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'[EE]: Unused pins on a CMOS OpAmp?'
2002\03\01@194253 by Rick Mann

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What should I do with the unused pins on a dual CMOS op-amp (specifically,
the National LM6482 rail-to-rail op amp)? If anything, is it enough to tie
the inputs of the unused op amp together? Should they also be grounded?

The datasheet had no recommendations, nor did the FAQ.

TIA,

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Rick

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2002\03\01@195724 by Dave Dilatush

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Rick Mann wrote...

>What should I do with the unused pins on a dual CMOS op-amp (specifically,
>the National LM6482 rail-to-rail op amp)? If anything, is it enough to tie
>the inputs of the unused op amp together? Should they also be grounded?
>
>The datasheet had no recommendations, nor did the FAQ.

You can tie one opamp input (doesn't matter which one) to one supply
rail, and the other opamp input to the other rail, and leave the output
open; or you can tie the output to the inverting input to make a
unity-gain follower, and then tie the non-inverting input to ground or
to either of the rails.

Either way, the output of the opamp is determinate and stable- which is
what you want so the unused opamp doesn't interfere with anything.

HTH...

Dave

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2002\03\01@210512 by Spehro Pefhany

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At 04:38 PM 3/1/02 -0800, you wrote:
>What should I do with the unused pins on a dual CMOS op-amp (specifically,
>the National LM6482 rail-to-rail op amp)? If anything, is it enough to tie
>the inputs of the unused op amp together? Should they also be grounded?

The best thing to do in general is to connect the op-amp as a voltage-
follower (output to inverting input) and connect the non-inverting input to
ground if you have +/- supplies, or a voltage that the output can follow
to if you have a single supply. Next best thing is to ground the non-inverting
input with a single supply, and have output connected to inverting input.

Some op-amps do strange things when you go outside the common-mode range or
apply a high differential voltage. They can draw excessive input current
(Bipolar types sometimes have the equivalent of back-to-back diodes across
the input) or they can draw excessive supply current. Unfortunately they
often don't publish the schematics of the op-amps any more, so it's hard
to tell without actually testing. This one is a CMOS one, so it's immune to
some of the ills that affect bipolar rail-to-rail op-amps, but if you follow
the above ideas, you'll have more flexibility in dropping in alternative
op-amps if required.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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