Signal Presence Detectors?

[danial stocks]

>
>(2) If you don't want to do (1), at least don't pollute the signal GND.
>      If you don't want an extra GND, derive it from the +/- supply rails.
>      A slow Opamp unity gain follower with input at signal GND, and
>      its output serving as Dirty_GND will often do. Do not bypass the
>      opamp output to signal GND with a cap, though. If you need
>      a cap, it goes to +Vb or -Vb.
>
dont bybass the op amp o/p with a cap to anywhere.. you'll regret it! they 
dont like cpacitive loads at all! any big cap >afew nf to anything other 
than a high impedance point ie audio o/p, summing node, next op amp stage 
etc really needs a few K res between the op amp and the cap..
Cheers,
Dan
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