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 Jelelnie 

    Thanks for your reply. 


    Unfortunately, I can't show you circuit diagrams as they will reveal sensitive company information, and I haven't got authority to do that. 
    What I can tell you is, that through a header with SPI signals and powersupply, the CN0326 circuit is connected through the J2 PMOD block like so: 

    CS_N - Connected to the MSP430 port 1 through a 3-to-8 line decoder (74HC138). 

    DIN -    Connected directly to the MSP430. 
    DOUT - Same as above. 

    Sclk    - Same as above. 

    Rcout - not connected. 

    Gnd / vcc - connected to the 3V digital voltage as every other chip in the entire circuit. 

    Note: On the SPI bus there is also connected a AD7718, two isolators (ADuM1310) before 4-20mA outputs for HART communication and lastly, the eeprom. 

    As far as I understand, the 1.05V for bias and reference is generated with the excitation current on IOUT2 (210uA +- 5-10%) and the 5k precision resister R1. Although that resister is a DNI, so I'm not sure how that works.. 

    CH0? I'm fairly sure that the AD has no such thing as a CH0. Not from the datasheet that is. Unless you declare +-AIN1 as CH0. 

    As of right now, my configuration register is set to 0x0010

    I will try the continous conversion you suggested and get back to you asap. 

    Thanks! 

Reply
  • Hi Jelelnie 

    Thanks for your reply. 


    Unfortunately, I can't show you circuit diagrams as they will reveal sensitive company information, and I haven't got authority to do that. 
    What I can tell you is, that through a header with SPI signals and powersupply, the CN0326 circuit is connected through the J2 PMOD block like so: 

    CS_N - Connected to the MSP430 port 1 through a 3-to-8 line decoder (74HC138). 

    DIN -    Connected directly to the MSP430. 
    DOUT - Same as above. 

    Sclk    - Same as above. 

    Rcout - not connected. 

    Gnd / vcc - connected to the 3V digital voltage as every other chip in the entire circuit. 

    Note: On the SPI bus there is also connected a AD7718, two isolators (ADuM1310) before 4-20mA outputs for HART communication and lastly, the eeprom. 

    As far as I understand, the 1.05V for bias and reference is generated with the excitation current on IOUT2 (210uA +- 5-10%) and the 5k precision resister R1. Although that resister is a DNI, so I'm not sure how that works.. 

    CH0? I'm fairly sure that the AD has no such thing as a CH0. Not from the datasheet that is. Unless you declare +-AIN1 as CH0. 

    As of right now, my configuration register is set to 0x0010

    I will try the continous conversion you suggested and get back to you asap. 

    Thanks! 

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