OFFSET TRIMMING

The INA128/INA129 is laser trimmed for low offset voltage

and offset voltage drift. Most applications require no exter-

nal offset adjustment. Figure 2 shows an optional circuit for

trimming the output offset voltage. The voltage applied to

Ref terminal is summed with the output. The op amp buffer

provides low impedance at the Ref terminal to preserve good

common-mode rejection.

INPUT BIAS CURRENT RETURN PATH

The input impedance of the INA128/INA129 is extremely

Ω. However, a path must be pro-

vided for the input bias current of both inputs. This input

bias current is approximately

±2nA. High input impedance

means that this input bias current changes very little with

varying input voltage.

Input circuitry must provide a path for this input bias current

for proper operation. Figure 3 shows various provisions for

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

inputs will float to a potential which exceeds the common-

mode range, and the input amplifiers will saturate.

If the differential source resistance is low, the bias current

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

couple example in Figure 3). With higher source impedance,

using two equal resistors provides a balanced input with

possible advantages of lower input offset voltage due to bias

current and better high-frequency common-mode rejection.

INPUT COMMON-MODE RANGE

The linear input voltage range of the input circuitry of the

INA128/INA129 is from approximately 1.4V below the

positive supply voltage to 1.7V above the negative supply.

As a differential input voltage causes the output voltage

increase, however, the linear input range will be limited by

FIGURE 2. Optional Trimming of Output Offset Voltage.

10k

Ω

OPA177

±10mV

Adjustment Range

100

Ω

100

Ω

100µA

1/2 REF200

100µA

1/2 REF200

V+

V–

R

G

INA128

Ref

V

O

V

IN

–

V

IN

+

FIGURE 3. Providing an Input Common-Mode Current Path.

47k

Ω

47k

Ω

10k

Ω

Microphone,

Hydrophone

etc.

Thermocouple

Center-tap provides

bias current return.

INA128

INA128

INA128

linear common-mode input range is related to the output

voltage of the complete amplifier. This behavior also de-

pends on supply voltage—see performance curves “Input

Common-Mode Range vs Output Voltage”.

Input-overload can produce an output voltage that appears

normal. For example, if an input overload condition drives

both input amplifiers to their positive output swing limit, the

difference voltage measured by the output amplifier will be

near zero. The output of A

inputs are overloaded.

LOW VOLTAGE OPERATION

The INA128/INA129 can be operated on power supplies as

low as

±2.25V. Performance remains excellent with power

supplies ranging from

±2.25V to ±18V. Most parameters

vary only slightly throughout this supply voltage range—see

typical performance curves. Operation at very low supply

voltage requires careful attention to assure that the input

voltages remain within their linear range. Voltage swing

requirements of internal nodes limit the input common-

mode range with low power supply voltage. Typical perfor-

mance curves, “Input Common-Mode Range vs Output

Voltage” show the range of linear operation for

±15V, ±5V,

and

±2.5V supplies.