[SI-LIST] Re: return planes in mixed analog/digital system
- From: "Sriram Subburathinam, TLS-Chennai" <sriramsr@xxxxxx>
- To: Gene Glick <gglick@xxxxxxxxxxxx>, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Thu, 1 Oct 2009 22:55:37 +0530
Hi Gene,
Please look at the article,
http://www.analog.com/static/imported-files/analog_dialogue/5467026043687049331665676350Grounding.pdf
This paper talks about Analog / Ground planes separation especially when used
with ADC / DACs.
Hope it helps.
Thanks,
Sriram SR
If you are unable to see the link, Google " GROUNDING IN HIGH SPEED SYSTEMS
site: analog.com".
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Gene Glick
Sent: Wednesday, September 30, 2009 9:02 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] return planes in mixed analog/digital system
(resend, sorry if it's a duplicate)
I'm not quite sure how to write this in a way that makes sense, but here
goes -
This design contains a mix of audio and digital circuits. Audio is
obviously low frequency stuff, but it is in particular meant to be high
quality audio, so cross-talk is very important. The digital stuff is
primarily for the ADC and DAC circuitry running in the vicinity of 25 MHz.
Due to how the layout has progressed, I am at a crossroads in the power
and ground plane topology. Without drawing pictures, this may be tough
to visualize. Take the DAC circuitry for example. It has separate
analog and digital supply pins (5V analog and 3.3 V digital). The
manufacturer data sheet says to use one single ground plane near the
part. But, I do not want any of the digital currents flowing in the
analog ground plane.
If this lousy ascii picture works, you'll have an idea of what I'm doing:
------------------------
| digital power plane|
| |
| |
| |------------\
| --clock--> | \---audio------>|
| --control-> | DAC / __|__
| |------------/ _____ Some Load.
| | |
| | |
| | GND
------------------------
------------------------
| analog/digital |
| ground plane |
| |
| |
| |------------\ |
--clock-->| | \---audio------>|
--control-> | DAC / | __|__
| |------------/ | _____ Some Load.
| | |
| | |
| |<----- GND
------------------------
My theory is that the audio signals will flow only from the DAC to the
load and back through the signal trace and ground return line. The 2
lines are routed just like a differential pair. The analog /digital
ground plane does not connect to the global ground plane any other place
than at the load (I've over simplified the real design - it's actually
more complex, but this is the gist of it). But what about the return
currents for the digital? If the digital return current can flow on the
digital power plane, then this should work good. Maybe need some good
decoupling between digital power and ground to assure this. Does this
make sense?
I've shown the 2 planes above overlapped but it doesn't have to be that
way. If there's a chance for capacitive coupling and the digital return
current going the wrong way, then this needs to change.
Any thoughts, improvements or criticisms?
thanks
gene
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