I am trying to use a 2500-ohm AD5162 digital potentiometer to "fake" the resistance of an RTD temperature sensor as input to the Maxim MAX31865 RTD-to-Digital converter, https://datasheets.maximintegrated.com/en/ds/MAX31865.pdf.
When the MAX31865 converter is not attached, the digital pot changes resistance exactly as expected. When I then connect it to the MAX31865, that resistance is seen and correctly converted to a digital temperature. But when the two are connected to each other, I cannot change the resistance of the digital pot. When I send the commands to set the resistance, nothing changes. When I disconnect the two parts, sending the same commands does change the resistance, which is then seen when I connect them again.
This happens with the MAX31865 bias voltage on or off. When it is off, it should just present a high-impedance load.
I've posed this question of a Maxim applications engineer, and he doesn't know what the MAX31865 could be doing that prevents the digital pot from changing.
The relevant parts of the two circuits, which are connected by a 4" long cable, are circled in yellow in the attached schematics. They share a common ground. They are running from separate 3.3V supplies that are within 0.05V of each other. There are solid-state switches between the AD5162 and the MAX31865, but I get the same result when I bypass those.
Does anyone have any ideas abPDFPDFout what could be going on, or what I could try?
