[sdiy] OpAmp Overvoltage / Power Up (was: Re: Pots read by microcontroller and sourcing question)

[Roman Sowa]

All those concerns are unimportant when you use rail-to-rail in-out 
opamp supplied from the same voltage as the micro, which is what I 
always do when there's external opamp involved in uC design.
This way whatever happens, opamp output will never exceed uC rails.

Roman

W dniu 2015-02-05 o 00:09, Michael Zacherl pisze:
>
> On 3.Feb 2015, at 10:11 , Oakley Sound <oakleylist at btinternet.com> wrote:
>
>> One problem with an op-amp per pot is that you need some sort of over voltage protection on the ADC or the Mux. An op-amp on power up, or partial power up, can do some horrid things.
>
> I tested OpAmp circuits with one rail disconnected and in most cases the output went up/down straight to the other rail.
> That indeed can be way too much for a 3.3 or 5V µC in a 12 or 15V system.
> For a good reason every now and then I can read warnings about making sure both voltages are present in a system otherwise parts can/would be damaged.
> I wonder how's that at power-up time?
> Due to tolerances etc. both supply voltages never would come up exactly at the same
> time, but that's probably a different situation with capacitors charging, rails poperly connected etc?
>
> Michael.
>
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