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| texane:
hi people, let say I have a device D0 I want to monitor the clock and data signals. The device is powered by a 5v/10Ma current source I have no access to (except to power D0 of course) To monitor the signals, I plan to use a pic microcontroller. Let call it D1. I will have to power it with another current source. Let say the D0 signal frequencies are far below the one the pic can handle, so no problem for that point. My question: since the ground is not the same in the 2 cases (cannot power D0 and D1 from the same source), how can I measure a D0 voltage from D1? Am I misleading? The question may be basic, but I dont have any clue... was thinking about something like a relay, but it would not be possible due to relatively high freqs... Thanks for answering, Regards, |
| jahonen:
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 |
| texane:
Hi, Thanks for replying. Some friends told me that by linkin the 2 grounds together, it will do the job, ie I will be able to measure both voltages. But I don t understand it...with this new mixed ground, how can the Vcc remain on the same value? Would you have any link, ref, course... handling this kind of topics? (ie. having 2 current and voltage sources inside the same circuit?) Regards, |
| mkissin:
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). |
| jahonen:
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 |
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