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difference between analog and digital gnd

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v_kumar

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help
can somebody tell me the difference between analog and digital gnd
wether we can short they or have to keep them seperate on the pcb.

thanx in advance
 

Digital ground is the return side of the signal loop for digital signals. Digital signals are mostly square wave, with fast rise and fall times. The characteristics of the signal waveshape and edge rates make the signal lines, including the gound return, a source of broadband noise.

Analog ground is the return side of the signal loop for analog signals. Analog signals depend on frequency, amplitude, and wave shape to transmit information. Music and voice are analog signals, for example.

If a digital signal is allowed to share a pathway with a digital signal, frequently the broadband noise from the digital signal will be induced into the analog signal path and cause noise and/or signal distortion.

Analog and digital signals can share the same ground plane - as long as the return paths for the two different types of signals aren't on the same part of the plane. Often, one side of the plane will be designated as analog, and the other as digital. No analog signal trace should be routed over the digital side of the plane, and no digital signal trace should be routed over the analog side of the plane.

Separate analog and digital gound planes can, and in many cases should, be connected together. The connection should be as close as possible to any common power source ground entry point. When analog and digital ground planes are connected together, it is essential that you look at the complete loop for each signal. Ensure that no signal return side has to flow through the common connection point to get to the opposite plane for return to its source. If signal current has to flow through the connecting via(s) to complete a circuit, you risk modulating other signals as they also try to go through the via - the result is noise and/or signal distortion.

There is a lot I have left out of the above; however, there is one common point that will keep you out of trouble. Always look at the complete signal path for your signals. Remember that power and ground lines carry the other half of the signal path represented by the trace or wire running between two points in a circuit. Every signal is a loop. Keep the return side of the loop out of noisy areas, just like any other signal path. Recognize that the loop is a real physical loop, and it will couple current just like any loop in any coil.
 

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