Grounding - should digital and analog ground be isolated?

[humbleson]

There are certainly differences between analogue and digital grounding.

But should our equipment have isolated ground between AC ground and DC ground?

Basically are there any differences on the two, as the DC regulated supply is supplied from the AC and with its chasis grounded to AC ground.

Pls advice.
I'm looking at possibilities where noise could be injected through supply lines to my hardware.

 

[ASIC]

humbleson,

It is always a good idea to isolate AC and DC ground, particularly since AC typically is high power/voltage and DC is low power/voltage signal paths.

As for noise injected through supply lines... I never had this problem before.

Mostly noise or spikes enters on the cables connected to inputs/outputs on the board. If your design works in a noisy environment, you should also seperate ground on the inputs/outputs from the ground of the main electronics. You may also want to add RC filters on (schmit-trigger'ed) inputs, some series resistors on IC input pins or even transorbs or MOVs to protect your inputs.

ASIC

 

[Alexg]

Good and reliable grounding both for signals and security is a complex topic but there are many good papers around talking about it. Do a search at google for grounding and audio signals and you should find more information than you can digest.

Best regards,
Alexg

 

[xmizi]

Sorry but I never notice grounding is such a big topic. Could you give me a brief description about the difference between AC and DC grouding?

 

[House_Cat]

As stated by others, grounding can be a subject of much discussion and disagreement. But there are some simple rules.

All signals have a return path. That return path generally follows the signal path as closely as it can along the nearest ground (or power) plane or trace. It is a simple matter of a signal following the lowest impedance route.

Digital signals have high frequency components - think of the Fourier representation of the leading and trailing edges. Those high frequency components of the pulse edges will readily capacitively couple into nearby components or signal traces. Low signal level analog circuits can thereby be subjected to the "noise" of digital signals.

You don't necessarily need separate gound planes for analog and digital, but you do need to look at both the signal path and the return path. You don't want the digital return path to cross or parallel the sensitive analog signal return path. You can control this with component placement such that the digital and analog circuits are physically separated, and each has a clearly defined path to return to the power supply.

You will hear and read arguments that separate ground planes should be used with a single point common "mecca". This is pure garbage when you look at both the theory and real world experiments that have been conducted. Single point analog/digital ground plane connections are, in fact, inviting introduction of noise onto the analog plane at the point of connection. They also ignore the real world fact that some return paths are through the power planes, and not the ground planes.

Learn to visualize the entire signal loop, including the return path, and keep the digital return away from the analog. You won't have any problems.

 

[Phytex]

You must be carefully with grounding electric cycles. If you make to much grounds it will be possible that a ground began to floating!

 

[Pootle]

I'm designing a high-speed mixed signal (100/400MHz) board and am following the advice of the esteemed. Dr. Howard Johnson.

From his excellent Signal Integrity website:

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In particular the first link describes an excellent approach to mixed-signal grounding.