I'm mesure 100 mV from Pt100. I set gane from 1 to 128 and using internal Vref.

I recived such 16 bit digital codes from ADC:

x1 - 5413

x2 - 10819

x4 - 11038

x8 - 21722

x16 - 25780

x32 - 26957

x64 - 28497

x128 - 56952

I do not understand why each successive value of the code is not equal to the previous multiplied by 2?


  • Hi,

    We are looking into this. We'll get back to you as soon as possible.

    Thanks and Best Regards,


  • FOR undeкstanding,  I get 16 bits from 24 bits by discarding the third byte.

  • Hi,

    Can you provide details on how the ADC is configured?(ex. Buffer enabled, excitation currents, etc.)


    Each time that the gain is changed, an internal FS calibration should be performed to calibrate the gain error due to the PGA. Can you repeat your test and include the internal FS calibration when the gain is changed.


    Also,  take note that the differential input voltage range of the AD7793 is between  +/-Vref/Gain. Once this limit is exceeded, the ADC will not perform properly.


    So for example, if we have a gain=128  internal voltage reference enabled; given the equation:


    AIN=+/- Vref/Gain


    AIN=differential input voltage

    Vref=voltage reference

    Gain= gain set in the PGA


    Hence,  the differential input voltage range of the ADC is limited:


    =+/- 1.17V/ 128

                                        =+/- 9.141mV

    Thanks and Best Regards,

  • 0
    •  Analog Employees 
    on Aug 2, 2018 2:47 PM
    This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

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    EZ Admin