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ADS122C04: System Monitor Issue - When monitoring the external reference voltage source (MUX[3:0] = 1100)

Part Number: ADS122C04

Hello.

I am developing a product using the ADS122C04 device.
I would like to use the "System Monitor" function to implement the self-diagnosis function.
I have set up the internal multiplexer to measure analog power supplyy (MUX[3:0] = 1101, 3.3V) and an external reference voltage source (MUX[3:0] = 1100, 3.0V).

The manual (ads122c04.pdf, page.29, 8.3.9 System Monitor) says that it is automatically converted to internal reference when using the monitoring function, but it does not work well when measuring an external reference voltage source.

When measuring an external reference voltage source, if the reference source is set as an external reference, it will be read correctly.

Please confirm.
thank you.

  • Hi Jungsoo,

    welcome to our e2e forum and thank you for your interest in our ADS122C04.

    When you want to measure the external reference voltage using the system monitoring function, you need to set MUX[3:0] = 1100b and VREF[1:0] = 01b.
    With an external 3V VREF applied you should then measure a voltage of ~3V / 4 = 0.75V.
    Do not select the internal voltage reference in the VREF[1:0] bit field for the system monitoring measurement of the external VREF. The device does however use the internal 2.048V reference for the ADC when using this system monitoring function to measure the external VREF.

    Regards,
    Joachim Wuerker

  • Hi Joachim.

    Thanks for the quick reply.
    Now the experimental results are as follows.

    When monitoring analog power supply (MUX[3:0] = 1101, 3.3V), it outputs 0.8215117V (3.3 / 4) regardless of the external reference voltage source setting. (Good)

    However, when monitoring an external reference voltage source (MUX[3:0] = 1100, 3.0V), the output voltage according to the reference source setting is as follows.

    ----------------------------------------------------------------------------

    reference source                       |       output voltage [V]
    ----------------------------------------------------------------------------
    Internal (2.048V, VREF[1:0] 00) |     0.2839001  ?????
    external (3.0V, VREF[1:0] , 01)  |      0.7499675V (3.0/4)

    ----------------------------------------------------------------------------

    * For voltage conversion, ADC value is converted based on 2.048V.

    The manual states as follows.

    "When measuring the analog power supply (MUX[3:0] = 1101), the resulting conversion is approximately (AVDD – AVSS) / 4. The device uses the internal 2.048-V reference for the measurement regardless of what reference source is selected in the configuration register (VREF[1:0]).

    When monitoring the external reference voltage source (MUX[3:0] = 1100), the result is approximately (V(REFP) – V(REFN)) / 4. The device automatically uses the internal reference for the measurement."

    However, I asked to confirm that the part below is correct.
    "When monitoring the external reference voltage source (MUX[3:0] = 1100), the result is approximately (V(REFP) – V(REFN)) / 4. The device automatically uses the internal reference for the measurement."

    고맙습니다. :)

    Regards,
    Jungsoo Kim

  • Hi Jungsoo,

    setting VREF[1:0] = 00b is not a valid setting when using the MUX[3:0] = 1100b setting. I actually don't really know what the device is measuring in that case.
    In order to measure the external VREF using the system monitor you have to select MUX[3:0] = 1100b and VREF[1:0] = 01b. And as you correctly did, you need to use the internal 2.048V VREF for the voltage conversion.

    Think of it that way. If you want to measure the external VREF you need to measure it against a different VREF, i.e the internal VREF. Would the system monitor use the external VREF for the ADC to measure the external VREF then your output code would always be FS/4 no matter what the external VREF voltage is.

    Regards,
    Joachim Wuerker

  • Hi Joachim.

    It would be nice if the contents of the manual could be modified.
    Thanks for checking it out.
    It helped a lot.

    Regards,
    Jungsoo Kim