PIC ADC Ground Problem

[elitel]

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]

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!!

You should still get an isolated signal... Actually! The vref+ will be constantly changing with reference to the PIC ground, so maybe not!!

 

[dknguyen]

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]

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]

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]

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?

 

[dknguyen]

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?

That's why we need more info.