Concept of multiple grounds and low impedance return path

[fala]

return path low impedance

Hello, In many circuits for lowering noise it has been advised to use multiple grounds, one for each segment of board and then connect them together at power supply ground. One problem rises with this scheme and that is some signals have to go from one segment to another segment, though usually this signal lines carry tiny amount of current but it also has been advised that ground return path of each line should not be discontinued anywhere. Can this be a problem, if so how should it be resolved or it can be overlooked because of tiny amount of current that signal lines carry?
Thanks

 

[IanP]

impedance return path

My gut feeling tells me this:

-if blocks are "independent" ie. don't send signals to each other, you can implenet this techniue ..

-if block send signals to each other the best way to go is a ground plane which provides a solid ground reference plane with the lowest possible impedance and shortest possible return paths for all signals ..

Regards,
IanP

 

[fala]

Thanks IanP, yes blocks are independent but inevitably some signals should be exchanged, I usually do this via differential signaling ie. Ground of one segment + output of one opamp will go to another segment and there they will be translated via a differential amplifier to a signal in reference to that segment’s ground. as you see because input impedance of diff amps are very high the resulting current is in matter of nA and sometimes uA, so amount of current is really tiny. Can this still pose a problem? You may say well you can merge this segments but I'm a bit reluctant to do that because for example in one segment I have purely sensitive analog signals(some of them high impedance unbuffered inputs) and in another segment I have DAC which inevitably has some latches, although latches are carefully decoupled but I do not like to place my sensitive quiet analog signals in vicinity of a noisy neighborhood of a latch so I do transfer buffered signal via differential signaling to that neighborhood. Do you think It would be better if I would merge these segments?
Thank you very much

PS. The above scenario may not seem very serious because the signals have been transferred along with their return path to another segment so let me add another scenario in which I have analog switches, analog switches have to switch sensitive analog signals but they have to be controlled by a digital signal so analog switches have been place in analog zone but digital signals that come from a digital zone will intrude analog zone at some point at this point their digital ground are discontinued. Although they carry tiny amount of current but can this pose a problem?

 

[IanP]

Re: Concept of multiple grounds and low impedance return pat

Working with small signals and preventing-shielding them from external noise(s) can be an art in itself ..
Sometimes it pays off to use optoisolators, and by this I mean both digital (switches) and analog (iso-opams) ..

Regards,
IanP