Re: PCB Design

I read in sci.electronics.design that martin griffith <martingriffith@xxxxxxxxxxxxx> wrote (in <hf9o51l25dn69cirp7d12vp190cvc5ae7p@xxxxxxx>) about 'PCB Design', on Tue, 12 Apr 2005:

slightly OT, look at the many data sheets of ADC's and DAC's and they ALL specify having seperate Digital and Analog grounds joined at one point only.

Every time this topic is brought up here, All the SED gurus say "DONT SPLIT THE GOUND PLANES"

This does look paradoxical, but that is because both statements are *deductions* from other, more fundamental statements:

1. Keep 'ground' as low impedance as possible. Everywhere;

2. Keep analogue currents out of digital areas of the board, and keep digital currents out of analogue areas.

Statement 1 leads to 'Don't split the ground plane.'

Statement 2 requires you to LOOK where currents are going to flow, and remember they flow IN LOOPS on the board. I was going to try to show in ASCII art what that implies, but it's too difficult.

Imagine that you have the device in the centre of a square board. with the Aground pin towards the 'west' and the Dground pin towards the east. They are connected together, but around the device is a continuous ground plane. Then ALL digital currents must flow substantially westwards to the Dground, and all analogue currents must flow substantially eastwards to the Aground.

Keep the sectors roughly from NW to NE and SW to SE for power management and other stuff that isn't sensitive signals. If you get it right, there should be very little analogue or digital current flowing in the strip of ground plane perpendicular to the bridge between the Aground and Dground pins.

But there may well be a lot of 'trash' current there, which would cause mischief elsewhere if you split the ground plane along the length of that strip.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
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