Grounding Rule of Thumb

[danner123]

Hi everyone. I was wondering if anyone had any good documentation on common grounding between different boards?

Basically, I am looking for a rule of thumb on grounding between different circuit boards. I have heard a little bit about the star method but it can be a bit confusing. Are there any good articles out there on how to efficiently ground?

Thanks!

 

[chuckey]

Grounding is a bit of a black art. There are two basic aims, one is to prevent huge current variations flowing through a PCB carrying low level signals and the second is to remove or shorten unwanted inductive leads in between high frequency circuit.
Take something like a high power audio amplifier running from the mains, it will have a reservoir capacitor in it that could be carrying 10A of AC current, it would be very foolish to run its earth wire back via the input amplifier board as 1mOhm of resistance would drop 10 mV of AC which would be comparable to the input signal. So you wire all the high power power supply circuits up and on to the high power amplifier all in thick wire. Some where in the high power amplifier it would be returned to the physical earth. Could even be at the input socket. if you earth everything to the chassis everywhere, you are at risk of forming "hum loops", when high currents flow, they can actually induce spurious voltages in adjacent wiring.
Frank

 

[Dan Mills]

Rules of thumb:

1: You are going to need to prototype it....
2: Currents flow in loops (Always), minimise loop areas to minimise coupling (Lowest inductance).
3: At anything more then a few kHz inductance usually dominates so current will, if given the choice, flow to minimise loop area. Use this with ground planes on your boards.
4: Any board containing internal single ended circuits (as in non differential) should have its reference plane connected to chassis at ONE point.
5: Differential IO is your friend, and if the differential pair is screened the screen goes directly to chassis via the shortest possible connection, never to the single ended reference plane, making all off board connections differential is a good plan.
6: Common impedance coupling is the enemy, draw your complete circuit (Including the ground nets) and verify that for critical sections the input current loop does not share significant impedaces with the output current loop if this is carrying meaingful power.
7: Opamps are 5 terminal devices, not 3 terminal ones, note well what the output current loop returns to (If you think it is the power supply you are probably doing it wrong).
8: Star points are old school audio stupidity, worked back when local rf sources were usually nonexistent, works less well today, planes and differential signalling are a much better idea.

Regards, Dan.