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Analog vs Digital Ground Planes

Posted on November 14, 2012

It is said that analog and digital ground lands should be connected together at just one loaction on a PCB. THis makes eminent sends

because it prevents AC currents in the digital ground land from flowing through the analog ground land..

[By the way, I call them lands rather than planes because they will be side by side - not one over the other. In fact, sandwitching the

analog and digital ground planes would undesireably cause digital ground plane AC voltages (yes, AC currents in the plane will cause

some AC voltages) to capacitively couple to the analog plane, and there could be magnetic coupling as well.]

For this separation of analog and digital ground lands to work, there must be no other ground loops. FOr example if several boards

plug into a backplane, it would create loops, if the board digital ground lands connect to a digital backplane ground and the analog

board ground lands connect to an analog backplane ground. this is because there is a connection between the analog and digital

grounds on each board thus many loops.

Also, I am always cautious when an interface chip (llike an A to D converter for example) has separate analog and digital ground

pins. These pins are suplsed be tied to the analog and digital board grounds respectively, but how much AC voltage can the chip

tolerate between them?

To minimize all the above effects what I do is the following:

On a given board, I locate the interface chips in a small area between the analog and digital sections. I use separate side-by-side

analog and digital ground lands, with 1/4 inch b etween, and I connect the two ground lands together with a wide land (maybe an inch

or two wide. Assuming a 10 x 10 board.

This connecting land goes under the interface chips, thus preventing significant AC voltages between the chips analog and digital

ground pins. Also I route all signals between the analog and digital sections over this connecting land. this provides transmission line

impedance continuity for single-ended signals, and also helps to avoid crosstalk even among differential pairs.

Where supply traces cross between the analog and digital sections, I either route them over the connecting land or I place series

inductors in them, over the gap between analog and digital lands.

And very importantly, if I have a system with multiple boards in a backplane, I lay out the board connecting lands to be near the board

edge that connects to the backplane. This way the backplane hca have a single ground plane that connects to the connecting land

rather than to wither the analog or digiral ground lands. A simple way to avoid loops!

Larry Miller

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