From: Spehro Pefhany
Subject: Re: Unused op amp inputs
References: <firstname.lastname@example.org> <email@example.com>
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Date: Thu, 19 Dec 2002 22:59:16 GMT
NNTP-Posting-Date: Thu, 19 Dec 2002 17:59:16 EST
On Thu, 19 Dec 2002 20:38:39 +0000, the renowned John Woodgate
>I read in sci.electronics.design that Spehro Pefhany
> wrote (in <firstname.lastname@example.org
>om>) about 'Unused op amp inputs', on Thu, 19 Dec 2002:
>>On Thu, 19 Dec 2002 17:54:13 +0000, the renowned John Woodgate
>>>Well, tell him what to do with them, then! Inverting input tied to
>>>output, non-inverting input to ground.
>>>This works for both bipolar and CMOS devices.
>>Yes, it works, but unless you've verified it in fact yields
>>lowest power consumption with every type of op-amp on the market ;-)
>>I don't think it exactly answers his quesetion.
>See my other response to a similar point. What is YOUR answer to his
As I said, *measure* it, as most op-amps have neither the
info in the data sheet nor do they have a schematic to analyze.
As was drilled into us endlessly in school, showing there is no
solution is often as useful or more useful than actually finding
Also I said to avoid piling on too much differential voltage input, if
it does turn out to be desirable to drive the output to one rail
or to the other, as some op-amps have an equivalent input circuit
something like back-to-back diodes in series with resistors. The
data sheets cagily give the maximum differential input voltage
but fail to mention that VERY substantial currents flow when
you put much differential voltage on there.
>>Note that "ground" may be outside the CM range for some amplifiers
>>and some definitions of ground.
>Yes: how complicated can we make the response to the OP? It seems to me
>that there is a competition on s.e.d. to pile up the complications until
>the OP's head spins and he/she departs in disgust.
There are not a lot of combinations of things to do with two inputs.
Ten or fifteen perhaps, worst case, with one of them being a voltage
follower within the CM range. Not *too* disgusting.
"it's the network..." "The Journey is the reward"
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