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Sunday, 3 February 2013

Debug – Inverting Op Amp

Let’s say you measured Vin = –1V, but Vout way off +1V, now what can be wrong? How do you debug it? Below are some suggestions for you to consider:
  • check the supply to see if the supplies are correct
  • visual inspect for
    • wrong polarity
    • wrong part (if manual assembly involved)
    • missing solder or “cold” solder
  • check if wrong resistors being loaded - power down – measure resistance of R1, R2 to see if any of R1, R2 is larger than 1KOhm – since this is in-circuit measurement – we don’t expect to see 1KOhm, but we definitely expect 1KOhm or less, since parallel resistance of any resistors is the minimum of those.
  • check if oscillation occurs - if everything looks good – get hold of a scope and confirmed there’s no oscillation on either pins mentioned above
  • check if op-amp already sourcing too much current - lift output pin, insert a small resistance resistor from pin to pad, measure the voltage drop to see if the amp is sourcing or sinking more that datasheet stated. It is possible that the next stage is demanding more current than expected.
So good luck with your troubleshooting Smile

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