[sdiy] analog or digital ground?

[Seb Francis]

Hi Harry,

Thanks for your reply.

I've seen lots of stuff saying that A/D and D/A should have both their analog and digital grounds connected to the analog ground plane.  This makes a lot of sense .. this article explains it quite well:
http://www.analog.com/library/analogDialogue/Anniversary/12.html

However, it seems not so simple for a MUX.  In a D/A the GND is used as an analog reference, so clearly the analog plane is the right choice.  But for a MUX I would imagine there is a fair degree of isolation (level conversion) between the logic and GND connections and the analog signal (since the analog signal can swing above logic level and below GND).  So in this case it _might_ be better to use the digital ground to save any noise injection into the analog plane from the digital part of the MUX.

Then there's also decoupling to consider.  V+ and V- are connected to the power rails via protection diodes to stop the S&H cap from killing the MUX when power is switched off.  I am decoupling the power rail end of the diodes to AGND to localise any current transients in the power rails.  DGND is not a viable choice because I don't want to couple the DGND noise into the +-15V power rails.

It seems strange to decouple the device to AGND, but connect the GND pin to DGND, so perhaps for this reason alone I should use AGND.

And then I get thinking about decoupling of the PIC itself.  Currently it's decoupled to digital ground, but then maybe that's bad because it is coupling noise into the +5V rail which is also used for the DAC.  I'm considering including a ferrite bead between the digital and analog sections of the +5V rail to help with this.

As always, further comments would be welcome (hey maybe I really should go back to education so I can bug my teachers with all these questions, rather than annoying the list ;-)

Seb



harrybissell wrote:

> My call is think of where the currents involved want to return to... then
> connect to the closest ground.
>
> Latest Analog Devices seminar said they considered A/D, D/A etc mixed
> signal to be ANALOG parts... they recommended some series resistors in the
> digital inputs to limit current and let the gate capacitance of the device act as
> a lowpass filter (hey its a couple pF....).   Sounded good to me.
>
> Since you are switching the analog signals, I'd say analog ground.
>
> H^) harry
>
> Seb Francis wrote:
>
> > 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?
> >
> > Seb