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ADS1234 Data Rates

Jared Becker
Genius 11845 points
Other Parts Discussed in Thread: ADS1234

Hi Team,

Is the 80 SPS per channel or the total throughput of the A to D conversion? I am thinking it is the total throughput, but would like to verify.

Thank you,

Jared

  • 0
    TI__Guru** 113765 points

    Hi Jared,

    The output data rate (conversion time) is 80sps, however if changing the mux you must also consider that the ADS1234 uses a sinc4 digital filter.  It will take 4 compete cycles before the data is valid on the 5th cycle.  There is also a mux setup delay time and this is shown in Figure 32 on page 17 of the ADS1234 datasheet.  So when changing the mux you need to throw away the first four conversions.  The total time before valid data will be 40-50us setup time plus 51ms with the SPEED pin high (80sps mode) and thereafter the data output will be at 80sps (12.5ms). So it takes a minimum of 51ms (invalid data) plus 12.5ms (valid data) for a mux change.

    If you take one valid reading then change the mux the best overall throughput to read all 4 channels will be about 3-4 per second (63.5ms times 4 channels).

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    TI__Genius 11845 points
    Hi Bob,

    Ok, so clarify, the 80 sps is per one single channel, then you multiply this 80 sps times 4 channels to get the total throughput, correct?

    Best regards,
    Jared
  • 0 in reply to Jared Becker
    TI__Guru** 113765 points

    Hi Jared,

    There is a single ADC and 4 input channels for the ADS1234.  The maximum data rate is 80 sps (one conversion cycle in 12.5ms) and you have to switch between channels so you divide by 4 to get the value in samples per second or multiply the period by 4 (which is the inverse relationship).  However as the ADS1234 is not single-cycle settling when changing the mux, it is not possible to achieve this throughput and get valid results as the digital filter has not yet settled in the first cycle being a sinc4 filter.

    I may have been a little misleading with my last post in that you need to wait for the 5th conversion data to be fully valid.  This would be the case for a step change on a single channel.

    What will happen when you change the mux on the AD1234 is immediately the DOOUT/DRDY pin will go high and stay high until data is valid.  The pin will go low when the data has settled and is valid.  This will take 51ms, which is approximately 4 conversion periods.  If you read the data and switch the mux again you will wait another 51ms.  So to read the data and cycle the mux for all 4 input channels it will take 4 * 51ms or 204ms total throughput.  As frequency is an inverse relationship to time you can measure all four channels in about 5 times per second.  This would be the same as 80sps /4(conversion cycles) /4 (input channels).

    Best regards,

    Bob B