Information regarding operation of ADC121C021 which uses the I2C communication protocol

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Joined Feb 10, 2019
I am using ADC121C021 which uses the I2C protocol for transferring the digital data. I am using eight of them to measure eight capacitor voltages in a power electronics circuit. Each of the eight ADCs is assigned eight different hardware addresses. According to the I2C protocol, an ADC transfers the digital data when the address on the SDA line matches with its corresponding hardware address. If it does not match, it generates a Not Acknowledgement (NACK).
Now, I am facing the following problems:
1) During some instants, the ADC generates NACK even though the address on the SDA line matches with its hardware address. The problem persists even after multiple attempts of REPEATED START.
2) The first four bits of the 16 bit digital data that ADC121C021 sends are supposed to be zero for the Non automatic mode of operation. However there are instants when the ADC sends ONEs in the first four bits.
3) There are instants where the ADC sends all 16 bits as zeros.

All these cases are interrupting the objective of sensing the capacitor voltages. Please note that there is electrical isolation provided between the high voltage capacitor voltage side and the sensing circuitry.
I will be really grateful if someone can point out the possible reason for the problems mentioned above.
Thanking you.