LT1168 datasheet(23/28 Pages) AD

23 / 28 page

LT6372-1

Rev. 0

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APPLICATIONS INFORMATION

Figure 9.

V–

V+

IN+

IN–

6372-1 F09

VOUT

10n

100n

1.54k

EXTERNAL RFI

FILTER

LT6372-1

f–3dB ≈ 500Hz

Adding a Simple External RC Filter at the Inputs to

an Instrumentation Amplifier Is Effective in Further Reducing

Rectification of High Frequency Out-Of-Band Signals

To avoid any possibility of common mode to differential

mode signal conversion, match the common mode low-

pass filter on each input to 1% or better. Here are the steps

to help determine appropriate values for the filter:

1. Pick R and CD to have a low pass pole at least 10x

higher than the highest signal of interest (e.g. 500Hz for

a 50Hz signal) using:

FilterFreqDIFF =

2πR(2CD+CC)

2πR(2CD+0.1CD)

4.2πRCD

2. Select CC = CD/10.

If implemented this way, the common-mode pole fre-

quency is placed about 20x higher than the differential

pole frequency. Here are the differential and common-

mode low pass pole frequencies for the values shown in

Figure 9:

FilterFreqDIFF = 500Hz

FilterFreqCM = 10kHz

LT1168 Datasheet(PDF) 23 Page - Analog Devices