Texas Instruments Extending the Common-Mode Range of Difference Amplifiers Data Sheet User's Guide

APPLICATION BULLETIN® Mailing Address: PO Box 11400 • Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd. • Tucson, AZ 85706 Tel: (602) 746-1111 • Twx: 910-952-111 • Telex: 066-6491 • FAX (602) 889-1510 • Immediate Product Info: (800) 548-6132 EXTENDING THE COMMON-MODE RANGE OF DIFFERENCE AMPLIFIERS

By R. Mark Stitt (602) 746-7445

Extending the common-mode range of difference amplifiers OFFSETTING THE INPUT COMMON-MODE allows their use in a wider variety of reduced power-supply RANGE WITH A CONSTANT VOLTAGE applications.

In many applications, the common-mode signal range is

The INA117 has a specified common-mode input range of known and the common-mode input range of the difference ±200V when operating on standard ±15V power supplies. At amplifier can be adjusted to coincide with the required power-supply voltages above ±13V, the INA117 input range range. For example, the ±60V common-mode range of the is limited to ±200V by the power capabilities of its internal INA117 operating on ±6V supplies could be shifted to range input resistors. On reduced power supplies, the input range from +0V to +120V, or +50V to +170V. is limited by the common mode input range of the internal

To offset the common-mode range, the reference connection

op amp.

of the difference amplifier is connected to an offsetting

The linear common-mode input range of the internal op amp voltage, V , instead of ground. With the reference connected

extends to within 3V of its power supply voltage. For to an offsetting potential, a second difference amplifier must example, with a ±15V power supply, the common-mode be used to refer the output back to ground.

input range of the internal op amp is ±12V. Because the

One way to offset the input voltage is to connect the

INA117 internal resistor network divides the input by 20, the

reference pins 1 and 5 to the negative supply voltage as

actual input range of the INA117 would be 20 • (±12V), or

shown in Figure 1. Another possibility is to derive the offset

240V, for ±15V power supplies. Similarly, reducing the

voltage from a zener diode connected to the negative power

power supply voltage to ±6V will limit the input common-

supply as shown in Figure 2. In either case, the total

mode voltage to ±60V.

common-mode range of the INA117 is unchanged and There are two approaches to boosting the common-mode shifted by –19 • (V

input range for reduced power supply applications: Offset- ting the common-mode range by a fixed amount, and dy- namically adjusting the common-mode range to follow the input common-mode signal.

V = V – VO 3 2

A2 –3V > VO > –10V

INA117

R3 R4 Swap A2 pins 2 and 3 for:

INA105

25kΩ 25kΩ 10V > VO > 3V also; ((V+) – 5V) > VO > ((V–) + 3V)

FIGURE 1. Offsetting the INA117 Common-Mode Input Range Using the Negative Power Supply as a Reference.

AB-0151990 Burr-Brown Corporation AB-015 Printed in U.S.A. August, 1990

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