Author Topic: Why does oscilloscope reads voltage even when prove ground is not connected?  (Read 2165 times)

0 Members and 1 Guest are viewing this topic.

Offline Rene

  • Regular Contributor
  • *
  • Posts: 67
Hello,

Stupid question.... Why is it that if I connect my oscilloscope to my signal generator using nothing but the hot leads I get a perfect voltage readout in the oscilloscope? (Please see attached picture for connection example).

I would have expected that it would be necessary to connect the ground lead to be able to read the voltage, after all, I am assuming that the signal generator signals (AC Signal in this case) is floating so there should be no ground reference right?

Thanks.
 

Offline c4757p

  • Super Contributor
  • ***
  • Posts: 7805
  • Country: us
  • adieu
Well, there still is a voltage there, it's just not referenced to anything. You can still read it, it might just not be a useful reading.

It's completely possible that it's referenced to ground through some other connection. Some signal generators have ground-referenced outputs just like the scopes have ground-referenced inputs.

Now, that means that the ground current has to travel all the way up the instrument's power cable, to the wall outlet and down the other instrument's power cable - meaning it's not a solid ground connection, and at higher frequencies it won't work. That's why you need a close, low-inductance ground connection directly at the probe tip for accurate probing.
No longer active here - try the IRC channel if you just can't be without me :)
 

Offline jpb

  • Super Contributor
  • ***
  • Posts: 1622
  • Country: gb
Signal generators vary. Some, like the Agilent ones, are floating whilst others like Sigilent (I think) are referenced to earth ground.

If both your generator and your scope are referenced to earth ground (via the mains connector) then they will both be referenced to the same point.
 

Offline AG6QR

  • Frequent Contributor
  • **
  • Posts: 804
  • Country: us
    • AG6QR Blog
Check with a multimeter.  You may find a very low ohms connection between those two disconnected ground leads in your photo.  As others have said, it's standard for an oscilloscope's shield to be connected to the mains earth ground, and it's not uncommon for signal generators to be wired likewise.

If the ground is connected through mains, it may be fairly low impedance at DC and low frequencies, but it'll be much higher impedance at higher frequencies.  You should still connect the ground leads if you need accurate waveform display as the frequency gets high.
 

Offline Rene

  • Regular Contributor
  • *
  • Posts: 67
Check with a multimeter.  You may find a very low ohms connection between those two disconnected ground leads in your photo. 

You guys are right, I checked for continuity and it showed that the ground cable of the signal generators are connected to ground and not floating... I am actually a little embarrassed that I did't think of testing that myself  :-[

I am not sure if I like this... I would much rather have a floating signal generator output since is one less thing to worry about. Oh well.

Thanks.
 

Offline Siglent America

  • Regular Contributor
  • *
  • Posts: 221
  • Country: us
Signal generators vary. Some, like the Agilent ones, are floating whilst others like Sigilent (I think) are referenced to earth ground.

If both your generator and your scope are referenced to earth ground (via the mains connector) then they will both be referenced to the same point.

Yes, the Siglent SDG800 and SDG1000 families do not have floating grounds but the SDG5000 series does have a floating ground.

Steve
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf