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AD7994: Trouble reading anything other than channel 1

Question asked by msoric on Sep 25, 2017
Latest reply on Sep 26, 2017 by jcolao

AD7994: Trouble reading anything other than channel 1. I'm using three AD7994s to read some amplified photodiode outputs. It seems that for each ADC, I can only read channel 1.

 

I'm using the AD7994s in Mode 2 (Command Mode). I initiate a write to the APR with the desired conversion channel and then subsequently, I read two bytes off the I2C bus. For each AD7994, I get valid data from channel 1 (0 - 4095) but once I try channels 2, 3, and 4, my data is always >> 4095 which isn't quite making sense to me. I've checked the I2C signals with a logic analyzer - I'm sending out the correct APRs,

 

ex: channel 1 (00010000), channel 2 (00100000), channel 3 (01000000), channel 4 (10000000)

 

The four LSB are all 0 so that it points to the conversion result register for the next read. I've no idea where I'm going wrong - any suggestions? I thought maybe it was a soldering issue but the fact that it's the exact same behaviour across all 3 ADCs I'm using is making me think I'm missing something in terms of communication. Any help would be greatly appreciated! ( I'm pretty sure I can attribute a record number of new grey hairs to this issue)

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