Ground loops
Ground loops
(OP)
I have a circuit where three amplifiers end up in a ground loop with the power supplies. An interesting thing is that when I pull 5A through my power leads and get a 100mV ground 'bounce', my amplifiers outputs shift by 200mV. I expect the PSRR would have kept this at the same 'bounce' voltage.
Note that I'm using 'bounce' loosely here, as this is a DC effect.
The three amps are independent driving three separate loads. One has a -3X gain, and the other are 1X and they all shift by the same amount (+/-20%).
Can you suggest any other parameters I should check?
John D
Note that I'm using 'bounce' loosely here, as this is a DC effect.
The three amps are independent driving three separate loads. One has a -3X gain, and the other are 1X and they all shift by the same amount (+/-20%).
Can you suggest any other parameters I should check?
John D





RE: Ground loops
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If we learn from our mistakes I'm getting a great education!
RE: Ground loops
The second page shows the amplifiers. I simplified it to only the output stages as the effect is the same with these inputs grounded.
John D
RE: Ground loops
If that didn't improve things, then I'd put on the thinking cap.
RE: Ground loops
RE: Ground loops
Glenn
RE: Ground loops
Don't equate the inherent CMRR of the IC as being that of the circuit. Component tolerances quickly degrade the CMRR of the overall circuit unless you're really careful.
VE1BLL has identified the easiest starting point - a star ground will at least eliminate interaction between circuit blocks and keep high currents out of the signal ground path. Do that and then see what happens.
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If we learn from our mistakes I'm getting a great education!
RE: Ground loops
Ditto.... Even on a large system having long cables, the above concept was the best and simplest of several fix attempts tried.
In this case it was AWG #6 green wire bonding that ran parallel all unbalanced audio cables.. Took out all discernable ground loop noise.
RE: Ground loops
Thanks for the ideas all. In hindsight differential signals would have been great for this circuit.
This ground loop has worked acceptably for multiple systems. Now I have a single system that is showing the problem. Interestingly enough it acted up last May too. It seems environmental effects but this system was on loan to a customer in Japan this May; last May it was in North America.
As a quick band-aid I'll try shifting the #22 ground wire to the other end of the #18 so my A/D and the sensor electronics will all ride together on the ground noise. That still doesn't get to the root cause but might let me ingore it for a while longer :S
John D
RE: Ground loops
In general, we would never design a system with ground loops. We would explicitly make sure that the subsystems and systems have single point grounds. And we would not allow any power supply return current to share a conductor with any signal return. Not to say mistakes aren't made, but these issues are covered by explicit design rules.
Good luck. Road trip to Japan...?
RE: Ground loops
The solution for most industrial applications is to use fiber optical cable.
RE: Ground loops
Thanks to everybody for your help!
John D