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ADS125H01: Sample at 60Hz with sync4 in pulse conversion mode?

Rick West
Prodigy 45 points
Part Number: ADS125H01

I have a number of these A/D's (ADS125H01) that I want to keep in sync with each other.  They are run off a common clock (7.3728MHz) and they have all their start pins tied together.  I was intending to start all the converter samples together in pulse mode so I could be sure they would stay in sync.  I wanted to use the sinc4 filter, but I don't know how this can work and the datasheet doesn't seem to address this, not directly, anyway.  I want to sample at 60Hz, but from what I'm reading, if the start pulse comes again before the sample is done converting, it will restart a new conversion.  How can the pulse mode be used with these filters?  It seems dangerous to just start them all together in continuous mode and expect them to stay in sync for long periods of time.  Granted, if they're all configured exactly the same way, run on the same common clock, and all start off the same start pin connection, they should stay in sync, but for some reason it makes me nervous.  Is there a way to use pulse mode with the digital filters, or should I abandon that method and use continuous?  Thanks.

  • 0
    TI__Mastermind 49921 points

    Hi Rick West,

    The normal procedure to follow would be to toggle the START pin, then wait for the DRDY pin to transition high to low, then clock out data. When you see DRDY drop low in this case, you can toggle the START pin again to restart conversions before you read data (because the data is buffered for one conversion cycle). This is basically shown in the datasheet, see the image below

    So to directly answer your question, using pulse mode with the digital filters is an intended way to operate this ADC

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 45 points

    So if I set the filter to sinc4, the sample rate to 60sps, the mode bit to pulsed, and pulse the start input at 60Hz, all will be well?  I assume each conversion will complete before the next start pulse, but I don't see a spec on how long that takes.  Is it there and I'm missing it?  Thanks.

  • 0 in reply to Rick West
    TI__Mastermind 49921 points

    Hi Rick West,

    No, this is not the case. You should set an interrupt to wait for the DRDY signal to transition high to low, not try to time your pulses. If you want to continuously sample, I would use the continuous conversion mode and then re-issue the START signal at some regular interval to ensure that all devices are synchronized

    The time it takes the ADS125H01 to output data is given in Table 9-6 in the datasheet. Note that at 60 SPS and Sinc4, the time to valid data between pulling START high and DRDY dropping low is 67.09 ms. This is valid for the first conversion in continuous mode or every conversion in pulse convert mode. Second and subsequent conversions are available at ~1/data rate in continuous conversion mode. This is described in section 9.4.1.3 in the datasheet. You can refer to this application note for more information about conversion latency: https://www.ti.com/lit/an/sbaa535/sbaa535.pdf?ts=1692026206273

    -Bryan