INA128 datasheet(10/20 Pages) TI

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INA128

INA129

SBOS051B − OCTOBER 1995 − REVISED FEBRUARY 2005

www.ti.com

OFFSET TRIMMING

The INA128/INA129 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.

10k

Ω

OPA177

100

Ω

100

Ω

1/2 REF200

V+

INA128

Ref

−

±10mV

Adjustment Range

V−

100

µA

100

µA

Figure 2. Optional Trimming of Output Offset

Voltage

INPUT BIAS CURRENT RETURN PATH

The input impedance of the INA128/INA129 is

extremely high—approximately 1010

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, 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 thermocouple 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.

47k

Ω

47k

Ω

10k

Ω

Microphone,

Hydrophone

etc.

Thermocouple

Center−tap provides

bias current return.

INA128

Figure 3. Providing an Input Common-Mode

Current Path

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 the output voltage swing of amplifiers A1 and

A2. So the linear common-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 A3

will be near 0V even though both 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.

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