I would like to use an op amp AD8618 for a design, and need the input offset voltage to be low. For this op amp it seems fine with maximum of 65uV, but looking at the charts on page 6 etc, it seems to indicate that the input offset voltage in fact changes depending on the common mode voltage (at the inputs?) to much higher than that. (https://www.analog.com/media/en/technical-documentation/data-sheets/AD8615_8616_8618.pdf)

I intend to bias the amp at 2.5V, which could mean very poor results!? Is this a characteristic of all op amps, could someone explain to me or provide me a resource on this phenomenon?

  • \$\begingroup\$ Recommendations are off-topic and even if they were on-topic nobody can guess what circuit and performance you are trying to achieve as a whole. Voting to close. Even recommending resources calls for opinions and questions that solicit opinions are off-topic too. \$\endgroup\$ – Andy aka Jan 27 '19 at 11:30
  • \$\begingroup\$ Ok Andy, I'll bite. The thread was posted more as a resource to other people with a similar question, as I could not find an immediate answer elsewhere. 90% of the post is a discussion about and a request for an opinion about the matter, which, if you didn't get it is: 'the relationship between input offset voltage and common mode voltage' \$\endgroup\$ – Himmel Jan 27 '19 at 11:48
  • \$\begingroup\$ I removed the "shopping" part of the question. However, the question may still be too broad. Also, you say you are asking for "an opinion about the matter" which is also off-topic for this site. \$\endgroup\$ – Elliot Alderson Jan 27 '19 at 12:00
  • \$\begingroup\$ This question actually seems to be on-topic. Specifically asks about the relationship between Input Offset Voltage (Ios) and Common Mode Voltage (Vcm). I take this as asking for help to understand that relationship and the consequences of changing the Common Mode Voltage. \$\endgroup\$ – Dwayne Reid Jan 28 '19 at 14:57
  • \$\begingroup\$ Most definitely, seems to be solved now, an op amp would need infinite CMRR for it not to affect the Vos. Cheers \$\endgroup\$ – Himmel Jan 29 '19 at 23:36

This is a common characteristic of rail-to-rail input op-amps. There are really two front ends and there is a transition between them at some common-mode voltage. R-R output is irrelevant.

If you care about Vos to that extent, you can choose another type of op-amp. Maybe a chopper type, but they have other subtle (and not so subtle) imperfections.

  • \$\begingroup\$ Cheers Spehro, yes the CMRR is definitely the issue here, so long as an op amp doesn't have an infinite CMRR this will always affect Vos. I have gone ahead and used a chopper amp as your recommended :) \$\endgroup\$ – Himmel Jan 29 '19 at 23:34

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