QI have a problem witch the AD7655AST.
I work in parallel Mode and have a 2,5V ref, all Voltages and Grounds are ok.
The IC is full funtion to a Voltage of 0,5V on INA and INB. Over this Voltage
the Databits D0 to D5 are allways low.
When I change the high and low Byte with the BYTESWAP pin I have the same
problem with D8 to D13.
AStuck bits are usually indicative of incorrect REF decoupling.
The ADC REF input needs a low ESR decoupling cap (we recommend a 47uF in the
datasheet) placed right at the REF pin.
If you lower the input to something small, the problem probably goes away.
A little background as to why this is:
During conversion, the internal comparator pulls current from the REF pin while
performing the successive approximation.
The larger the input on the T/H, the more current is pulled from REF and
without sufficient decoupling, the REF bounces.
Others possible causes could be incorrect decoupling on any of the supplies.
We strongly recommend a multilayer PCB with separate analog and digital GND
planes connected together under the ADC.