Search FAQs on the left to see if your question has been answered. Click on the dropdown to view all of the documents associated with the product. If you can't find your question, click on Ask a Question

AD7655: Stuck bits

I 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.


Stuck 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.