OUT

RG

Figure 3. Providing an Input Common-Mode Path

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 balanced input. The advantages are lower input offset

voltage due to bias current flowing through the source

impedance and better high-frequency gain.