How does ground loop voltage change with ground loop impedance?

[Farad22]

How does ground loop voltage changes with ground loop impedance

Hi,

I have a question about ground loops. One thing I don't understand is how an "isolation" resistor can eliminate ground loop problems. Take the following setup:



If a certain voltage (Vg) is induced in the ground loop (for example by 50Hz induction) then the loop current (Ig) is determined by induced voltage and the resistance of the loop, according to Ohm's law. Let's say this induced voltage is 1V. If we only have a ground loop wire impedance of 0.1 Ohm then the ground loop current is 10A. If we put in an isolation resistor (10 Ohms) then the current will be 1V/(10+0.1 Ohms) = 0.1A. So the current in the loop can be reduced, but the induced voltage is the same in both situations. If the ground loop noise voltage (Vg) stays the same then what difference does the isolation resistor makes?

Thanks

 

[Farad22]

Re: How does ground loop voltage changes with ground loop impedance

Anyone has any idea? According to Faraday's law a voltage is induced proportional to the loop inductance L and the rate of change of the magnetic flux. So the induced voltage is apparently not a function of loop impedance. So changing loop impedance (with isolation resistor) would only change the ground loop current and not the induced voltage in the ground loop, right?

 

[FvM]

Re: How does ground loop voltage changes with ground loop impedance

So changing loop impedance (with isolation resistor) would only change the ground loop current and not the induced voltage in the ground loop, right?

Yes, did someone claim anything different?

To get rid of the ground voltage drop interference, you'll either use differential signaling or a constant current driver.

 

[schmitt trigger]

Re: How does ground loop voltage changes with ground loop impedance

That is the reason you have common mode voltage rejection specs.

I remember doing tests, back in the days where balanced audio meant coupling transformers, that with high quality transformers one could have 120VAC difference between two connected audio systems and the hum would be barely noticeable.

 

[mtwieg]

Re: How does ground loop voltage changes with ground loop impedance

It usually won't decrease the total induced voltage, but it will force most of the voltage drop to be at a specific point in the circuit. That way it can be dealt with by using differential signaling or isolators at that one location, instead of all over the place. The worst case is when the total impedance is so low that the induced emf is distributed throughout your circuit ground reference.

And sometimes the ground loop current itself matters, and must be attenuated with increased loop impedance, even if this does nothing for the induced voltage.

 

[Farad22]

Re: How does ground loop voltage changes with ground loop impedance

Thanks guys, your answers made it clear now!