INA125 datasheet(12/19 Pages) TI2

12 / 19 page

INA125

amount of supply voltage headroom available to the refer-

ence. Approximately 180

µA flows out of the V

REFCOM

terminal, therefore, it is recommended that it be connected

through a low impedance path to sensor common to avoid

possible ground loop problems.

Reference noise is proportional to the reference voltage

selected. With VREF = 2.5V, 0.1Hz to 10Hz peak-to-peak

noise is approximately 9

µVp-p. Noise increases to 36µVp-p

for the 10V reference. Output drive capability of the voltage

reference is improved by connecting a transistor as shown in

Figure 4. The external transistor also serves to remove power

from the INA125.

Internal resistors that set the voltage reference output are

ratio-trimmed for accurate output voltages (

±0.5% max). The

absolute resistance values, however, may vary

±25%. Adjust-

ment of the reference output voltage with an external resistor

is not recommended because the required resistor value is

uncertain.

SHUTDOWN

The INA125 has a shutdown option. When the SLEEP pin

is LOW (100mV or less), the supply current drops to

approximately 1

µA and output impedance becomes approxi-

mately 80k

Ω. Best performance is achieved with CMOS

logic. To maintain low sleep current at high temperatures,

VSLEEP should be as close to 0V as possible. This should not

be a problem if using CMOS logic unless the CMOS gate is

driving other currents. Refer to the typical performance

curve, “Sleep Current vs Temperature.”

A transition region exists when VSLEEP is between 400mV

and 2.7V (with respect to V

REFCOM) where the output is

unpredictable. Operation in this region is not recommended.

The INA125 achieves high accuracy quickly following wake-

up (VSLEEP ≥ 2.7V). See the typical performance curve

“Input-Referred Offset Voltage vs Sleep Turn-on Time.” If

shutdown is not being used, connect the SLEEP pin to V+.

LOW VOLTAGE OPERATION

The INA125 can be operated on power supplies as low as

±1.35V. Performance remains excellent with power sup-

plies ranging from

±1.35V to ±18V. Most parameters vary

only slightly throughout this supply voltage range—see

typical performance curves. Operation at very low supply

voltage requires careful attention to ensure that the com-

mon-mode voltage remains within its linear range. See

“Input Common-Mode Voltage Range.” As previously men-

tioned, when using the on-board reference with low supply

voltages, it may be necessary to connect V

REFCOM to V– to

ensure VS – VREF ≥ 1.25V.

SINGLE SUPPLY OPERATION

The INA125 can be used on single power supplies of +2.7V

to +36V. Figure 5 shows a basic single supply circuit. The

IAREF, VREFCOM, and V– terminals are connected to ground.

Zero differential input voltage will demand an output volt-

age of 0V (ground). When the load is referred to ground as

shown, actual output voltage swing is limited to approxi-

mately 150mV above ground. The typical performance curve

“Output Voltage Swing vs Output Current” shows how the

output swing varies with output current.

With single supply operation, careful attention should be

paid to input common-mode range, output voltage swing of

both op amps, and the voltage applied to the IA

REF terminal.

VIN+ and VIN– must both be 1V above ground for linear

operation. You cannot, for instance, connect the inverting

input to ground and measure a voltage connected to the non-

inverting input.

FIGURE 5. Single Supply Bridge Amplifier.

1000

Ω

+3V

1.5V –

∆V

1.5V +

∆V

+3V

INA125

Ref

Amp

to load

(transducer)

V+

Bandgap

REF

INA125

REFCOM

REFBG

REF2.5

REF5

REF10

REFOut

TIP29C

10V

FIGURE 4. Reference Current Boost.

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