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PGA2500

Mehrdad Bradaran
Intellectual 430 points
Other Parts Discussed in Thread: PGA2500

Hello,

We have been using the PGA2500 in one of our products for more than 5 years without any problems.  All of a sudden, on this last production run, about 2% of the chips are failing.  The failure symptom is a very high (up to 4 or 5 volts) DC offset on the outputs.  The chip has a fancy DC offset circuit that requires 2 external caps, but the datasheet doesn't give any details about how the circuit works.   I am suspecting something like flux residue on the boards that is affecting the DC offset circuit.  Unfortunately, cleaning the board with alcohol doesn't fix the problem.

Can someone give me ideas about what might make the DC offset circuit misbehave.  How sensitive is it to leakage current of the external caps, or leakage current to power or ground that might be caused by flux residue?

Thanks,

Mehrdad.

  • 0
    TI__Mastermind 27715 points

    Hello Mehrdad,

    I have a few follow-up questions regarding the PGA2500 failure you are experiencing:

    • What is the total number of units in your last production run?
    • Does the output DC offset issue follow the part (replace the failing PGA2500 with new PGA2500 and issue goes away / install failing PGA2500 on previously good system and issue is observed)?
    • What kind of voltages are observed at the input to the PGA2500 when the output DC offset issue occurs?
    • Have you tried cleaning the failing PCBs in an ultrasonic bath followed by a moderate temperature bake in order to fully remove moisture? A surface-only alcohol scrub will not remove flux contamination from underneath the surface mount components.

    Best regards,

    Ian Williams
    Linear Applications Engineer
    Precision Analog - Op Amps

  • 0 in reply to Ian Williams
    Intellectual 430 points
    Thanks, Ian.

    The customer has replied back and the problem has been resolved. "What kind of voltages are observed at the input to the PGA2500 when the output DC offset issue occurs" was the key. Normally, on a good circuit, the input voltage is about -0.6 on both inputs, as described in the datasheet. On the bad circuit, they were measuring -3.77 volts on the minus input and +2.45 on the plus input. So, that led them to suspect the clamping diodes, MBRA120. On 2 boards so far, they took the diodes out, and the DC offset went away.

    Mehrdad.
  • 0 in reply to Mehrdad Bradaran
    TI__Mastermind 27715 points

    Hello Mehrdad,

    I'm glad to hear the customer was able to resolve their issue by tracing the root cause to clamping diodes at the input. Thanks for reporting back!

    Best regards,

    Ian Williams