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ADS1285: sync and sps

Massimo Zennaro
Prodigy 10 points
Part Number: ADS1285

Tool/software:

Hello all,

 

I am using three ads1285 to sample two geophone sensors, so I am using sync signal + mux command.

 

My output data is the sinc filter, so, with sps=256 (8KHz) I am getting about 1.2KHz.

 

Our customers asks for “round” numbers, like 1KHz: can I achieve this goal?

  • 0
    TI__Mastermind 46880 points

    Hello Massimo,

    Welcome to the TI E2E community.

    Can you explain what '1.2kHz' refers to?  Is this the useful input signal bandwidth?

    Operating at 8ksps, the -3dB bandwidth of the SINC filter will be 1.632kHz.  You can adjust this frequency by adjusting the clock frequency.

    fCLK=1kHz/1.632kHz*8.192MHz=5.0196MHz

    Also, the ADS1285 SINC filter is only intended to be used with additional external filtering that limits the input bandwidth.  The effective bandwidth of the FIR filter is 0.375 times the data rate.

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Prodigy 10 points

    Hello  

    in the following picture you will find some explanation

    we have a shift register (SR) for the activation of spi ss (spi to parallel)

    D0 is the spi clock, followed by SR signals. The last clock "spike" is the command to switch the mux on the other input

    The last two are drdy and sync (pulse mode). In this picture we are activating sync before every other activity but the same happens if we move sync after changing the mux.

    As you can see, between the two drdy there are 817 us = 1.2 KHz (612 Hz per input)

    Our customer wants round numbers, like 500 Hz, or 1 KHz, so the question is: can I achieve this goal?

    Thank you

  • 0 in reply to Massimo Zennaro
    TI__Mastermind 46880 points

    Hello Massimo,

    When you change the input channel, and then send a SYNC pulse, this resets the internal digital filter, which takes more than 1 data rate period to settle (between 5.5 and 6 data rate periods for the SINC filter, but I do not have an exact number).  I think this is the behavior that you observe.

    The ADS1285 typical use case reads multiple conversion results from a single channel, in which case the data rate will be as indicated in the data sheet.  If you change your timing to send the SYNC pulse after the mux change and with an interval of 1000 us, you can achieve an exact 1ksps data rate.  In other words, regardless of the ADC data rate setting, you can control the data rate with the SYNC period.

    Regards,
    Keith