>>>> Looks to me like if you have a design that must be low 
>>>> power, the
>>>> best bet is to avoid having the uncommited op amp in 
>>>> the first place.

>> I thought the general conclusion was to configure a 
>> voltage follower
>> whose input was well within the range allowed for that 
>> particular op-
>> amp.

Yes - but not having it at all is even better power wise 
:-).

>> For most modern op-amps, doesn't that mean you can 
>> parallel the
>> input with any of the USED amps in your package ?

> Yes, or possibly not exactly parallel an op-amp input but 
> another analog
> level point in the circuit that's guaranteed to stay 
> within limits. If you
> have an extreme low-power design with op-amps, I think 
> it's quite likely
> that you have a point somewhere that's not too sensitive 
> WRT the extra
> load, that's within the limits and that doesn't change too 
> fast.

Agree.
ie if you parallel another amplifier with both inputs you 
both add bias currents and whatever power is dissipated by 
having the amplifier change level with time. If you drive no 
load with the second output then incremental power may be 
low but non zero. However, if you have another amplifier 
with one input which is varying less than the other then 
configuring your 'spare' as a follower and then connecting 
non inverting input to the low variation point will do an OK 
job - you are still potentially adding bias current. So, as 
Gerhard says, IF you can find a point that varies less than 
anything connected to an opamp input that may be better. It 
needs to be of low enough impedance that the opamp input 
doesn't overly affect it.

However, the main point here was correctly terminating the 
amplifier inputs and it bears restating the advice in 
reasonably complete form:


TERMINATION OF UNUSED OP AMPS:

Connect unused opamps as a follower and then connect the 
non-inverting input to a point that is within the common 
mode range of the amplifier (preferably well within).

- Where there is a choice

    - Select a connection point with minimum AC variation.

    - Select a point where the amplifier will have minimum 
impact on the original functionality.

    - Consider creating a simple 2 resistor divider where 
the above conditions cannot be well met.

    - Not having any spare sections may be the best choice 
if possible.



        Russell McMahon


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