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ADS1258 Sampling Rates

Justin Forrester
Expert 1015 points
Other Parts Discussed in Thread: ADS1258

I have been reviewing the ADS1258 datasheet and I notice that it can be configured to use several set data sampling rates.  From what I understand, the lower the sampling rate, the more digital filtering is done on the signal data.  We may need to sample at a variety of rates, some lower than those listed as a configurable option.  What would the be the best way to sample the ADC lower than the configured rate and what implications would there be in doing so? 

Thanks!

  • 0
    TI__Mastermind 23235 points

    Mr Forrester -

    In trying to understand your question better, are you trying to run slower than the minimum sampling rate of the converter? Or are you trying to take a sample then maybe power down and/or do some other process before reading a sample again.  Conversions are controlled with the START pin of the device, so you can control when the device performs a conversion.  Details of the START pin and different conversion methods are detailed in the data sheet on page 27.

  • 0 in reply to Greg Hupp
    Expert 1015 points

    By having a lower sampling rate, I simply mean obtaining a reading from the ADC at a lower rate than the configured SPS. 

    So, if I were to use the START pin method and I toggle that line at a rate of 10 Hz to obtain a reading that frequently, would that have any impact on the quality of the data that I obtain than if I were to sample at the configurable SPS rates?

  • 0 in reply to Justin Forrester
    TI__Mastermind 43915 points

    Hi Justin,

    There are a couple ways that you can lower the data rate depending on what you want to do. As Greg mentioned, the most common and easiest way would be to simply use the START pin to perform more of a "single-shot" conversion method. Just note that if you use this method, there is going to be an increased initial delay as shown in Figure 56 and Table 11 on page 28. A second option would be to adjust the data rate bits to set the data rate accordingly. Doing, so will change the number of averages used for each conversion and effectively change the digital filter behavior. If that is what you want to do, then that is the route to go. The last option would be to adjust the master clock to slow the modulator sample rate and decrease the conversion rate. Doing so would let you keep the device set up for continuous conversions at the data rate that you choose. The data rate will scale with the master clock as all the data rate limits are calculated using the 16MHz clock. You would just need to calculate to see if it is possible to slow the master clock down enough in order to meet the data rate that you would like. I believe the slowest that the master clock can run is 100KHz according to the data rate specs.Equations 2 and 3 in the data sheet can be used to determine how the data rate scales with the master clock.

    My recommendation is to use the START pin, it would be the most convenient want to do it.

    Regards,

    Tony Calabria

  • 0 in reply to Tony Calabria
    Expert 1015 points

    Thank you for the thorough response. 

    When using the different methods of obtaining samples at different rates, what impact does that have on the effectiveness of the internal digital filtering?

  • 0 in reply to Justin Forrester
    TI__Mastermind 43915 points

    Justin,

    It has to do with where the digital filter notches appear in the Sinc filter. See Figures 42 and 43 for more information.

    -Tony

  • 0 in reply to Tony Calabria
    Expert 1015 points

    Okay, I have a couple questions regarding these figures. 

    Is the number of averages automatically set when you change the data rate values? 

    When you are sampling in autoscan mode and are scanning across four channels, how does this effect the response of the digital filter, if at all? 

    Lastly, if we use the START pin and sample lower, lets say at 500 Hz, where would the 3db point be for the filter? Would that change depending on how many channels we sample on?

    If you think this is the wrong application for the ADC, would you be able to recommend an alternate?

  • 0 in reply to Justin Forrester
    TI__Mastermind 43915 points

    Hi Justin,

    1. The oversampling rate and modulator sample rate does not change, so the number of average is what changes when the data rate is changed. It is automatically set.

    2. It does not effect the response of the digital filter. Digital filter response is based off of the selected data rate since that is what determines how the input is sampled.

    3. The 3dB point is dependent on the data rate which you have the converter set to. Using the Start pin is not going to change the sample rate or modulator rate, it is just going to effect when you read a conversion back.

    4. Continuous monitoring is common for Delta Sigma converters. SAR converters are generally better off and more common for a "one-shot" type of measurement. That being said, I do not really know what the application is so I cannot really comment if the ADS1258 is the right choice. Could you give me a little more info on what you are trying to do?

    Regards,

    Tony

  • 0 in reply to Tony Calabria
    Expert 1015 points

    Thank you for the explanation.  That does clear things up.

    Regarding the application, this is the type of sampling we want:

     

    Upper Range: 10,000-20,000Hz on at least 2 channels

    Lower Range: 10-500Hz on multiple channels (up to 8)

    For most of our applications, we would be aiming for the lower rates, although sometimes we need higher.

    In the past, we have been providing all the filtering circuitry.  At this point, we would like to be able to largely eliminate all of that and go with an ADC that has internal filtering.  However, it is important that we are able to achieve a filter point that makes sense for our sampling rate.