INA125 datasheet(11/18 Pages) TI1

11 / 18 page

INA125

The 60k

Ω term in equation 1 comes from the internal metal

film resistors which are laser trimmed to accurate absolute

values. The accuracy and temperature coefficient of these

resistors are included in the gain accuracy and drift specifi-

cations of the INA125.

The stability and temperature drift of the external gain

setting resistor, RG, also affects gain. RG’s contribution to

gain accuracy and drift can be directly inferred from the gain

equation (1). Low resistor values required for high gain can

make wiring resistance important. Sockets add to the wiring

resistance, which will contribute additional gain error (pos-

sibly an unstable gain error) in gains of approximately 100

or greater.

OFFSET TRIMMING

The INA125 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

the IAREF terminal is added to the output signal. The op amp

buffer is used to provide low impedance at the IAREF

terminal to preserve good common-mode rejection.

FIGURE 2. Optional Trimming of Output Offset Voltage.

10k

Ω

OPA237

±10mV

Adjustment Range

100

Ω

100

Ω

100µA

1/2 REF200

100µA

1/2 REF200

V+

V–

INA125

REF

–

INPUT BIAS CURRENT RETURN

The input impedance of the INA125 is extremely high—

approximately 1011

Ω. However, a path must be provided for

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

flows out of the device and is approximately 10nA. 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 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 input common-mode range of the INA125 is shown in

the typical performance curves. The common-mode range is

limited on the negative side by the output voltage swing of

A2, an internal circuit node that cannot be measured on an

external pin. The output voltage of A2 can be expressed as:

02 = 1.3VIN – (VIN – VIN) (10kΩ/RG)

(voltages referred to IAREF terminal, pin 5)

The internal op amp A2 is identical to A1. Its output swing

is limited to approximately 0.8V from the positive supply

and 0.25V from the negative supply. When the input com-

mon-mode range is exceeded (A2’s output is saturated), A1

can still be in linear operation, responding to changes in the

non-inverting input voltage. The output voltage, however,

will be invalid.

PRECISION VOLTAGE REFERENCE

The on-board precision voltage reference provides an accu-

rate voltage source for bridge and other transducer applica-

tions or ratiometric conversion with analog-to-digital con-

verters. A reference output of 2.5V, 5V or 10V is available

by connecting VREFOUT (pin 4) to one of the VREF pins

(VREF2.5, VREF5, or VREF10). Reference voltages are laser-

trimmed for low inital error and low temperature drift.

Connecting VREFOUT to VREFBG (pin 13) produces the

bandgap reference voltage (1.24V

±0.5%) at the reference

output.

Positive supply voltage must be 1.25V above the desired

reference voltage. For example, with V+ = 2.7V, only the

1.24V reference (VREFBG) can be used. If using dual sup-

plies V

REFCOM can be connected to V–, increasing the

–

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

47k

Ω

47k

Ω

10k

Ω

Microphone,

Hydrophone

etc.

Thermocouple

Center-tap provides

bias current return.

INA125

–

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