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Original question:

ADS131E08EVM-PDK: Not able to see correct ADC input on ADS131e08 EVM software scope

ADS131E08EVM-PDK: ENOB calculation

Tejaswini Shirsath
Prodigy 170 points
Part Number: ADS131E08EVM-PDK
Other Parts Discussed in Thread: ADS131E08, ADS131E08S

Hi Team,

We want to calculate or check the ENOB for ADS131E08 EVM-PDK kit.

For particular PGA Gain and Data rate we will have certain ENOB as mentioned in Table 1(Pg. no 13)  of ADS131E08S Datasheet.

Also on ADS131E08 EVM software we can check the SNR so we tried to calculate the ENOB from SNR using this formula:

 ENOB = (SNR – 1.76) / 6.02 dB.

Following readings we got & which are not matching with Table 1 (Pg. no 13)  of ADS131E08S Datasheet.

As input voltage (Vpp) increases SNR decreases so ENOB also decreases.  

Is our approach is correct or not? 

Regards,

Tejaswini

  • 0
    TI__Guru* 97825 points

    Tejaswini,


    The calculations that are used for the ENOB values in table 1 are given in equations 1 and 2 just above the table in the datasheet.



    If you look at these equations, you'll see that the ENOB is a bit different than you might be used to. For some of our lower speed ADCs, we use ENOB as a metric of the noise performance of the device compared to the full scale range of the input signal (with some scaling for periodic signals). That's why the VREF voltage is part of the equation.

    Note that this data is taken with a low noise reference. Any noise in the reference is reflected in the ADC measurement. In the datasheet, it mentions the REF5025 as an external reference, but I think you'll need the REF5040's 4V reference to maximize the signal compared to the noise. The EVM has an unpopulated spot for the external reference.



    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 170 points

    Hi Joseph,

    Thanks for your response,

    In REF5040 datasheet Low Noise: 3 μVPP/V is given which same for REF5025 as they are in same family. So which Low noise reference IC is mounted on EVM board.

    In our case , we have made the jumper settings for 2.5V Vref (internal Vref).

    Is there any chance that we can calculate the actual ENOB when we give some analog input through function generator. 

    As I told you in earlier mail that we are getting SNR on EVM software , But though Data Rate and PGA gain is constant still SNR is changing as Input voltage changes as shown below:

    Can you tell me what can be the reason for that?

    Regards,

    Tejaswini

  • 0 in reply to Tejaswini Shirsath
    TI__Guru* 97825 points
    Tejaswini,



    The ADS131E08EVM has open spots for two references and they match the pinout of the REF50xx references. Either one is pin selectable based on the build of the EVM.

    As for the SNR of the EVM measurement, I'm not sure about the method of calculation. I'll need some time to see what the output comes out from the software. What kind of function generator are you using for the source?


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 170 points

    Joseph,

    We are using AFG1022 Function Generator (Arbitrary, 2 Channel, 25 MHz, AFG1000 Series) for giving analog inputs.

    Regards,

    Tejaswini

  • 0 in reply to Tejaswini Shirsath
    Prodigy 170 points

    Hey Joseph,

    Any update on ENOB or SNR?

  • 0 in reply to Tejaswini Shirsath
    Prodigy 170 points

    Hi Joseph,

    Anything you got?

    Regards,

    Tejaswini

  • 0 in reply to Tejaswini Shirsath
    TI__Guru* 97825 points

    Tejaswini,


    I'm sorry about missing this, I forgotten until recently that you were looking for a response.

    Unfortunately, I'm still not sure about how this EVM software evaluates this EVM. I did run a test looking at an input and calculating the SNR with a sine wave generator. Looking at the input, I got the following input waveform.



    Then running the FFT, I got the following FFT, and SNR calculation:





    Now when it comes to the SNR calculation, it looks like it's not removing the fundamental input frequency, and may be calculating the SNR based on the FFT as if the signal is a component of the noise. This would explain why as the output signal got larger the SNR got increasingly smaller.

    Until I figure out how this EVM software removes the fundamental frequency of the input signal you may need to use an alternate method. I talked this over with a colleague and he mentioned that you may be able to use a different FFT calculator to determine the SNR. He mentioned that with the Analog Engineer's Calculator, you can run some similar calculation. It's a fairly large download because it uses the same LV engine and that's why it looks similar in construction.

    A link to the Calculator is here:

    www.ti.com/.../ANALOG-ENGINEER-CALC

    And here is a screen shot of the FFT page:




    I'd note that this is example data, and that it's not exported from the ADS131E08, but this calculator came from my current group and it's not legacy software, like that from the ADS131E08EVM. This was mentioned at the end of the day, when I talked to another manager, so I haven't had a chance to try this.

    I'd note that I'm not at my home site, but rather on travel. I might have some delays with a response, I am closer here with some people that might have more background on the ADS131E08EVM software.


    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Guru* 97825 points
    Tejaswini,


    Were you able to download the data and use the Analog Engineer's Calculator to run the FFT? As I mentioned before I didn't know how the ADS131E08 software derived the calculations for the SNR. I wasn't able to find more information about how the software made the calculation.

    I'll close this post for now, but if you have more questions, please post back.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 170 points

    Hi Joseph,

    Sorry for late response,

    We will try to calculate SNR and ENOB using Analog Engineer's Calculator.


    Regards,

    Tejaswini