>> Some people would say that's real sloppy. A precision analog
>> circuit would like its ground to be protected from glitches
>> caused by other parts.
>
> If you have a "precision analog circuit", then you most likely
> shouldn't be making assumptions about the cleanliness of the
> board's main supply, and therefore filter it properly local to the
> analog circuit.

I mentioned ground plane, not a power supply. Digital IC's current
would flow through the ground plane and therefore produce some voltage
distribution picture around analog parts. You don't need much current
through the ground plane to spoil 1V signal measured with 18-bit ADC.
--=20
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