The AD7865 is specified to operate from an AVDD supply of 5V +/-5%. What would happen to the sampled data if it is operated outside of these 5% limits?
In a legacy design, the AVDD pins is connected via a simple RC low pass filter, R is 10 ohms and C is 100nF. A DC measurement was 4.76V at ambient, indicating 24mA of current. At datasheet maximum current of 32mA, Avdd would be 4.68V. We use an external Vref (2.6V) for the device and DVDD and Vdrive are connected to +5V directly.
I'm seeing some odd results in the analogue signal chain and need to rule out options.
It was only the AVDD supply of the AD7865 that was at the lower limit, this was due to the 1st order RC filter. The rest of the analogue signal chain had a 5V +/-20mV supply.
It looks like some PCB issues have caused the problem with the AD7865, in our case the external Vref signal disconnected under temperature causing a 4% shift (from 2.6V to 2.5V), thus causing the issues I initially reported. I'll mark this as closed.