9

®

INA148

FIGURE 2. Optional Offset Trim Voltage.

FIGURE 3. Preferred Offset Trim Circuit.

THE GAIN EQUATION

An internal on-chip resistor network sets the overall differ-

ential gain of the INA148 to precisely 1V/V. It’s output is

accordance with the equation:

VOUT = (VIN

+ – V

IN

– ) + V

REF

(1)

COMMON-MODE RANGE

The 20:1 input resistor ratio of the INA148 provides an input

common-mode range that extends well beyond its power

supply rails.

The exact input voltage range depends on the amplifier’s

power-supply voltage and the voltage applied to the Ref

terminal (pin 1). Typical input voltage ranges at different

power supply voltages can be found in the applications

circuits section.

OFFSET TRIM

The INA148 is laser-trimmed for low offset voltage and

drift. Most applications will require no external offset ad-

justment.

Since a voltage applied to the reference (Ref) pin (pin 1) will

be summed directly into the amplifier’s output signal, this

technique can be used to null the amplifier’s input offset

voltage. Figure 2 shows an optional circuit for trimming the

offset voltage.

To maintain high common-mode rejection (CMR), the source

impedance of any signal applied to the Ref terminal should

be very low (

≤5Ω).

A source impedance of only 10

Ω at the Ref pin will reduce

the INA148’s CMR to approximately 74dB. High CMR can

be restored if a resistor is added in series with the amplifier’s

positive input terminal (pin 3). This resistor should be 19

times the source impedance that drives the Ref pin. For

example, if the Ref pin sees a source impedance of 10

Ω, a

resistor of 190

Ω should be added in series with pin 3.

1M

Ω

50k

Ω

52.6316k

Ω

50k

Ω

10k

Ω

10k

Ω

+15V

–15V

10

Ω

2.7778k

Ω

1M

Ω

190

Ω

V

IN

7

3

2

6

41

A1

INA148

–V

S

V

O

+

V

IN

–

+V

S

V

O = (VIN – VIN) + VREF

–

+

±15mV Offset

Trim Range, RTI

V

REF

1M

Ω

50k

Ω

52.6316k

Ω

50k

Ω

2.7778k

Ω

1M

Ω

V

IN

7

3

2

6

4

1

A1

INA148

–15V

V

O

+

V

IN

–

+15V

±15mV Offset

Trim Range, RTI

100k

Ω

100k

Ω

100

Ω

+15V

–15V

V

REF

OPA237

V

O = (VIN – VIN) + VREF

–

+