Electronics > PCB/EDA/CAD

star grounding vs ground plane for DC board

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ftransform:
So I have a dual layer board board consisting of OPAMPS, linear regulators, passives and an ADC only, the micro controller will be connected to this board via a cable.

The board is dealing with DC signals only. I managed to route everything on the top plane and I am wondering what kind of grounding system I should use for lowest noise.

Should I use the entire bottom layer as a big ground plane or should I try to implement star routing centered around the ADC ground? I read some people on audio forums saying that a star ground is better, but other people just recommend using a solid ground plane. I could implement the star routing using thick traces, like 50-100 mils.

Smokey:
One big thing to keep in mind is you want to keep high current stuff from one section of your board from flowing through your sensitive circuits.  For instance, you don't want the return from your power supply flowing through the same ground traces that feed your ADC and analog section.  Having a star ground system segregates the current paths of each section while keeping them approximately the same potential since they all meet a one point.  There are a lot of ways to do this, with some more practical than others.

The extent to which you need to keep ground current separate depends on how much resolution/accuracy your circuit is capable of.  If you are using an 8-bit ADC with no external reference, or your board is going inside a machine next to a giant switching power supply with no shielding, or in some cases where all currents are really low and signals are slow, then you probably would never be able to tell the difference.  Unless you know you are doing a high sensitivity design and have specifically planned ahead for your error budget and routed accordingly, then in my opinion, you can probably get away with a solid ground plane.  We aren't usually talking huge potential differences in a solid plane here unless the currents are really high or the signals are really fast or you are looking for super accuracy.

One easy thing you can do, which I guess is a variant on single point for the whole system star grounding, is have ground islands for your sensitive circuit sections.  Just keep all the analog and ADC components in their own local ground plane/pour just big enough for the section, and then connect that plane to the outer global ground plane that is surrounding it with just a single point connection of one trace.   Make sure the connection trace can handle the full current of your isolated section and is big enough to not have a significant resistance relative to that current.  That way anything high current in the global plane will presumably go around your island instead of through it, and all your ADC stuff will be at about the same potential.  That works pretty well and still allows you to make mostly solid planes.

This is one of those topics that has a lot of details.  Check out this app note.  It's dense, but full of good stuff.
http://www.analog.com/library/analogDialogue/archives/46-06/staying_well_grounded.pdf

Gall:
That's simple. Short answer: generally both are incorrect, and the choice depends upon your circuit.

Do not think about "ground". Think about signal return path. Every signal uses two wires.

On DC, think resistors. Every wire is a resistor. If the return current can choose the path, it chooses the shortest one. You cannot keep the resistance "small enough" but you can use separate wires for separate signals to minimize crosstalk. Or you even may use something like Kelvin connection between parts on the board to minimize resistance effect. In most cases this results in well-known "star grounding" but there are exceptions (more than one star and careful elimination of "ground loops"). A notable example is routing of a stereo amplifier board with RCA inputs; the input ALWAYS forms a ground loop, so at least three "grounds" are used to eliminate the problem, each having its own star and no direct connection with others.

On high frequency AC, think inductors. Every signal wire pair forms an inductor. If the return current can choose the path, it chooses the one that follows the forward path (forms a pair). To allow for path choosing, you will want to use a solid "ground plane". Or jist route everything in wire pairs.

Inbetween think both. Remember that a DC circuit may catch AC interference too. Wire pair routing theoretically works for every circuit but is virtually impossible in practical cases (mostly due to the loops caused by power supply connection), so a reasonable compromise should be found.

How sensitive is your circuit?

ftransform:

--- Quote from: Gall on March 18, 2013, 01:54:54 pm ---That's simple. Short answer: generally both are incorrect, and the choice depends upon your circuit.

Do not think about "ground". Think about signal return path. Every signal uses two wires.

On DC, think resistors. Every wire is a resistor. If the return current can choose the path, it chooses the shortest one. You cannot keep the resistance "small enough" but you can use separate wires for separate signals to minimize crosstalk. Or you even may use something like Kelvin connection between parts on the board to minimize resistance effect. In most cases this results in well-known "star grounding" but there are exceptions (more than one star and careful elimination of "ground loops"). A notable example is routing of a stereo amplifier board with RCA inputs; the input ALWAYS forms a ground loop, so at least three "grounds" are used to eliminate the problem, each having its own star and no direct connection with others.

On high frequency AC, think inductors. Every signal wire pair forms an inductor. If the return current can choose the path, it chooses the one that follows the forward path (forms a pair). To allow for path choosing, you will want to use a solid "ground plane". Or jist route everything in wire pairs.

Inbetween think both. Remember that a DC circuit may catch AC interference too. Wire pair routing theoretically works for every circuit but is virtually impossible in practical cases (mostly due to the loops caused by power supply connection), so a reasonable compromise should be found.

How sensitive is your circuit?

--- End quote ---

I want it to be as good as possible as it is for a 2 layer board !
My board is a home made ohm meter. which uses a current source and a unity gain differential amplifier connected to a 16 bit ADC with a kelvin connection to the DUT.
The voltages being read are from 0.2 to 4 volts.

Maybe I should have a ground plane for all my OPAMPS and reference and then do "star grounding" on my voltage regulators?
I tried to fit the regulators as close to the opamps as possible (they are low noise linear regulators)

Or should every OPAMP and its assosiated decoupling capacitors/filter capacitors have a ground leading the the ADC ground?

I guess consider it a thought experiment more then a practical project.  :-+

How is a ground handled in a 8.5 digit multimeter?  I just wanna push it to the limit.


And just to be clear, I read through that article but I felt that it did not go into specific about the actual analog grounding system, just about separating digital and analog grounds.
I guess I want more nitty gritty on just the analog section. Should I do a current measurements, see which parts are drawing the most current and then single those out (I.E. the differential amplifier is drawing alot so it should be separated from the inverting amplifiers ground, but since the voltage reference and low pass filter are drawing hardly any current they can be on the same plane? (THIS IS JUST EXAMPLE I HAVE NO IDEA HOW MUCH THESE PARTS DRAW)

Gall:
Here's the idea of how this could be done.

I intentionally use "worst-case" discrete opamps everywhere. In practice you'll probably want to use readily-made instrumentation amplifiers or even differential input ADCs. I also "forgot" to draw digital power circuit, obvious decoupling capacitors and local feedback of IC4A. Also extra capacitors in feedback circuit may be needed to ensure stability.

Here I use two star connections - C1 and C2. Each capacitor should be as small as possible so that it could be center of the star. Everything else is just Kelvin-connected. And if I draw two wires like a pair, they should be routed like a pair.

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