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AD7656 missing lower bits

Question asked by AlanPE on Jul 1, 2012
Latest reply on Jul 6, 2012 by ClaireL

I have a design that uses a AD7656.  I am reading the device in parallel byte mode.  When I read the device I get a valid value for the high byte (I have HBEN =1) and what seems to be a valid value for the lower byte, except the lowest 3 bits are always 0.  I am in +/- 10 volt range and sampling 1Ksps and reading the values back using a slow micro so there is not issues with the timing.  When I look at the data lines on a scope, I see the upper byte bits and then nothing where the lower byte bits should be.  I am using 5 channels and all the channels are acting the same.  I get all 8 bits on the high byte read, but when reading the lower byte and get what appears to be good values expect the lower 3 bits are always zero.

 

I have replaced the chip, tried different read timing, checked voltages, etc.  It is working (almost) so it seems like I am missing something.

 

Thank you for your help.  Feel free to contact my via phone.

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