On Jul 4, 3:20=A0am, angus.oli...@[EMAIL PROTECTED]
wrote:
> Hi All,
>
> Having found dual op amp packages around a fifth the price of
> comparable single packages from online distributors, I think i'll go
> for the double.
> (BTW, I'm using supply of +16, and ground).
>
> I don't want the unused channel drawing current unnecessarily though,
> or bouncing the output around.. - If I tie the non-inverting input to
> +3.3V, and have a high resistance feedback resistor to the inverting
> input (with no other input to that pin), would that be as good as
> anything? I figure the output won't have to 'try' very hard to modify
> the voltage of an input pin with no other voltages on it..
>
> Am I right?
>
> Thanks..
As long as you've chosen a single supply op amp (like the LM358) with
input range extending to the negative rail, just grounding the non-
inverting input on the unused op amp/voltage follower should be OK.
But even if you've got an op amp like the LM1458, 3.3V should be
fine. If you've got an LM358, GND will be lower current, because it
doesn't have the noise of another voltage input.
The "high resistance feedback resistor" won't help. At worst (for
high resistance values), it can encourage oscillations, which will
dramaticaly increase quiescent current (charging and discharging the
internal compensation cap). Just connect the inverting input directly
to the output to make a voltage follower, and save the price of the
resistor.
By the way, make sure you change the circuit layout for the new op amp
pinout.
Cheers
Chris
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