The Unused Op Amp—what to do?

RE: The Unused Op Amp—what to do?

Bruce Trump — Fri, 23 May 2014 14:45:43 GMT

Christian-- Interesting question. An op amp that is not unity-gain stable should not be connected in unity gain, even if it is not being used to pass a signal. The oscillations that would likely result could easily couple to other amplifiers. You could, of course, use two additional resistors to set an acceptable gain for that amplifier and connect the non-inverting input to a node within its common-mode range. If adding these resistors is undesirable, it would be wise to check with an expert on our E2E forums. For any specific amplifier, there may be other acceptable options. The OPA4022 is supported on the high speed amplifiers forum and an expert on this device can provide advice. -- Bruce

RE: The Unused Op Amp—what to do?

Christian Kuhlmann — Fri, 23 May 2014 13:18:29 GMT

I wonder whether the unity gain approach still applies to parts like THS4022 which are not unity gain stable. If not, what would you recommend in this case?

RE: The Unused Op Amp—what to do?

Bruce Trump — Mon, 03 Dec 2012 17:44:15 GMT

Dox-- Rarely (I'd say never, but there might be some exception) would an op amp be designed to turn off internal bias circuitry when an amplifier is forced to saturate. This would likely prevent or greatly delay recovery in real signal overload conditions. Far more likely is the case described where the current increases as it "strains" to accomplish the signal condition that has been applied. Best approach--keep an unused op amp in linear operating range.

RE: The Unused Op Amp—what to do?

Jeff Melvin — Mon, 03 Dec 2012 17:31:57 GMT

While I always use the "unity gain follower" approach, I sometimes wonder if it would make more sense to force the input and/or output stage to saturate in a particular direction to turn off internal current mirrors and shut down the opamp. Any advice on this approach?

If this makes sense, shouldn't the approach be explicitly described in the datasheet?

RE: The Unused Op Amp—what to do?

Robert Groh — Sun, 02 Dec 2012 16:48:04 GMT

If I have the space and the extra cost doesn't cause the marketing dept to have a fit, I will usually put a couple of resistors in - one from output to one of the inputs and then resistors from each input to (as mentioned) a node that is in the I/O range. I'll use resistor values that are already used someplace else (so number of part types being used doesn't increase). Setup like this, it makes it much easier to 'mod' the board in production if needed.

RE: The Unused Op Amp—what to do?

David Williams — Sun, 02 Dec 2012 14:49:28 GMT

very intresting learn someting every day