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Strange behavior of AD8421

Question asked by Marcus_nth on Sep 8, 2014
Latest reply on Sep 12, 2014 by Marcus_nth

Hi all.


I built up a PCB including an AD8421 integrated circuit. The board circuitry is as shown in the screenshot.

I used the OP1177 as recommanded in the data sheet for offset purposes. Not shown in the screenshot is the decoupling of V+ and V- with 100nF X7R and 22µ Tantalum each. The voltage supply is +/-12V.




The positive input signal is constant around 3V. The negative input span lies between 1.6V an 3.8V.


My old PCB used an discrete instrumentation amplifier and that worked fine. In order to shrink the PCB size and cost I wanted to use an integrated instrumentation amplifier.


The output of a typical operation situation with the old circuit is show in the next screenshot.

good signal.jpg

This is what I expect as well from the new circuit. Using the same input signals like in the old circuit I get the follwing result with the AD8421.

bad signal.jpg


Scope setting are the same.


What have I done so far? I swapped the input signals. No success. I added a 10nF cpacitor between the input pins. No success.

I removed R22 to yield a gain of one. No success. In contrast the DC level with an open gain input lets the output raise to a higher voltage.


Any ideas what I have done wrong?


Kind regards