RE: Analog and Digital Ground Pins

gervasi — Mon, 07 May 2012 22:14:36 GMT

I agree with DAB's comments.

My main point is it's tempting to want to avoid a good signal path on the board between analog and digital ground pins, trying to keep the noise confined to the digital side. This temptation is completely wrong. Paradoxically you want to connect those pins to the same plane. The plane can split off from there, but it should come together at the ADC.

The worst case I've seen was board with separate analog and digital grounds completely isolated. AGND and DGND pins on the ADC are not isolated, so the ADC was the only path for currents to flow between them. This caused various failure modes depending on the temperature.

Tie and analog and digital grounds to the same ground plane using separate vias.

RE: Analog and Digital Ground Pins

DAB — Mon, 07 May 2012 20:47:29 GMT

Plus use decouping capacitors and opto-isolators everywhere. Make sure you have a huge amount of ground areas under the circuits.

It is still amazing to me that what I learned was analog levels are now considered digital frequencies and not RF.

Back to the post, depending upon your operating frequencies, you can get mini-antennas develop from your components. Why do we take all of this trouble? Noise, Noise, Noise. When you are trying to detect low level signals, the last thing you need is a bunch of RF noise coming in from your analog or digital circuitry. Sometimes you spend a lot of board space building filters just to get a passable performance.

I have looked at some new research where they use fractal patterns on the circuit boards to break up the straight lines and sends to noise into higher frequencies, which are usually easier to filter out.

As you get into more sophisticated circuits, you will find that keeping the Digital and Analog sectioins separate is just good practice. Take a look inside an old VCR. You will see metal cages around the high frequency areas. They went to that length just to keep the RF noise down. Learn from their efforts and you will see how to keep the cross talk subdued.

Just a thought,

DAB