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AD7793: Not getting correct datacode from channel 2.

Question asked by Carlcs1990 on Jan 24, 2017
Latest reply on Jan 27, 2017 by Carlcs1990

Hi there. 


I am using the cn0326 evaluationboard in a bigger system, to measure pH and temperature. And Im having some conversion issues with the ad7793.  


As of right now, I can measure pH perfectly fine on channel1 (AIN1+ - AIN1-). 

But when i change to channel2 (AIN2+ - AIN2-) to measure temperature, i almost get fullrange output. The errorbit in the statusregister is not set, so it is not a clamped 1's output, it is however, very close to a 0xFFFFFF value. 
And that doesn't make sense to me.

I have removed resistor R4, which is placed just before the AIN2+ input. Instead, I have placed a potentiometer to represent a RTD. My setup right now should give me a reference of 1.17V (internal), 390mV on AIN2+ through the pot-meter. A difference of over -700mV. With my gainsettings, this shouldnt give me close to fullrange output (i get 15.8 mil bits out of 16.7 mil). At all. The exact opposite. I should be closer to zero output. 


My register values when i initiate a single conversion is:
CONF:      0x0091
IO:             0x02

MODE:      0x204F


All ny signals on the scope are correct. Everything looks how it should. Except for the output code from the ADC. 

Do I need to zero/fullscale calibrate when i change channel/gain settings? 

If I do, and that doesn't help on my recieved datacode, what could be the reason? 


Thanks in advance.