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AFE4420: AFE4420 - Negative dark current

Part Number: AFE4420
​We are using afe4420 chip in our system. 
Connection - photodiode cathode is
connected to the positive of TIA and anode to negative of TIA. With this we expect ADC to always swing in positive direction as per the datasheet
With PD disconnected configuration, we are getting negative ADC codes and also in pd connected case we are getting negative ADC codes for ambient phase
This does not happen for all the boards, few boards show positive for the same configuration
We are trying to understand why is this happening and how to handle this. This negative ADC codes in ambient is causing issues in our design
Would appreciate help on this
I've also sent an email at biosensing_afe-support@list.ti.com
Regards,
Sneha