®

INA131

8

INA131

V

IN

V

IN

–

+

10k

Ω

V

O

OPA177

Ref

±10mV

Adjustment Range

100

Ω

100

Ω

100µA

1/2 REF200

100µA

1/2 REF200

V+

V–

INA131

47k

Ω

47k

Ω

INA131

10k

Ω

Microphone,

Hydrophone

etc.

Thermocouple

INA131

Center-tap provides

bias current return.

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

OFFSET TRIMMING

The INA131 is laser trimmed for very low offset voltage and

drift. Most applications require no external offset adjust-

ment. Figure 2 shows an optional circuit for trimming the

output offset voltage. The voltage applied to Ref terminal is

summed at the output. Low impedance must be maintained

at this node to assure good common-mode rejection. This is

achieved by buffering trim voltage with an op amp as

shown.

FIGURE 2. Optional Trimming of Output Offset Voltage.

INPUT BIAS CURRENT RETURN PATH

The input impedance of the INA131 is extremely high—

Ω. However, a path must be provided for

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

is typically less than

±1nA (it can be either polarity due to

cancellation circuitry). 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

if the INA131 is to operate properly. Figure 3 shows various

provisions for an input bias current path. Without a bias

current return path, the inputs will float to a potential which

exceeds the common-mode range of the INA131 and the

input amplifiers will saturate. If the differential source resis-

tance is low, bias current return path can be connected to one

input (see thermocouple example in Figure 3). With higher

source impedance, using two resistors provides a balanced

input with possible advantages of lower input offset voltage

due to bias current and better common-mode rejection.

INPUT COMMON-MODE RANGE

The linear common-mode range of the input op amps of the

INA131 is approximately

±13.75V (or 1.25V from the

power supplies). As the output voltage increases, however,

the linear input range is limited by the output voltage swing

of the input amplifiers, A

gain of the INA131 reduces this effect. Compared to the

INA114 and other unity output gain instrumentation ampli-

fiers, the INA131 provides several additional volts of input

common-mode range with full output voltage swing. See the

typical performance curve “Input Common-Mode Range vs

Output Voltage”.

Input-overload often produces an output voltage that appears

normal. For example, an input voltage of +20V on one input

and +40V on the other input will obviously exceed the linear

common-mode range of both input amplifiers. Since both

input amplifiers are saturated to the nearly the same output

voltage limit, the difference voltage measured by the output

amplifier will be near zero. The output of the INA131 will

be near 0V even though both inputs are overloaded.

INPUT PROTECTION

The inputs of the INA131 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 (approxi-

mately 1.5mA). The typical performance curve “Input Bias

Current vs Input Voltage” shows this input current limit

behavior. The inputs are protected even if no power supply

voltage is present.