INA322EASLASH2K5 datasheet(10/18 Pages) TI2

10 / 18 page

INA322

SBOS174B

SETTING THE GAIN

The ratio of R2 to R1, or the impedance between pins 1, 5,

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:

G = 5 + 5(R2/R1)

The INA322 is designed to provide accurate gain, with gain

error specified 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 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:

VOA1 = 5/4 VCM – 1/4 VREF

(See Typical Characteristic 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:

VOA2 = VREF + 5(VIN+ – VIN–)

VOA2 should be less than VDD – 1.2V.

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

With a high input impedance of 1013

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,

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

FIGURE 3. Providing an Input Common-Mode Path.

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

(1)

NOTE: (1) V

B is bias voltage within

common-mode range, dependent

on REF.

INA322

V+

Shutdown

IN–

V–

IN+

REF

INA322

V+

Shutdown

IN–

V–

IN+

REF

47k

Ω

Microphone,

Hydrophone,

etc.

(1)

INA322

V+

Shutdown

IN–

V–

IN+

REF

OUT

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