We’ve been using the AD7730 for years without any issues. However, after redesigning the PCB—using the same hardware and software but in a more compact layout—we're seeing ADC drift. What could be causing this change?

Answer:

During the redesign, ESD protection diodes were added to the REFIN+ and REFIN− pins of the AD7730, alongside the external voltage reference.
The leakage current from these diodes was adding to the ADC reference path, causing the ADC reference to drift with temperature and introducing ADC errors.
This highlights the importance of carefully reviewing the characteristics of protection diodes—especially leakage current and temperature dependency—when using them on analog or reference input pins of an ADC.