®

INA141

9

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

OFFSET TRIMMING

The INA141 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 2. Optional Trimming of Output Offset Voltage.

INPUT BIAS CURRENT RETURN PATH

The input impedance of the INA141 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 INA141 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

INA141 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 to in-

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

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

the complete amplifier. This behavior also depends on sup-

ply 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 INA141 will be near 0V even

though both inputs are overloaded.

LOW VOLTAGE OPERATION

The INA141 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 through 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 require-

ments of internal nodes limit the input common-mode range

with low power supply voltage. Typical performance curves,

“Input Common-Mode Range vs Output Voltage” show the

range of linear operation for

±15V, ±5, and ±2.5V supplies.

INPUT PROTECTION

The inputs of the INA141 are individually protected for

voltages up to

±40V. For example, a condition of –40V on

one input and +40V on the other input will not cause

damage. Internal circuitry on each input provides low series

impedance under normal signal conditions. To provide

equivalent protection, series input resistors would contribute

excessive noise. If the input is overloaded, the protection

circuitry limits the input current to a safe value of approxi-

mately 1.5 to 5mA. The typical performance curve “Input

Bias Current vs Common-Mode Input Voltage” shows this

input current limit behavior. The inputs are protected even if

the power supplies are disconnected or turned off.

10k

Ω

OPA177

±10mV

Adjustment Range

100

Ω

100

Ω

100µA

1/2 REF200

100µA

1/2 REF200

V+

V–

INA141

Ref

V

O

V

IN

–

V

IN

+

47k

Ω

47k

Ω

10k

Ω

Microphone,

Hydrophone

etc.

Thermocouple

Center-tap provides

bias current return.

INA141

INA141

INA141