®

INA2128

9

OFFSET TRIMMING

The INA2128 is laser trimmed for low offset voltage and

offset voltage drift. Most applications require no external

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.

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

voltage swing of amplifiers A

mon-mode input range is related to the output voltage of the

complete amplifier. This behavior also depends 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 the INA2128 will be near 0V even

though both inputs are overloaded.

LOW VOLTAGE OPERATION

The INA2128 can be operated on power supplies as low as

±2.25V. Performance remains excellent with power sup-

plies 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.

47k

Ω

47k

Ω

10k

Ω

Microphone,

Hydrophone

etc.

Thermocouple

Center-tap provides

bias current return.

INA2128

1/2

INA2128

1/2

INA2128

1/2

10k

Ω

OPA177

±10mV

Adjustment Range

100

Ω

(For other

channel)

100

Ω

100µA

1/2 REF200

100µA

1/2 REF200

V+

V–

R

G

INA2128

1/2

Ref

V

O

V

IN

–

V

IN

+

FIGURE 2. Optional Trimming of Output Offset Voltage.

INPUT BIAS CURRENT RETURN PATH

The input impedance of the INA2128 is extremely high—

Ω. However, a path must be provided 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 of the INA2128 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

INA2128 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 the output