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Abnormal digital output obtained from LTC2203

Question asked by ned2018 on Jul 19, 2018
Latest reply on Aug 14, 2018 by Clarence.Mayott
Hello there,
Recently I have designed a circuit using the LTC2203. 
During the testing, I obtained some weird results. I have tried the circuit with three LTC2203s and they all gave similar results.
Is there any methods to test whether the ADC is malfunction or not. 
At the beginning, I kept the AIN+ and AIN- of the LTC2203 open, the output was normal.
However, when I used a jumper to connect AIN+ and AIN- (to the AGND), the digital output was neither all 0's using 2's complement coding nor a stable value (±4LSB).
Instead, the 10 LSBs were changing.
Another observation is the higher the clock frequency, the less LSBs will change.
 
10 LSBs were changing with a clock pulse of1Mhz.
7 LSBs were changing with a clock pulse of2Mhz.

 
6 LSBs were changing with a clock pulse of 5Mhz.
Here is the setting of the LTC2203 during testing.
Is there any methods to test whether the ADC is malfunction or not? I have tried to solve this problem for more than a week but still no clues. Thanks a million.

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