[sdiy] analog or digital ground?

[Magnus Danielson]

From: Seb Francis <seb at is-uk.com>
Subject: [sdiy] analog or digital ground?
Date: Sun, 10 Nov 2002 21:24:56 +0000

> Hi,
> 
> I'm just routing the PCB for my MIDI2CV, and I've been pondering which ground plane to connect the S&H MUX to.
> 
> It's a MAX306 (+/-V supply and analog signal range, TTL/CMOS compatible digital inputs).  Digital inputs come direct from a PIC (so they're pretty noisy).
> 
> My first thought was to wire the GND pin to the digital ground (assuming there might be a bit of coupling to ground from the digital inputs), but the datasheet doesn't give much clue whether GND is in any way internally connected with the analog signal - the digital ground plane is itself a source of noise.
> 
> Any thoughts?

Well, the GND is not of much use for the analog side really, but for the
digital side it is mandatory. Assuming it's CMOS inside, then nothing much is
actually happening in there, but to be sure, you want to do the propper thing
with decoupling caps, having minimal traces between the digital power and
ground so that you acheive minimal series-induction.

You'd want to do "The Right Thing" on all chips actually. PIC included.

I'd have to look at the MAX306 specifics, but my tip from the top-of-my-head is
to wire your GND to the digital ground and do the decoupling properly.

Actually, I recall some very interesting appnotes from MAXIM that gave insight
to how their analog switches worked, down to the transistors, so that should
help you out.

Cheers,
Magnus