Author Topic: [ different grounds ]  (Read 4929 times)

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Offline texaneTopic starter

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[ different grounds ]
« on: February 19, 2010, 02:09:28 pm »
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,
 

Offline jahonen

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Re: [ different grounds ]
« Reply #1 on: February 19, 2010, 03:25:32 pm »
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
 

Offline texaneTopic starter

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Re: [ different grounds ]
« Reply #2 on: February 20, 2010, 07:15:14 am »
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,
 

Offline mkissin

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Re: [ different grounds ]
« Reply #3 on: February 20, 2010, 10:41:39 am »
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).
 

Offline jahonen

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Re: [ different grounds ]
« Reply #4 on: February 20, 2010, 11:50:53 am »
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
 

Offline texaneTopic starter

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Re: [ different grounds ]
« Reply #5 on: February 21, 2010, 10:20:18 am »
Hi,

Thanks for replying, your answers are of great
help since I have hard time understanding some
key concepts (more of a software guy...)

I think I understood all this, I will confirm by
practicing a bit...

Regards,

Fabien.
 

Offline Simon

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Re: [ different grounds ]
« Reply #6 on: February 26, 2010, 09:30:33 pm »
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.
 


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