I'm using the EVAL board for the ADuCM360 (see ADuCM360_361_schematic.PDF). I can measure the on-board RTD easily. To measure an off-board resistor, (quick and dirty) I can disconnect the jumpers J6, J7, short AIN0 and AIN6, and put the resistor to measure between AIN0 and AIN1.
When this resistor is 100 Ohms, I get a reasonable measurement (as is the case when I measure the on-board RTD). However when this resistor is ~ 500 Ohm I get ~430 Ohm. When it's 1 kOhm, I get ~830 Ohm.
These results are irrespective of the IEXC - the currents are small. All PGA gains are 1 and buffers off.
What's going on? Does the Reference resistor (R5 5k6) not provide enough headroom? Do I need to use a 56k reference resistor?