I'm designing/building a small quantity of voltmeters for a customer based on the LTC2400. As a base for comparison tests I'm using your Eval boards (DC573A and DC590B) and software. If I take Vref (5V) from the DC573A and feed it back into Vin and to my board, your software and mine both say 5V. Mine has a stability figure of around 80uV. If I then put a resistor divider on Vref (900K/100K) so that Vin to both boards is 0.5V. My board is showing noise in the order of 600uV or more. As soon as I pause your software (i.e. stop the SPI bus), my noise level drops to around 30uV. Is there a known issue with the 573/590 combo that can cause SPI generated noise?
Note that I have modded the 573 as per the datasheet for lower noise operation (cut p2 track and power LTC2400 from the 5V reference ic) but I was getting similar noise anomalies before.