ADC Vss/Vdd on PICs(AVR/ARM is ok too) for external measurements?
I can read voltages from withing the PIC's board, 5v, 3.3v... But if i try using an external power supply i can't read.
If i just probe the (+) wire, i can't read...if i plug the (-) wire in the gnd/vss of the board/pic, there's a voltage drop on the supply's voltage, and i can read it, but it sounds wrong.
I also already tried to use the vref+/vref- pins, and didn't make any difference, i think.
Like, 3v from the supply drops to 0,4v on a multimeter and in the pic adc value. It looks like it's right, but that voltage drop isn't "ok".
I read something about single-ended and differential, but i couldn't find any real explanation.
So, the bottomline i need to use external voltage sources, and i will read 4-20mA sensors so, they will use external power, i can't use the boards sources and gnd/vss.
Can someone shed some light on the matter?