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ads7828 Analog Ground

I am trying to get some of the noise out of my ads7828 pcb, and have been confused by the datasheet. The particular point of confusion concerns the AGND pin, described as analog ground in the pin description. There does not seem to be a digital ground pin. The datasheet also suggests tying the A0/A1 pins directly to digital ground. A diagram then depicts the A0/A1 pins being connected directly to the AGND pin. My PCB ground plane is currently split with an opening. The analog side contains pins 1-11. I include 11 because that is the COM pin for the eight analog inputs. If I were to connect the ends of the splits with a line, it would pass between pins 11 and 12. So the AGND pin is located on the analog side. Should I take AGND directly to the ground plane at the pin, or would it attract digital signals which would then go past COM increasing noise? Should I make a lead going from AGND into the digital section and via it to the dig grnd plane? Any suggestions would be great. Thanks

Russell

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russell
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The A to D converters grounds want to be the same ground. You can bring the pins directly to ground and tie the ground planes together under the chip. Better would be to place the entire converter on the digital plane and bring both the signal and the analog ground in differentially. You just treat the analog ground as one of the signals.

Paul C

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PaulCsouls

Hello Russell,

Ok, here is one. I have never seen split ground systems like that work. And it's been a good 20 years now. Try to link the two planes together at as many places as you can, especially around the ADC. But no long pigtails here. Use wide braid or copper tape. None of the links should be more than 1/2 inch long. That is the way I was able to convince the most skeptical clients not to use split grounds. It also won me quite a few bets when in college which usually meant free booze for me that night.

Regards, Joerg

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Joerg

I would agree with Joerg

My technique when I have analogue and digital grounds on the same device is to take them direct to a plane layer, but put a small void in the plane such that the analogue and digital ground paths can not meet under the analogue section of the device.

Same ground plane, just a little current steering.

Cheers

PeteS

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PeteS

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