You can connect Vref to a pin (you did, obviously). But then it will be subject to
- the internal routing resistances (look at the analog viewer in the design wide resources, it has an ohm calculator)
- the limited current capability of the reference voltage buffers
According to the TRM, the Vref buffers are good for 5µA. So you really need to add an output buffer, and make sure its connected via its dedicated output pin (for lowest resistance). But even then, I would suggest to measure the output voltage with a DAC too.
But why don't you use an IDAC to get a defined output current? This would be much easier to handle, since all the internal resistance are then don't make any difference.
Regarding the PGA: it has a rather high offset voltage (20mV), which will be amplifier with your gain. It also is not really accurate. It might be better to just a delta-sigma-ADC with its input buffer, this could give much better accuracy.
Here is one approach to low Z measurements (sync detection) -
Project attached, contact Chris Kees at email@example.com
If you use DelSig the input buffer must be disabled to get R-R performance
on its inputs.
That being said you would be better off measuring the R not ground ref-
erenced, eg. get A/D inputs into their CM range. That way you could use the
input buffer at its max gain of 8 and 20 bits and possibly get the dynamic
range and resolution you are looking for.
Here is a milliohmeter done in PSOC 1. The project is under review
by Cypress, not sure why.
milliohmmeter.zip 674.4 K