[ different grounds ] - Page 1
Forum section: electronics / beginners

author: texane
author_rank: Regular Contributor
timestamp: February 19, 2010, 02:09:28 pm
content: hi people,let say I have a device D0 I want tomonitor the clock and data signals.The device is powered by a 5v/10Macurrent source I have no access to(except to power D0 of course)To monitor the signals, I plan to usea pic microcontroller. Let call itD1. I will have to power it withanother current source.Let say the D0 signal frequencies arefar below the one the pic can handle,so no problem for that point.My question: since the ground is notthe same in the 2 cases (cannot powerD0 and D1 from the same source), howcan I measure a D0 voltage from D1?Am I misleading?The question may be basic, but I donthave any clue... was thinking aboutsomething like a relay, but it wouldnot be possible due to relatively highfreqs...Thanks for answering,Regards,

author: jahonen
author_rank: Super Contributor
timestamp: February 19, 2010, 03:25:32 pm
content: How much voltage difference you have between the grounds? If it is not extreme (less than 1 kV), then maybe AD/TI digital isolators are the solution you are looking for. Optoisolator is also possible but problem there is a rather large drive current required by the optoisolator LED.Or, if the ground voltage difference is only a few volts, then even a RS-485/422 receiver / ordinary comparator may work.Regards,Janne

author: texane
author_rank: Regular Contributor
timestamp: February 20, 2010, 07:15:14 am
content: Hi,Thanks for replying.Some friends told me that by linkin the2 grounds together, it will do the job, ieI will be able to measure both voltages.But I don t understand it...with this newmixed ground, how can the Vcc remainon the same value?Would you have any link, ref, course...handling this kind of topics? (ie. having2 current and voltage sources inside thesame circuit?)Regards,

author: mkissin
author_rank: Regular Contributor
timestamp: February 20, 2010, 10:41:39 am
content: As long as you connect the ground of D0 and D1 together, your PIC will be able to read the other digital signals from D0 just fine. It actually doesn't matter that they are being powered by two different sources.All voltages in electronics are relative. As long as all your devices agree on what voltage to call zero, it works. Only when they cannot agree on the zero voltage level do you need an isolation barrier.I'm assuming, though, that your PIC is also being powered with a 5V source. If it's not, then you would also need to be careful that it will accept the 5V logic levels used by D0 (i.e. a 3.3V PIC probably wouldn't accept 5V logic, unless you used current limiting resistors or level translators).

author: jahonen
author_rank: Super Contributor
timestamp: February 20, 2010, 11:50:53 am
content: Connecting the grounds together is easiest solution, but that is not always possible due to isolation/ground voltage difference or safety reasons.Vcc does not need to be exactly same for both circuits, but one must make sure that levels are compatible so that logic levels are correct. Small voltage differences are usually not significant.Regards,Janne

author: texane
author_rank: Regular Contributor
timestamp: February 21, 2010, 10:20:18 am
content: Hi,Thanks for replying, your answers are of greathelp since I have hard time understanding somekey concepts (more of a software guy...)I think I understood all this, I will confirm bypracticing a bit...Regards,Fabien.

author: Simon
author_rank: Unknown
timestamp: February 26, 2010, 09:30:33 pm
content: you should be able to connect the two grounds together, he you could even connect VCC of one to the ground of the other but that would make it difficult for the the circuits to interact, ground is essentially just that the 0 and reference point, provising it is actually negative (or even if not) you can connect it to your main earth, providing you don't connect two power delivering lines together at the same time. if your sampling one circuit with you pic circuit ensure you are not in a position where two voltages will clash, this is ually avoided by making a connection between the two via a pullup resistor and/or IC's that have an output transistor with an unconnected collector so that the two supplies never "meet" directly.