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Old 22nd October 2002, 09:01 AM   #1
MWP is offline MWP  Australia
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Default Multiple PCB layout & grounding.

I want to build my DAC using multiple small PCBs.
The main reason for this is that i can change componets and rebuild PCBs if needed a lot easier in the future.

But its got me thinking about ground loops.

Obviously, each PCB is going to have its own leads to the power supply, which will then give all the boards a center grounding point.

But, do i run a ground inbetween connected boards?
For example, i will have a DAC board and a headphone amp board.
I run the L&R channel wires between them... but do i also run a ground?

If not, would i run a ground between the SPDIF reciever board and the DAC board that only has digital signals transfered between the two?

Im planning to use two torids for the power supply.
One for analog supplies.. will give AGND, 3.3V, 5V, +15V, -15V.
One for digital supply giving DGND, 3.3V, 5V.

Your thoughts?
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Old 23rd October 2002, 06:08 AM   #2
haldor is offline haldor  United States
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One key point to keep in mind. Electrical signals flow in loops. When you send a signal from one component to another, the current flow that makes that signal has to somehow return to the source. If you are sending high speed signals any distance (makes no difference if you are talking about a PCB or a harness between PCB's) then you must make provision for the return path. If the return path is not in close physical proximity to the signal path then you are creating a large loop antenna area that will both radiate electrical noise and also pick up noise from other circuits.

Making sure this return path is always in close proximity to the signal trace (along with controlling trace impedance) is the main reason why high speed boards have ground planes. If you don't have a ground plane (not uncommon in DIY) then to maintain signal integrity you must route a return trace immediately alongside the signal trace to connect the two devices grounds directly together. Continue the return path when a signal leaves the PCB. Include a return wire in the harness to connect the transmitting devices ground to the receiving devices ground.

I would still use a star ground for power supplies. As long as each board has it's own direct, low impedance ground return to a central point then ground loops should not be a problem.

Professional sound reinforcement often has problems with ground loops caused by equipment on stage being powered from a different source (with a different ground potential) than the mixer console that can be located hundreds of feet away from the stage. The simplest solution to this is to run a grounded extension cord from the stage to the mixer and power the mixer from stage power. This does not cause a ground loop even though there are other ground connections between the stage gear and the mixer (shields on audio cables, etc.) because the grounds on the stage gear and the mixer ground all end up connecting to the same grounding point on stage.

Phil
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Old 23rd October 2002, 01:25 PM   #3
MWP is offline MWP  Australia
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Thanks for that...
I do understand the whole ground-loop problem.
Its just that i dont know if i have to run ground between boards to keep other (non groundloop) noise down.

So from what youve just said... if my boards are grounded well to the power supply, grounds between the signal boards (SPDIF in, DAC, headphone amp, etc) are not needed?
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Old 23rd October 2002, 04:19 PM   #4
weissi is offline weissi  Europe
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Default 2 or 1 ground plates?

Hi!
i actually do the same you mentioned...... just because I "like" to solder my boards to death, so itīs better if they are splitted... Or only to be flexible if I get some newer parts.

but I also have an essential question: should the pcb have ground plates on all two layers (of course with large amount of viaīs between them to reduce differential voltages) or should I only use one ground plate? for low frequency applicationīs thereīs no real need for an, but the DAC would love one(or two)? thatīs the next, splitted or not? would only one heavy ground plate be enough?
Since pcbīs are the most expensive of my project I donīt want to experiment with it....
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Old 23rd October 2002, 04:39 PM   #5
haldor is offline haldor  United States
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Quote:
Originally posted by MWP
So from what youve just said... if my boards are grounded well to the power supply, grounds between the signal boards (SPDIF in, DAC, headphone amp, etc) are not needed?
My previous explaination must have been poor. All high speed digital signals have to be accompanied by a signal return (ground) path that is in close physical proximity to the signal path from beginning to end. If you depend on the power supply star ground for the signal return then every signal wire will become an antenna and you will potentially have lots of problems with signals interferring with each other.

My comments regarding ground loops was just to say that if you have a star grounding scheme for the pwer supply connections to the PCB's then you don't need to worry about the extra signal ground connections between PCB's causing ground loops.

Phil
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Old 23rd October 2002, 05:12 PM   #6
sam9 is offline sam9  United States
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Default Try "Chaining" ?

MWP -- Have you considered laying out the boards so that you can "Chain" then. I.e. each board has a set of "In" terminals for power plus ground and a set of "Out" terminals. You run the leads from the power supply and system ground to the "In" terminals on board No.1, then from the "out" terminals on the same board to the "in" terminals on board No.2.

I've seen this done on a power amp so that there were separate boards for the input section, the VAS and the output section. Reason was the same as yours. The electrical effect was that the three boards together were equivalent to a single board. Though little goofy looking this mooted the concerns you have.

I would think it wise to keep the leads between boards as short as you can and use a guage of interconnect one grade thicker than usual.

Just a thought.
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