Hey,

I'm wondering is there different analog and digital grounds for the PIC18?

The device does have two grounding pins but they are both labeled the same (VSS). Also the data sheet refers to VSS in discussions and specifications on the ADC module (as well as digital parts). On measuring the resistance between the two ground pins I'm getting abut 2 Ohm implying the are indeed the same net.

I dont think there is too much point in extending my AGND to the PIC if it is wired to DGND internally, perhapps this could even be bad for my ground isolation...

any insights??

Thanks

The datasheets usually just list both VSS pins as "Ground reference for logic and I/O pins".

It does not indicate that there is any difference between them.

I have seen Microchip application notes circuit diagrams that show them requiring to be connected together, e.g AN822 and so would recommend you do the same.

The same applies to multiple VDD pins (plus they sometimes also show separate 0.1uF decoupling capacitors across each pair of VDD, VSS pins on the 40pin chips which is also a good idea).

Ok just to follow up.

It 'appears' that the ground pins are in the same net. However 1 Ohm (measures 1.05 with a more accurate meter) is not a negligible for any significant current as pointed out above

So the decision: Connect both VSS pins into my DGND plane. Connect the Vpos (3V analog) to the Vref+ (AN3) and my AGND to the Vref- (AN2). I do see from the data sheet that the ADC inputs are diode clamped to VDD and VSS could this cause problems with different reference voltages? They are more or less the same (but not exactly as there is RC decoupling on Vpos)

Our ground scheme will change. AGND will consist of the analog circuitry up to the Vref- input of the ADC. Under the PIC we will connect AGND to DGND via an inductor and from there on DGND will be the system ground including the power supply and the RF stuff.

Anyone see any obvious flaw in this?

Thanks

C