Grounding a scope

[antknee]

I like to think I'm quite competent at using a scope after getting my first around 3 months ago. The one thing I've never been able to learn is how to ground the scope. So I've never been certain whether my signals were above or below ground. It just didn't matter until now.

The scope I'm using is handheld so I have a button I can press and hold for grounding. But when am I supposed to do that? Do I attach both terminals of the scope to the ground and then hold the button? Why doesn't the scope just take zero as ground?

Thanks.

 

[JimB]

I like to think I'm quite competent at using a scope after getting my first around 3 months ago. The one thing I've never been able to learn is how to ground the scope. So I've never been certain whether my signals were above or below ground. It just didn't matter until now.

I had the awfull thought that you were abaout to ask about grounding the case of a mains powered scope!
Just dont go there!
Arguments as hot as ohms law!

The scope I'm using is handheld so I have a button I can press and hold for grounding. But when am I supposed to do that? Do I attach both terminals of the scope to the ground and then hold the button?

What does the user manual say about it?
At a quick guess the button just connects the probe tip to the ground clip so that you can set the trace to a convenient reference point on the graticule.

Why doesn't the scope just take zero as ground?

What is "ground" and what is "zero" ?

The trace on the scope displays the voltage difference between the probe tip and the "ground clip".

Where you connect the ground clip is up to you, bearing in mind what that ground clip may be connected to inside the scope.
If it connects to a real earth via the scope mains cable, then don't connect it to any point with voltage on it.

Your question is not all that clear, can you explain a bit better please?

JimB

 

[antknee]

Well in the past I've known whether I had a signal above or below zero before I used the scope. Now I need to test whether I have a sine wave staying above zero volts or alternating above and below zero volts.

I guess the heart of the problem is that I'm used to zero and ground being the same - zero. And of course they aren't. The user manual doesn't actually mention grounding at all. It says 'there is a ground button and points at it', they leave the concept for the reader to work out.

I think I have hold of the concept now. Attach both ground clip and probe to my reference point, for example ground. Press ground button, now leave the clip where it is and use the probe to test...

 

[Nigel Goodwin]

I suspect you're simply confusing yourself - there's no such thing as 'ground' on a portable scope (and in my case not on a mains scope either).

I presume all the button does is short the input to the common/ground connection, that's all. It's so you can adjust the reference to where you want on the screen, without been confused by any input. So connect the 'ground' lead to the chassis of the unit under test, and the probe to where you're measuring - then press the ground button and set the reference point on the screen.

All my mains powered scopes have a three way switch, AC, DC, and Ground - so in that case you set it to Ground to set the line in position, then back to DC (not AC) to measure the DC offset.

 

[antknee]

Hi Nigel, yes I just did it, thanks! I grounded the signal and set it to DC offset. I can now see that my signal is quite visibly and correctly a square wave going from 0 to 4V and riding above the maker line I have. That is exactly what I wanted.