do probes have common ground !! --- DS1052E - Page 1
Forum section: electronics / beginners

author: pheonix
author_rank: Newbie
timestamp: October 23, 2011, 01:24:17 pm
content: Hi guys,I just bought a DS1052E and a newbie to oscilloscope.... still in process of understanding the functionalities.Today I was just testing a circuit and noticed if I have the probes attached to same point in circuit and one probe with ground attached (and other not) both channels were displaying exactly same waveform. Do both probes have common ground !! I wasn't expecting any waveform from a channel which hasn't got ground attached.Also, the resistance (with oscilloscope switched off) between the ground clips of the probes is zero !!I am not sure if I messed up some setting or there is some problem with oscilloscope/probes !Any suggestions please !!cheers

author: Unknown
author_rank: Unknown
timestamp: October 23, 2011, 01:29:30 pm
content: In 95% of oscilloscopes, ground is common between all channels, and connected to safety ground on the mains plug. The reason why individual ground leads are important is that the impedance of this ground lead at higher frequencies is high due to inductance, so signal fidelity will be reduced. You tend to see ringing after fast edges if you don't have a proper ground connection.

author: amspire
author_rank: Super Contributor
timestamp: October 23, 2011, 01:36:39 pm
content: Yes, nearly all oscilloscopes have a common ground for the probes. There are a few specialty scopes that do not, such as ones with fully differential inputs for working with mains single and three phase circuits.In spite of this, the better each probe tip is grounded near the signal source, the more accurate the waveform will be at high frequencies.It is something you have to watch our for - if you are testing some hardware that has the 0 Volts connected to mains ground, then you cannot clip your oscilloscope ground lead to any part of the circuit that wasn't also grounded.Richard

author: pheonix
author_rank: Newbie
timestamp: October 23, 2011, 02:27:22 pm
content: Thanks for a quick reply alm and richard,I am just testing couple of simple ac-dc rectifier circuits ... 5v, 9v, 12v, and variableI am trying to compare input/output wave and connecting one probe (with ground) at the transformer's output and other (with ground) at the output after rectification.As in this scenario if I am connecting ground clips to input and output of the circuit, I am actually short-circuiting the 2 grounds !!Code:/ Probe1| |--------| / Probe2)-----| |---------- +) ~ | ac-dc |)-----| |---------- -) | |--------| \ Probe2 gnd\ Probe1 gndIs there any way to see input/output waves simultaneously on a oscilloscope !!

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 23, 2011, 06:59:55 pm
content: dont do my mistake. i clipped the ground probe on some +V potential track.

author: pheonix
author_rank: Newbie
timestamp: October 24, 2011, 10:10:00 am
content: Isn't ground just a reference point.. and reference could be +v, -v .. etc !!Or am I mixing Voltmeter and Oscilloscope!I wonder how some multi-channel oscilloscope is *really* helpful if we can't see waveforms in reference (ground) to two different points in a circuit.lets say I want to see waveform across two different components of the circuit !!Isn't oscilloscope not more than a device which shows voltage (and off course showing other parameters... vpp,freq,etc) in the form of a graph !!

author: elliott
author_rank: Regular Contributor
timestamp: October 24, 2011, 10:35:46 am
content: Quote from: pheonix on October 24, 2011, 10:10:00 amThat is what the math functions are for.

author: amspire
author_rank: Super Contributor
timestamp: October 24, 2011, 10:52:53 am
content: Quote from: pheonix on October 24, 2011, 10:10:00 amThe ground on an oscilloscope is not just a reference point, it is hard wired to the ground pin on the power connector on the back.Now if the circuit you are testing is totally floating, then in theory you can connect the ground anywhere.But if the circuit you are testing is also connected to earth ground, or it connects via cables to other grounded devices, then you can only connect ground to ground.Now most of the time you get perfectly useful waveforms in spite of this.When you do need to see the waveform across a component, you can use the scope in differential mode, where channel 2 is inverted, and the two channels are added together. This means the scope only shows the difference. The Rigol can do this, but you have to be careful that a channel is not limiting, or that the difference is not so small that there are inadequate bits of resolution to be useful.Better still, differential probes are available designed for just this job. these perform much better then subtracting the channels. Unfortunately they are not cheap. They usually cost much more then the Rigol scope.Richard

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 24, 2011, 11:11:40 am
content: Quote from: pheonix on October 24, 2011, 10:10:00 amyes its true until your are probing circuit connected to mains ground such as PC usb device. or until you are probing with 2 channel 1 ground is at circuit ground and 2nd ground is at +V track. you will be lucky if you dont short something.Quote from: pheonix on October 24, 2011, 10:10:00 amdifferential probe! search differential probe! you will figure out how lucky you are with your $300 dso.edit: to relieve some of your pain, get a DMM with continuity check. connect between the 2 probe's ground or to/between mains ground in power socket outlet. either case you'll hear a nice continuous beep. that should clear things up.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 24, 2011, 11:20:47 am
content: Quote from: elliott on October 24, 2011, 10:35:46 ammaybe i translated OP msg incorrectly. 2 channel only able to math the diff V of ONE component. if you need to probe N components, you need 2*N channels common ground oscilloscope.

author: imranhussain
author_rank: Newbie
timestamp: January 09, 2012, 07:14:40 pm
content: What is the reason of having a common for both channels in oscilloscope and in signal generator? Please help me solving this. Thanks.

author: jahonen
author_rank: Super Contributor
timestamp: January 09, 2012, 07:27:01 pm
content: Common ground arrangement is much easier to do and yields to considerably better performance. Most measurement applications don't actually require separated grounds. And if that is needed, one can always get a proper differential probe.Regards,Janne

author: vk6zgo
author_rank: Super Contributor
timestamp: January 10, 2012, 04:07:15 am
content: Quote from: pheonix on October 23, 2011, 02:27:22 pmYes,I can see how you would have a problem with a bridge rectifier.A half wave rectifier would be OK,or a centre tapped fullwave rectifier,where the centre tap would be where you would place your earth clip because you need to look at the two sides of the secondary separately.I'm just going to go away & scribble some circuit diagrams to get this clear in my mind,but I think that commoning one side of the secondary in a bridge rectifier setup with the dc common out,would just change the circuit to a half wave rectifier.I'll be back!----------NOPE!!!!! I was wrong!!!.It will put one of the diodes directly across the secondary,& cook it!!!!!As you get a bit longer in the tooth,you have to check these things.One fairly crappy workaround is to look at the secondary,save the waveform,then compare that with the rectified waveform.OK,it isn't realtime,but it gets around your problem,& is probably how most people would do it in the field.VK6ZGO