[sdiy] analog or digital ground?

[Seb Francis]

Hi Magnus,

Magnus Danielson wrote:

> > 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.
>

Interesting you mention decoupling because this adds a further twist .. there is no "digital" power on this MUX.  It has only +V/-V which I guess is very much connected to the analog side of things.  I'm thinking it is a bad thing to decouple to AGND, and connect the GND pin to DGND.  On the other hand decoupling to DGND will for sure introduce noise into the analog +/-15V rails.

So perhaps AGND is the right choice for the GND pin (yes the MUX is CMOS BTW, so digital currents are small).

>
> 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.

I've found this appnote:
http://www.maxim-ic.com/appnotes.cfm/appnote_number/684

Figure 3 shows that the GND is used only as a reference for the logic level comparator section.  So from this point of view it seems pretty safe to connect GND to digital ground.

The appnote also says that high-frequency noise on the power rails can couple to the analog signals, therefore power supply decoupling is necessary (presumably to analog ground).

But the question remains whether is it ok to use DGND for the logic GND level, but decouple the power rails to AGND.  Presumably the digital section of the MUX can't cause too much current transients in the power rails or this would couple into the analog signals even with external decoupling - so from this point of view it seems ok not to decouple to the same ground that the digital reference uses.

It's so hard knowing what to do on the PCB .. there's no point even trying to measure the effect of using different grounds when the whole circuit is built at the moment on a solderless breadboard.  Maybe I should just toss a coin .. heads for AGND, tails for DGND :-)

Thanks for the pointers,
Seb