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AD7793 output clamped to all zeros

Hey guys. 

I'm trying to include analog devices CN0326  pH-measuring circuit in an already existing measuring circuit controlled by an MSP430. 
For now, I have removed the isolator (ADum5401) because it rendered my LED display in an unpowered state. But this shouldnt affect any signals, since it only isolates between two digital areas. 

Anyhow, the problem at hand is, that the adc only outputs zeroes. 
I have solid SPI communication with the chip, and I can update and read from registers without any issues. I'm running on a very low bus-speed just to make sure my problems aren't SPI related (allthough I haven't ruled that out 100% yet). 

I start my software like so: 

Read ID register (I get the correct value) 

Internal fullscale calibration 

Internal zeroscale calibration

Set the mode register (Idle mode, 64kHz internal clk and 4.17Hz update rate). 

Set the config register (bipolar, Gain 1, buffered mode, ch1)   

I read both mode and config register back, and the correct values are stored. 

Then a lot of other stuff happens (regarding the other circuit) and when its time to convert, I change the mode reg to a single conversion, the config register to the fitting channel (Ch1 for pH, Ch2 for temperature). 

I'm supposed to measure ph in a -420 mV to 420mV span, but right now, I have tried every kind of voltage and configuration and I still can't get a conversion. 

I might not have configured it correctly even once. At this point I kinda blinded myself by trying so many different things. 

So for a +-420 mV range, how should i configure the adc with bias voltages, references etc etc. 

If you need any more info, please let me know. 

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  • Hi,

    Do you have any RTD connected to P1? The DNI resistor R1 is a replacement to RTD that needs to be populated if your application doesn't require a temperature compensation. Open wire RTD or R1 will not generate a voltage reference to REFIN+ and REFIN- pins due to open circuit connections, thus the ADC will no longer have a valid reference input which may results the ADC conversion to clamped to all 0s or 1s. The board is recommended to work with an RTD input.

    Thanks,

    Jellenie

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  • Hi,

    Do you have any RTD connected to P1? The DNI resistor R1 is a replacement to RTD that needs to be populated if your application doesn't require a temperature compensation. Open wire RTD or R1 will not generate a voltage reference to REFIN+ and REFIN- pins due to open circuit connections, thus the ADC will no longer have a valid reference input which may results the ADC conversion to clamped to all 0s or 1s. The board is recommended to work with an RTD input.

    Thanks,

    Jellenie

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