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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. 

  • Hi again. 

    I figured out that removing my power supply to fake the input & reference voltages fixed my clamping problem. I was most likely having power supply noise from the different things attached to the front end (scope, power supply, converter from motherboard etc). Voltage level was fine, but I was most likely having some grounding noise / altercations, which messed up the adc. 
    Either way, as of now, I'm not having any errors. I have much more clean signals, and I'm not getting any unknown spikes. 

    But now, the AD wont start a single conversion. I still have the same setup (config / mode), i still do the calibrations. Hte only difference is that i know use potentiometers to create the needed input/reference voltages. 
    Haven't tested it through yet, but I suspect that the potentiometers create some input impedance issues, practically rendering the adc in idle mode all the time. 

    Also, it seems that the excitation current is not outputted correctly. As in: either it is not, even though I set it to 210uA, or it just fluctuates across all 4 settings. 
    Will get back to you when I'm out of ideas. 


  • Hi,

    Good to hear that you have solved the issue. If you have any other questions just let us know and we'll see how can we help you.



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