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

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MRW - 26 Jul 2007 19:01 GMT

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.

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DJ Delorie - 26 Jul 2007 19:49 GMT

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.

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MRW - 26 Jul 2007 23:42 GMT

> 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
[quoted text clipped - 4 lines]
> 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.

Awesome, thanks, DJ!

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Chris Jones - 26 Jul 2007 20:04 GMT

> Hi all. I remember posting a question before regarding leaving opamp
> inputs open. From what I read, the inputs should be connected to ground
[quoted text clipped - 15 lines]
>
> --

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

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MRW - 26 Jul 2007 23:42 GMT

> > Hi all. I remember posting a question before regarding leaving opamp
> > inputs open. From what I read, the inputs should be connected to
[quoted text clipped - 30 lines]
>
> Chris

Thanks, Chris! The text is clearer to me.

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cpemma - 29 Jul 2007 22:29 GMT

> Hi all. I remember posting a question before regarding leaving opamp
> inputs open...

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

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Electronics Forum / Basics / July 2007

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