[SI-LIST] Re: Ground Article
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: <krhoads@xxxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Wed, 18 Jan 2017 09:52:16 -0800
Still looking for a physical example of this. Been designing PCBs with all
of those circuits on them for a long time and have seen no occasion where
this was needed.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Kevin G. Rhoads
Sent: Wednesday, January 18, 2017 8:50 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Ground Article
On 1/18/2017 9:16 AM, Istvan Novak wrote:
in mixed-signal boards and on some boards where naturally very noisy
circuits (like high-current DC-DC converters) have to coexist with
sensitive circuits, like SerDes, PLLs, etc, splitting the ground plane
between the domains can improve the isolation and thus improves
performance, as long as we make sure that we dont create SI and/or EMI
problems by crossing the splits with signals that otherwise need a
solid reference.
First the disclaimer -- I mostly follow the SI list to learn.
Next the other disclaimer -- the board I speak of is a one-off for
university research use.
We had a multi-use board (i.e., K101) for instrumentation support on
sounding rocket science flights. Among other things, it digitized three
axis analog magnetometer data and formatted that to feed by synchronous
serial to NASA telemetry.
Four layer board
16 bit ADCs
10 MHz synch serial channels (NASA WFF 93 standard) SOLID ground plane
Nominal +28 from NASA batteries through DC-DC for +/-12 and +5 in box
First gen used filtering -- digitized channels fairly noisy -- even though
the +5 was decoupled to death -- distributed decoupling across the board --
both electrolytics and 0.1 microF-s
Second gen split the +5 supplies -- generating one from +12 via LM7805 and
that one supplying the ADCs and +5 op-amp front ends.
MUCH less noise
So -- the SI issues are truly serious. I have nothing to add to that except
solid ground plane with separate power supplies can work well in some
situations. But noisy power supply rails (feeding digital
circuitry) will not do you well when they also feed sensitive analog.
If anyone wants to see the details (links are "public"):
Cascades 2 board photos
https://www.facebook.com/kgrhoads/posts/4915482088395
Cascades 2 instrumentation
http://www.facebook.com/album.php?aid=2050033&id=1338500958&l=c23c77bae3
Cascades2 schematics
https://www.facebook.com/media/set/?set=a.4916478913315.1073741831.133850095
8&type=3
Sincerely
Kevin
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