[SI-LIST] Re: analog and digital partitioning
- From: Todd Hubing <HUBING@xxxxxxxxxxx>
- To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Mon, 21 Jun 2010 23:42:27 -0400
Gene,
You've left out some relevant information (e.g. what/where are your external
analog/digital connections to the board, how many layers do you have available,
how many mixed signal devices do you have, ...). Nevertheless, it is pretty
safe to say that you should not be gapping your ground plane in this situation.
If the board is mostly digital, provide a solid digital current return plane
and route your audio analog current returns on a different layer. Your audio
and digital return conductors should only be connected at one place (near your
ADC). Be sure there are not sneak paths through externally attached devices or
the power supply.
You say that creating a single-point connection between the analog and digital
areas is unlikely given the topology. That may be true if you try to keep both
analog and digital current returns on the same layer, but it should not be too
difficult if you route them on different layers. You can always cut up your
power layer under the analog sections of the board if you need to. Maintaining
control of your audio frequency current paths should be a top priority in your
layout.
Todd Hubing
Clemson University
http://www.cvel.clemson.edu
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Gene Glick
Sent: Monday, June 21, 2010 11:20 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] analog and digital partitioning
hi all,
I am struggling with some partitioning decisions, maybe you can help.
This card has an analog section (audio), digital (uP running around 200
MHz, SDRAM, etc.), mixed (ADC, DAC). For the most part, the analog and
digital sections are completely separate. The mixed signal devices
straddle the partition. Some digital signals pass over the boundary,
but are psuedo-static control lines. One clock signal (1 MHz) also
crosses the boundary.
Creating a single-point connection between analog area and digital area
is very unlikely given the topology. I wanted to moat around the analog
sections, but effectively will wind up with slots in the ground plane.
So the question is, how bad is it to have slots in the ground plane? Are
slots acceptable if no return currents or eddies flow around them?
Somehow, I don't believe it possible to be 100% certain of this.
An alternative, is to have one solid ground plane, keeping the signals
routed such that no return currents flow where they should not. But I
am concerned with keeping the audio noise floor at a very low level.
thanks
gene
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