+

20 k

380 k

380 k

380 k

19 k

−15 V

15 V

4

7

2

3

1

6

5

−IN

+IN

+

15 V

−15 V

100

100

100 µA

½ REF200

100 µA

½ REF200

10 k

(1)

INA149

SBOS579A

– SEPTEMBER 2011 – REVISED NOVEMBER 2011

www.ti.com

COMMON-MODE REJECTION

Common-mode rejection (CMR) of the INA149 depends on the input resistor network, which is laser-trimmed for

accurate ratio matching. To maintain high CMR, it is important to have low source impedance driving the two

inputs. A 75-

Ω resistance in series with pins 2 or 3 decreases the common-mode rejection ratio (CMRR) from

100 dB (typical) to 74 dB.

Resistance in series with the reference pins also degrades CMR. A 4-

Ω resistance in series with pins 1 or 5

decreases CMRR from 100 dB to 74 dB.

Most applications do not require trimming. Figure 45 shows an optional circuit that may be used for trimming

offset voltage and common-mode rejection.

(1) The OPA171 (a 36-V, low-power, RRO, general-purpose operational amplifier) can be used for this application.

Figure 45. Offset Voltage Trim Circuit

14

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© 2011, Texas Instruments Incorporated

Product Folder Link(s): INA149