[SI-LIST] Re: Mixed signal: partitioning GND or unique plane ?
- From: "Javier DeLaCruz" <jdelacruz@xxxxxxxxxxxx>
- To: <tkrzesaj@xxxxxxxxxxxxx>
- Date: Tue, 3 Jun 2003 15:59:05 -0400
Thomas,
The simple answer to your question is that digital sections of your =
device will generate noise which is easily transferred through the =
ground planes (ground bounce.) Analog sections of you device generally =
require a quiet ground. This assumes that you have simultaneously =
switching digital buses, which generate noise. Therefore, in order to =
keep much of the digital noise from affecting the analog section, the =
ground planes should be separated. If you run a web search on keywords =
"ground bounce," a bunch of articles describing phenomenon this will be =
listed. =20
There are other many other occasions where ground plane splitting would
=
be desired. One example of these are in neighboring digital sections of =
the device that have different power supply voltages. A bus that swings =
0 to 5V will generate more noise on ground, than one that only switches =
from 0 to 1.8V. Additionally the lower voltage bus will have a lower =
threshold for noise and would be adversely affected by the noise from =
the 5V bus.
Depending upon your application, it may be a good idea to keep IO =
ground and core ground separated as well.
The faster the rise time or fall time, the more noise will be =
transferred to the plane(s.) Buses with fast edge rates should be =
isolated from the more sensitive signals/buses.
Be careful not to over-split your planes, and consequently raise the =
inductance of the ground. This will limit the ground plane's ability to =
dissipate the noise. Another rule-of-thumb for keeping the ground quiet =
is to add redundant ground vias, balls, etc., especially neighboring the =
signal/bus path. These same rules can be applied to power planes as =
well. =20
There are software tools available which can simulate this. Let me =
know if you need recommendations, which I can send separately.
Regarding your comment on return paths, it is important to not split =
your planes in such a manner that your transmission lines cross from one =
ground or power plane to another. Use plane splits which similar to =
pizza slices, with a core slice in the center. This topic has been =
coved several times in the SI-list and I recommend reviewing the earlier =
messages. Chapter 5, in Dr. Johnson's "High Speed Digital Design" =
covers this topic very well, too, it you have it handy.
Thanks,
Javier =20
-----Original Message-----
From: tkrzesaj@xxxxxxxxxxxxx [mailto:tkrzesaj@xxxxxxxxxxxxx]
Sent: Friday, May 23, 2003 1:44 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Mixed signal: partitioning GND or unique plane ?
Hi,
I read some books and papers saying that one should divide the PCB =
ground
plane into analog, digital, I/O areas...(e.g. "PCB design techniques =
for
EMC compliance" M. Montrose) and some other papers are saying that the
best practice is juste using a unique ground plane but placing the
components into block areas controling the return curent flow... (e.g.
"Grounding and Shielding techniques" R.Morrison and, "Best practice in
circuit board design" T. Jarvis)
Is there a best practice ? Can I have some comments ?
Thanks,
Thomas.
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