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DDC114: measurement difference between channels

Part Number: DDC114

Hi,

Below is customer's schematic. 

This is the ADC output versus time of different channels. Red and blue curve are AIN1 and AIN2. Gray curve is AIN3. All three channels measure the same constant optical source through three respective fibers.They are not straight lines, because there is noise and error. However, you can see AIN1 and AIN2 are almost synchronous, and AIN3 is quite different from them. We swapped the photodiodes, but the phenomenon remains, so it is not due to the difference of photodiode.

Could you pls help analyse the reason? Is AIN1 and AIN2 synchronous sampling, and AIN3 and AIN4 are another synchronous pair?

Thanks.