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PIC ADC Ground Problem

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elitel

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I want to read a kind of rectified square wave in a circuit with PIC 12F675 ADC input. The rectified voltage can be read without problem by a multimeter, but when I use the common ground of the circuit the value changes dramatically. I must have a seperate ADC ground in order to connect to this floating ground. But the ADC input has no seperate ground in PIC. What can I do to measure the potential difference between two points in the circuit with PIC 12F ADC without using common ground?
 

Ian Rogers

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I haven't tried this, but try connecting the ADC ground to the AD input pin and the ADC signal to the vref+.. then read the opposite!!

upload_2018-4-8_20-24-1.png
You should still get an isolated signal... Actually! The vref+ will be constantly changing with reference to the PIC ground, so maybe not!!
 

dknguyen

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Show your circuit and what is the application? I suspect it doesn't actually need to be floating or can be easily worked around if it does. Why does it "need" to be floating?
 

elitel

New Member
The ground reference of the rectified square wave is really floating. For example it changes according the real ground of the power supply of the circuit between 1-2vdc. The positive end of the rectified wave also changes according to the real ground, but the potential difference between these two points is very stable. So I found a solution I think; to use a differential opamp. I will try at least. Thanks for the comments.
 

dknguyen

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The ground reference of the rectified square wave is really floating. For example it changes according the real ground of the power supply of the circuit between 1-2vdc. The positive end of the rectified wave also changes according to the real ground, but the potential difference between these two points is very stable. So I found a solution I think; to use a differential opamp. I will try at least. Thanks for the comments.
That's not the whole picture though. Your entire MCU circuit could also be floating (such as if it was being powered from a wall-wart, battery, or other isolated supply) which can often be the case. If it did not need to connect to another piece of equipment that is grounded (like a PC) then you would not need to do much of anything.

Differential op-amps won't help if your circuit can float such that it exceeds the common mode range of the diff-amp. If it's sufficiently high frequency, you might be able to get away with just using capacitive isolation, or of course, a transformer.
 

large_ghostman

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I know your using a pic 12, but some the other pics have a separate connection for the ADC, but i dont see why you couldnt battery power the pic, that would float it in relation to your signal?
 
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