This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

about DC input histogram of ADC datashet

Other Parts Discussed in Thread: ADS1248

Hi,

     From the ADC datasheet ,i see there is a graph that describe the DC input histogram.

     Take ADS8881X's datasheet as example,and show below:

Maybe it is a statistical description.But what does the X axis and Y axis mean?

could some describe the graph and tell me how to test it?

Thanks a lot

Robert

  • Robert,


    You are correct, this is a statistical representation of the noise for a measurement.

    If you take a DC input measurement, there is noise on the output reading. Here the output data is a code of 15 which is given as the ADC reading on the x-axis. Counting the bins, it looks as if the output code spread +/- 4 codes.The y axis is the percentage of readings for each code represented.

    I attached another graph for the ADS1248 which shows the exact same thing. The x-axis shows the ADC output code, while the y-axis shows the number of times that the code came out for the output (out of 12,000 samples).

    Here, on the x-axis, the LSB is the output code minus the offset. The y-axis uses counts instead of a percentage of the readings.


    I hope this answers the question. Let me know if you have any other questions.


    Joseph Wu

  • From the code, it varies ,such as 20LSB varies, how about its DNL INL?

    Seems its INL and DNL is big?

    is my understanding right?

    But generally, the INL and DNL is small,generally below 2LBS.

  • Robert,

    In the specification, we separate noise from the measurement and typically average the result to get an accurate INL/DNL measurement.

    In delta-sigma ADCs, the DNL is generally not specified. Since these ADCs have high resolution (often 24 bits), the noise is larger than the DNL value, which is typically very low. For the INL, again we often take averages of many readings to remove the effect of noise from  the INL measurement.

    In SAR type ADCs, the ADC is often trimmed for INL/DNL so both are important and specified, Also, since these are lower in resolution, the noise is smaller compared to the LSB of the reading.

    Joseph Wu

  • Hi Joseph,

        Thanks for your patient reply.

        It really helps!

     Robert