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ADS131M08: Data rate

Theo Keinarth
Intellectual 325 points
Part Number: ADS131M08
Other Parts Discussed in Thread: ADS127L18, ADS112U04, , ADS1220, ADS122U04

Tool/software:

Hi TI Team

I have a question/confirmation on how to use this ADC correctly. I want to have a Samplerate of 10SPS to FIlter 50/60Hz and to achieve it I need to use a clock of 327680 Hz on the CLKIN pin. I tested it with the EVM Board and it seems to work. Can you confirm that this scenario is OK?! And is it also safe to use 25MHz on the SCLK to read the Data from the ADC?

Thank you in advance.

Regards Theo

  • 0
    TI__Guru 52546 points

    Hi Theo Keinarth,

    You must meet the requirements in the Recommended Operating Conditions, shown below. A value of 327680 is within this range

    The maximum SCLK speed is 25 MHz, so this is acceptable, but of course leaves no margin for tolerances. I also wonder why you need such a fast SCLK rate if your data rate is 10 SPS. 

    -Bryan

  • 0 in reply to Bryan Lizon86
    Intellectual 325 points

    Hi Bryan

    Sorry for my late reply. I need a lot of analog inputs (40+) for a plc project and I also need to filter out 50/60Hz noise. So, using this ADC with 10SPS seems to be the easiest way.  I also thought of the ADS127L18 but its far to complicated and expensive for such low data rates. The ADS112U04 would also be nice but for 40 channels I would need 40 ADCs. The only downside with the ADS131M08 is that I can only have +/-1.2V maximum input voltage.

    We probably will use a SCLK rate of 6.25Mhz. Was just curoius to know.

    Regrads Theo

  • 0 in reply to Theo Keinarth
    TI__Guru 52546 points

    Hi Theo Keinarth,

    Understood, thanks for the additional explanation

    -Bryan

  • +1 in reply to Bryan Lizon86
    TI__Guru 52546 points

    Hi Theo Keinarth,

    To be clear, the filter response at the lowest OSR has a sinc1 response, which will not give you much rejection at 50 and 60 Hz. Ideally the rejection is infinite, but any variation in the input clock frequency or the line cycle frequency will shift the input frequency away from the rejection null, resulting in less rejection

    You can see this in the second image below where the first notch for the sinc3+sinc1 filter (red plot) is very narrow. Any small deviation in the input frequency relative to the exact data rate (10 SPS in your case) will quickly move up the frequency response plot and away from the infinite null. Looking at the plot a 1Hz deviation might provide 75dB of rejection

    If that is insufficient, can you use multiple external multiplexers + 1-ch ADCs, instead of using a simultaneous sampling ADC? For example, an 8:1 mux + ADS122U04 (or ADS1220)? These devices offer much better 50/60Hz rejection at 10SPS compared to the ADS131M0x

    -Bryan

  • 0 in reply to Bryan Lizon86
    Intellectual 325 points

    Hi Brian

    I know of the sinc1/sinc3 response of the ADS131 and I thaught of mutliplexing the ADS122 but I need to read all analog inputs within 100ms. So multiplexing is not an option. Thats why I want to use the ADS131 or the ADS127L18. The ADS131 would be the better choice because it is quite easy to handle and has CRC and test functionalty. I need it for a safety application and would use always 2 of them for one analog signal. The clock I use schould be within +/-0.01% and is testsed with the help of the DRDY signal. So far it looks promising.

    Regards Theo

  • 0 in reply to Theo Keinarth
    TI__Guru 52546 points

    Hi Theo Keinarth,

    Understood, we just wanted to make sure this was clear. Glad the M0x is working out in your system so far

    -Bryan

  • 0 in reply to Bryan Lizon86
    Intellectual 325 points

    Hi Brian

    One final question. Does it make a noticeable difference using an external reference like the REF7012 instead of the internal? Or is the ADC the limiting part here?

    Regards Theo

  • 0 in reply to Theo Keinarth
    TI__Guru 52546 points

    Hi Theo Keinarth,

    Can you explain what you mean by "noticeable difference"?

    The VREF is typically one of the main error sources in the signal chain, but it might not be noticeable depending on the accuracy your system requires as well as the other signal chain components that you choose

    I'd suggest performing an error analysis with the existing system to see if it meets your requirements, then making adjustments if it does not 

    -Bryan

  • 0 in reply to Bryan Lizon86
    Intellectual 325 points

    Hi Brian

    I tried it. At 25°C it does make a difference. But at 60°C the rest of the componets become an issue.

    So I will leave room for one but I think we will not use it. 

    Thank you for your help. Really appriciate it.

    Regards Theo