"Ghost" Parallel Impedance: Possibly from an Op-Amp

Thread Starter

stasbala

Joined May 24, 2021
6
1633466607126.png
Hello Everyone,
I am having a fairly strange problem where parts that are validated to be within tolerance for this circuit are displaying significantly lower resistance when placed on the board. Through trial and error I believe I have tracked the issue down to the Op-Amp, but I do not really have a way of testing other than that the problem does not present itself when the Op-Amp is not present on the board.

For instance: R8, R17, R18, R35 are all reduced to values of 4.2 Meg when on board, but after removing are still their proper values of 5.6 Meg.
Other symptoms include a lowered input voltage to the Op-Amp, gain of 1.2 when it should be just a buffer, and a randomized reading from the other amplifier in the package.

The reason that I am believing the Op-Amp to be at fault is that the 2nd amplifier in the OPA2348AIDR package is also having problems in this circuit, but if either problem is present then the other one is as well.
The second Amplifier circuit:
1633466775961.png
Unfortunately I am unable to provide any high-quality images of the boards themselves at this point, but the problem occurs randomly and once it does occur, it stays.


I'm not even sure what exactly I'm looking for as evidence towards how to solve this problem at this point.
Could I ask for some pointers on how to look into verifying the Op-Amps are not damaged/counterfeit?
Is there something that I should do to provide a more robust design in this case?

Thank you all in advance :)
 

Thread Starter

stasbala

Joined May 24, 2021
6
The opamps must be powered for all you measurements.
Are they?
Hello,
The Opamps have not been powered when I was measuring impedances as I was just using a basic meter which cannot handle measurements while there is a power.
When taking voltages, currents power has been applied to the circuit.

I have not measured the op amps themselves for any qualities as of yet.
 

Thread Starter

stasbala

Joined May 24, 2021
6
[Q
Then that's likely your problem.
An opamp may have a low input impedance when it is not powered,
Ah I see,
Thank you very much for the knowledge.
Turns out I was barking up the wrong tree, as an old issue that we thought had been resolved did not have the solution make it through to production, causing the circuit to receive an incorrect input.

Again, thank you very much for your time Crutschow!
 
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