Dear TI technical support team,
We have designed a DAQ board based on ADS4129 which works perfectly for over 2 years. However, the last batch we produced last month do not function properly. It turns out the Vcm pin of ADS4129 does not work as expected. The voltage at Vcm is 1.65V, instead of 0.95V as specified by the datasheet.
In our design, the Vcm output of ADS4129 directly and solely drives the Vocm input of a differential op amp, ADA4937-1.
For working boards:
The voltage at Vocm of ADA4937-1 is 0.95V as expected. After removing the op amp Vcm is still 0.95V.
For defective boards:
Vcm voltage is 1.65V. If we remove the op amp, the voltage is 0.5V initially and falls to 0.3V in seconds.
If we keep the op amp and cut off the connection between Vocm of the op amp and Vcm of ADS4129, Vocm is still 1.65V.
Apparently no driving power is present at the Vcm pin of ADS4129.
The design has been thoroughly tested and working for us for years. We wonder if the problem is caused by defective ADS4129.
Thank you very much!
Mian Liu
