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ADS1255: Filter performance in one shot mode

Rick West
Prodigy 45 points
Part Number: ADS1255

I have an application where I need a very low noise A/D converter and I need to be able to sync it with 6 other converters so they sample at exactly the same time and at the same rate.  I'm looking at the ADS1255.  It has a sync input on it, but very little on how this works.  Is there an app note? Or is there another converter better suited?  Need 0 to 5V input range.

The datasheet says how to do this, but makes no mention of how this affects the filtering frequencies.

Thanks.

  • 0
    Prodigy 45 points

    I want to add, I cannot use a single multiple input A/D.  I'm aware they exist.  This A/D, along with the 6 others, will reside each on their own PCB and will communicate with a main PCB using RS-485 drivers.

  • 0 in reply to Rick West
    TI__Mastermind 34991 points

    Hi Rick,

    Is there something specific you want to know about the SYNC functionality on the ADS1255? It basically does what it says - it starts the conversion process after the command is clocked in (or the pin is toggled). If you have multiple ADCs on the same SPI bus, you can send the command to each of them by connecting a single DIN trace to the DIN pin on the ADS1255. Assuming the communication traces are similar in length, inductance, etc. (i.e. no delays), each ADC will receive the command and begin converting at the same time. If each ADC has the same output data rate selected, then you can use one of the DRDY signals to indicate when all data is ready.

    I am not sure I understand your question about how this would affect filtering frequencies.

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 45 points

    Hi Bryan,

    Is the past I've used SAR converters and tied their control signals together.  Here, I can either one shot it over and over or sync it once and hope it stays in sync.  I'd run a single system clock for all A/D's so that would probably work, but doing it with continual one shots would guarantee it.  My question has to do with how the sinc filter and digital filters work.  What kind of frequency response I would get using the continuous one shot method?  Does the frequency response have anything to do with the period between one shots, or is that purely system clock?  There's nothing in the datasheet addressing this.

  • 0 in reply to Rick West
    Prodigy 45 points

    In addition, would this have any effect on the noise performance?

  • 0 in reply to Rick West
    TI__Mastermind 34991 points

    Hi Rick,

    The frequency response is shown in the datasheet on pages 18-20. The filter 3dB bandwidth is included in Table 12. The settling time (time between toggling the SYNC pin and DRDY dropping low) is in Table 13. This settling time is also the conversion period, in that the ADC is continuously converting during this time.

    So if you selected the 1000 SPS data rate in the ADS1255 and toggled the SYNC pin, DRDY would drop low to indicate that new data is ready in ~1.18ms. As noted previously, you should monitor DRDY to identify when new data, or you can always try to time the data read with an external timer (or timer in an MCU). This latter method is best when combined with the SYNC pin so you know exactly when conversions starts. Otherwise the external timer and the ADC might get out of sync over time.

    The period between conversions does not have any affect on the digital filter or frequency response, since neither the delta-sigma modulator nor the digital filter are active during this time. Moreover, this does not affect the noise, for the same reason.

    I will also point you to our Precision Labs - ADC content that provides a lot of the fundamental information regarding the behavior and operation of SAR and DS ADCs: https://training.ti.com/ti-precision-labs-adcs

    -Bryan