®

INA114

9

OFFSET TRIMMING

The INA114 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 INA114 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 INA114 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 INA114 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

INA114 is approximately

±13.75V (or 1.25V from the

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

the linear input range will be limited by the output voltage

mode range is related to the output voltage of the complete

amplifier—see performance curve “Input Common-Mode

Range vs Output Voltage.”

A combination of common-mode and differential input

signals can cause the output of A

terms of a common-mode and differential input voltages.

Output swing capability of these internal amplifiers is the

same as the output amplifier, A

input common-mode range must be maximized, limit the

output voltage swing by connecting the INA114 in a lower

gain (see performance curve “Input Common-Mode Voltage

Range vs Output Voltage”). If necessary, add gain after the

INA114 to increase the voltage swing.

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 nearly the same output

voltage limit, the difference voltage measured by the output

amplifier will be near zero. The output of the INA114 will

be near 0V even though both inputs are overloaded.

INPUT PROTECTION

The inputs of the INA114 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 Common-Mode Input Voltage” shows this input

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

INA114

V

IN

V

IN

R

G

–

+

10k

Ω

V

O

OPA177

Ref

±10mV

Adjustment Range

100

Ω

100

Ω

100µA

1/2 REF200

100µA

1/2 REF200

V+

V–

INA114

47k

Ω

47k

Ω

INA114

10k

Ω

Microphone,

Hydrophone

etc.

Thermocouple

INA114

Center-tap provides

bias current return.