Ground Channel Coupling

[jj5]

I'm reading the manual for my new Rigol MSO5074 scope (this is my first scope and I'm just starting to learn what it's capable of and how to use it). On page 59 the manual explains three supported channel couplings:

• When the coupling mode is "DC": the DC and AC components of the signal under test can both pass the channel.
• When the coupling mode is "AC": the DC components of the signal under test are blocked.
• When the coupling mode is "GND", the DC and AC components of the signal under test are both blocked.

I don't understand GND channel coupling. If you block both the DC and AC components, what's left?

[ledtester]

GND coupling just means that the scope input is connected to ground. You should just get a straight line on your scope.

On analog scopes I think this was useful so you could tell where on the screen where the ground of an input was set to. On digital scopes there is a little marker on the side which shows that.

[jj5]

GND coupling just means that the input is connected to ground. You should just get a straight line on your scope.

Thanks. So that straight line will be at zero volts? What's the point of having ground coupling? I don't understand when it would be useful.

[AnalogTodd]

When you set the channel for GND coupling, it does just that--it grounds the channel. The idea is that you will know exactly where the ground potential for the channel is on the scope. It is very useful when you are trying to set where the zero potential for that channel is. You don't always want zero to be the center line of the scope; sometimes you want it to be a number of divisions up or down so you can utilize more of the screen to view your signals.

[jj5]

To be clear: if you use ground channel coupling to get a reference for ground on the scope, there's no point in attaching a probe to that channel?

[TimFox]

If a probe or other cable is connected to the channel that is set to "GND" coupling, its signal will be ignored.
When you set that channel to "AC" or "DC", the signal will be displayed.
With analog CROs, it was a common practice to set the channel to GND, move the vertical position control to get "0" at the desired location (typically center line or bottom line of the screen), then switch back to AC or DC coupling.
Note that on both CROs and DSOs, "GND" input setting does not short the input BNC to ground, so you won't short out the cable to your device under test.