9

®

INA118

INA118

47k

Ω

47k

Ω

INA118

10k

Ω

Microphone,

Hydrophone

etc.

Thermocouple

INA118

Center-tap provides

bias current return.

NOISE PERFORMANCE

The INA118 provides very low noise in most applications.

For differential source impedances less than 1k

Ω, the INA103

may provide lower noise. For source impedances greater

than 50k

Ω, the INA111 FET-Input Instrumentation Ampli-

fier may provide lower noise.

Low frequency noise of the INA118 is approximately

0.28

µVp-p measured from 0.1 to 10Hz (G≥100). This pro-

vides dramatically improved noise when compared to state-

of-the-art chopper-stabilized amplifiers.

OFFSET TRIMMING

The INA118 is laser trimmed for 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. 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 INA118 is extremely high—

Ω. However, a path must be provided for

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

is approximately

±5nA. 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 INA118 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

INA118 is from approximately 0.6V below the positive

supply voltage to 1V 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

linear common-mode input range is related to the output

voltage of the complete amplifier. This behavior also de-

pends 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 INA118 will be near 0V even

though both inputs are overloaded.

LOW VOLTAGE OPERATION

The INA118 can be operated on power supplies as low as

±1.35V. Performance of the INA118 remains excellent with

power supplies ranging from

±1.35V to ±18V. Most param-

eters vary only slightly throughout this supply voltage range—

see typical performance curves. Operation at very low sup-

ply 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 a various

supply voltages and gains.

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

10k

Ω

OPA177

±10mV

Adjustment Range

100

Ω

100

Ω

100µA

1/2 REF200

100µA

1/2 REF200

V+

V–

R

G

INA118

Ref

V

O

V

IN

–

V

IN

+