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Analog/Digital GND


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It is necessary in a layout which uses digital devices and analog devices to use a different voltage buss on the circuit board for each. These ground busses are then connected at one axis. This keeps the digital circuitry clean of the transients introduced by the analog circuitry.

MP

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This keeps the digital circuitry clean of the transients introduced by the analog circuitry.

Siddharth,
Actually, the opposite is true. When a digital circuit changes its output state from a "0" to a "1" and back, a current pulse is created in the ground wiring which causes voltage spikes (digital hash) along it. The digital circuits are not affected by reasonable (10% to 30% of the supply voltage) voltage spikes in the ground wiring due to their noise-immunity specification.
However, analog circuits (audio or video) do not have noise-immunity. If you supply an analog circuit with a digital ground wire then you will hear or see digital hash.
By separating the analog ground wiring from the digital, this keeps the analog circuitry clean of the transients introduced by the digital circuitry.
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audioguru, I work with them every day. It is not the opposite. It is both. I only mentioned the side that I work from. It can happen both ways, which is why they are separated.
Add a motor to a microprocessor circuit without the separation and tell me that the microprocessor can handle it. Not.

MP

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Point taken. Not a good example but made the explanation easier. The difference is this: In analog, you could add a little filtering to the board to get rid of any digital glitching since it is all built around a specific clock frequency of the on-board electronics. Many do not know this. You can just find the clocking frequency and tune it out. On the other hand, any of the many differences that can be caused to the voltage buss from analog circuitry cannot be compensated by a simple filter circuit for a specific frequency on the digital buss. There is no specific frequency. Since the items are analog, they make an infinite number of changes to the voltage buss. A simple microprocessor circuit with brown out protection will continuously go into reset from activity on the analog side without it's own voltage/gnd buss.

Kevin: Yes, all circuitry uses the same ground, but where it is connected makes a difference. You use different ground planes for analog and digital, then you have one connection to tie the two together.


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