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I wanted to ask if there is a difference between 2 scenarios on the illustration i've included. If we have 2 or more boards interfacing with each other should we connect every output with it's own signal return path or can we simply ground every board connecting it to a common point and carry the signal with single wires?

It's much more convenient to do it like in scenario B, but i've seen a lot of PCBs where every input/output was a pair of pads: "hot" pad and a grounding pad. Why can't we just use one pad to ground a pcb and use only "hot" wires?

grounding

Voltage Spike
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cubix
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  • It is relevant to mention what such signals "are". As in, are these simple digital gpios, or are they high-speed comm interfaces, or perhaps analog signals, etc. If they are just low-frequency digital IOs, scenario B should be fine. Distance between boards may also play a part in the final decision. – Vicente Cunha Jul 29 '19 at 17:32
  • depends on the speed. If 1nanosecond edges and 1 inch wires, you may be OK. If 1nanosecond edges and 12 inch wires, you need either twisted pairs or coaxes, or those flat grey cables with alternating signal/ground/signal/ground.....and some terminiations. – analogsystemsrf Jul 29 '19 at 17:34
  • i'm talking about analog signals up to 20kHz (audio application) – cubix Jul 29 '19 at 17:35
  • i found this: https://electronics.stackexchange.com/questions/37578/sending-ground-along-with-digital-connections-between-boards?rq=1 the answer closes the discussion i guess – cubix Jul 29 '19 at 18:20

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It depends. Scenario B is effective and commonly used for many situations.

Scenario A may be needed when the signals have high frequency content, or when there is a special need to achieve low cross-talk.

What frequencies are "high" depends on the length of the cable.

The Photon
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  • so more crosstalk occurs when signals have common return path, right? – cubix Jul 29 '19 at 17:33
  • Yes, and when the loops formed by the data and return wires overlap. – The Photon Jul 29 '19 at 17:33
  • thank you! and how can i estimate what frequencies are "high" in my situation? – cubix Jul 29 '19 at 17:34
  • You'd need to calculate the mutual inductance between the loops and see if it is significant at your frequency. For your 20 kHz signals, cross-talk due to return wire resistance might become significant before inductive cross-talk does. – The Photon Jul 29 '19 at 17:49