Author Topic: Oscilloscope Ground  (Read 2512 times)

0 Members and 1 Guest are viewing this topic.

Offline ABCD

  • Contributor
  • Posts: 23
Oscilloscope Ground
« on: August 05, 2011, 10:14:45 AM »
My Hitachi V-222 scope has a ground jack on the front panel.  When would I use this?  Right now, my electronics work is all DC on breadboards with a power from an AC adapter (if you're working through the Make: Electronics book, it is the power plug with multiple voltage settings).

The experiment I was working on is the one where the PUT charges a cap which will then discharge into an LED, making it flash.  When I connect the scope's probe to the cap to measure its charge/discharge curve, the LED stays on and the voltage stays constant.  I am guessing that it's something to do with grounding, but I'm not sure what exactly is going on.

alm

  • Guest
Re: Oscilloscope Ground
« Reply #1 on: August 06, 2011, 06:22:28 AM »
Check the manual of the scope. It's probably for connecting an anti-static wrist strap or anti-static work mat. It's usually not a safety ground, these tend to be on the back.

The issue with the LED staying lit sounds like leakage through the EMI filter caps to ground through the LED. Try a power supply that's properly isolated from mains or a battery (guaranteed practically zero leakage to ground) to verify.

Offline Bored@Work

  • Super Contributor
  • ***
  • Posts: 3940
  • Country: 00
Re: Oscilloscope Ground
« Reply #2 on: August 06, 2011, 07:08:58 AM »
It is a convenience thing. If you need to string together multiple ground connectors from several instruments and from parts of your DUT then the 4mm banana socket on the front makes it easier to connect the oscilloscope, too. You don't have to crawl behind the instrument and look for a ground screw.

However, and that is a big however, this is no substitute for properly connecting the ground on your probe to the DUT, especially not for high-frequency signals. It is also no proper substitute for any safety feature. The oscilloscope itself should be properly connected to PE, independent of the use of that socket. Further, with a bunch of 4mm banana test leads it is easy to set up all sorts of funny ground loops and build undesired antennas.
I delete PMs unread. If you have something to say, say it in public.
For all else: Profile->[Modify Profile]Buddies/Ignore List->Edit Ignore List

Offline ABCD

  • Contributor
  • Posts: 23
Re: Oscilloscope Ground
« Reply #3 on: August 06, 2011, 10:17:58 AM »
It is a convenience thing. If you need to string together multiple ground connectors from several instruments and from parts of your DUT then the 4mm banana socket on the front makes it easier to connect the oscilloscope, too. You don't have to crawl behind the instrument and look for a ground screw.

When would I want to do this?  (I'm asking out of ignorance.)

Quote
However, and that is a big however, this is no substitute for properly connecting the ground on your probe to the DUT, especially not for high-frequency signals. It is also no proper substitute for any safety feature. The oscilloscope itself should be properly connected to PE, independent of the use of that socket. Further, with a bunch of 4mm banana test leads it is easy to set up all sorts of funny ground loops and build undesired antennas.

PE?  The oscilloscope itself is grounded to mains, and the breadboard (I assume) is grounded via the wall adapter.  I do connect the ground clip to ground and put the probe tip on the pin I'm trying to measure.  So I think I have things grounded the right way.

Offline Bored@Work

  • Super Contributor
  • ***
  • Posts: 3940
  • Country: 00
Re: Oscilloscope Ground
« Reply #4 on: August 06, 2011, 06:03:06 PM »
Last time I needed to do it was when I had a SMPS in the setup. The SMPS powered the DUT with DC. It was one of those SMPS where the designer went the cheap and common route to reduce EMI by having a Y cap between the primary and secondary site.

Those Y caps are legal (if done right), but they couple the secondary site to mains. You get some high-impedance AC coupled into the system, which also depends on which site (live or neutral) the Y cap happens to be connected inside the SMPS. And the site can change, depending which way you plug in the europlug.

Thanks to the Y cap I had something like 86 Vrms towards ground at some point in the system. Properly grounding the secondary site of the SMPS and, while at it, also connecting the ground sockets of all instruments to that point made it go away.
I delete PMs unread. If you have something to say, say it in public.
For all else: Profile->[Modify Profile]Buddies/Ignore List->Edit Ignore List

Offline ciccio

  • Frequent Contributor
  • **
  • Posts: 359
  • Country: it
  • Designing analog audio since 1977
    • Oberon Electrophysics
Re: Oscilloscope Ground
« Reply #5 on: August 06, 2011, 07:18:01 PM »
It is a convenience thing. If you need to string together multiple ground connectors from several instruments and from parts of your DUT then the 4mm banana socket on the front makes it easier to connect the oscilloscope, too. You don't have to crawl behind the instrument and look for a ground screw.

When would I want to do this?  (I'm asking out of ignorance.)


I use it often: it is more practical, for fast debugging or for operational check, to connect the oscilloscope ground to a suitable ground point in the DUT (device under test) with a banana to alligator cable and leave it connected, while searching other test points with the scope probe point, without need to move also the probe ground.  Think about debugging a large board, such a PC motherboard: the probe's ground cable is short, and it will be an inconvenience to move it from one test point to the others, and sometime there is no practical ground point near the test point. You can also remove the ground cable from the probe, and you'll reduce the risk of inadvertently shorting something to ground.
If the rise time of the signal is low, sometimes you get a non correct waveform, or maybe you see more noise, but it is for debugging purposes only...
Ciccio

Strenua Nos Exercet Inertia

Offline ejeffrey

  • Super Contributor
  • ***
  • Posts: 1365
  • Country: us
Re: Oscilloscope Ground
« Reply #6 on: August 06, 2011, 07:41:29 PM »
Last time I needed to do it was when I had a SMPS in the setup. The SMPS powered the DUT with DC. It was one of those SMPS where the designer went the cheap and common route to reduce EMI by having a Y cap between the primary and secondary site.

Ugh.  I hate these.  I have seen devices with 5 milliamps of AC current flowing through the Y caps to ground.  Even with more sensible power supplies if you have many instruments with 100s of microamps or a milliamp of leakage you can quickly get 10s of milliamp of ground current running every which way to wreak havoc on any sort of low noise or high impedance measurement system.


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf