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What do you do with unused opamps in a quad package?

Hi all. I remember posting a question before regarding leaving opamp inputs open. From what I read, the inputs should be connected to ground or some voltage source to have some meaningful output.

So, if I used an IC with 4 opamps and only utilized 2 of them, does that mean that I have to ground the inputs of the other two and just leave the output open? Or do I connect the unused opamp output to a grounded resistor?

... or do I connect the unused opamp inputs together to insure that both inputs have the same voltage? Or do I just configure the unused opamps as voltage followers?

My gut feeling is that I should connect the inputs of the unused opamps together to make sure that they have the same voltage. I think that if I don't, then noise might turn the output on and off and maybe "eat" up on the voltage source.

Reply to
MRW
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As I posted earlier, the right way to tie off unused op amps is to connect IN- to OUT, and tie IN+ to a resistor pair - one to V+, one to V+ - so that IN+ is about halfway. You need only one resistor pair for all the unused opamps, and they can be really big resistors.

Doing this guarantees that all the inputs are in spec. If you just tie the two inputs together, any input offset in the op amp will be amplified to the output, which may put the output out of spec.

Reply to
DJ Delorie

I connect them as unity gain followers (connect the inverting input to the output), and then make them buffer some convenient voltage that is within their common-mode range, e.g. connect the non-inverting input to some resistive divider that I already have in the circuit, say at mid-rail or something.

The above procedure is probably not necessary with most op-amps, but it doesn't cost anything extra, and it prevents any of the current sources in the op-amp from going into saturation that could conceivably upset the other op-amps in the package by pulling down shared bias rails or injecting substrate currents (if the opamp design were not very clever).

Chris

Reply to
Chris Jones

Awesome, thanks, DJ!

Reply to
MRW

Thanks, Chris! The text is clearer to me.

Reply to
MRW

Google AN1957 for a Maxim Application Note on the subject; it explains the pitfalls of *not* doing it as DJ Delorie & Chris outlined

Reply to
cpemma

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