external power supply going to the AtoD on the uC32.

[btb4198]

I have a Extech instrument Multi-function Process Calibrator(MPC)  that outputs 4 -20mA but at 25V. I have a comment ground with the board and I have a 165 ohm resistor between power from the MPC  and the atoD but  the A to D is not working.  when I say it is not working, I mean the AtoD is not reading the values from the resistor.

 

I have a pot for testing and the atoD works great with the pot, so it is not the code.  

Does the Pic have to power up the device that is sending a signal to the AtoD for it to work ? 

I have the - coming from the MPC going to the ground on the uC32.

[JColvin]

Hi btb4198,

As per page 179 of the PIC32 Family Datasheet by Microchip, the absolute voltage range that the Analog to Digital Converter can accept is A_VSS - 0.3V to A_VDD + 0.3V, which correlates to -0.3V to 3.6V. With the setup you described, your 165 ohm resistor will only drop the incoming voltage (presuming we are providing 20 mA) by 3.3V, so you would be applying 21.7 V to the input of the Analog to Digital Converter. As Microchip notes on page 151 of the same document linked above: "Stresses above those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. This is a stress rating only and functional operation of the device at those or any other conditions above those indicated in the operation listings of this specification is not implied. Exposure to maximum rating conditions for extended periods may affect device reliability." Likely, the pin you were using on the Analog to Digital Converter (or that whole section of the PIC32 chip) has been irreparably damaged. 

So you do not need to have the uC32 power the device sending the signal, but they do need to share the same ground (as you have done) and the device needs to send a signal within the 0V to 3.3V range.

Let me know if you have any more questions.

Thanks,
JColvin