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ADS1252 data valid after switching analog input

Gordon Su
Intellectual 285 points
Other Parts Discussed in Thread: ADS1252

Hi,

I am using ADS1252 to sample multiple channels with external analog front-end and multiplexer. I'd like to know how soon data will be valid after I switch from one channel to another channel. do I need to wait for 2 conversion cycles (2*384*clk) before I serial out the data? Upon power up data is valid on the sixth conversion cycle . I guess I should not have to wait that long since the IC is powered. please advise.

as I am interfacing this IC with FPGA I wonder whether you've a simulation model available?

 

thanks

BR

Gordon

  • 0
    TI__Mastermind 44550 points

    Hi Gordon,

    The ADC conversion data passes through a SINC5 digital filter. This filter essentially averages the last 5 conversion results. Therefore, if your input signal presents a step response to the ADC, it will require at least 5 conversions for the digital filter to output a "settled" value. Otherwise, your data will be some averaged combination of past and present data.

    We say to allow for at least 6 conversion cycles, because the multiplexer or step input is most likely to occur "mid-conversion", meaning that the first conversion result is also some form of past and present data. Therefore, allowing for 6 conversions after switching the multiplexer will ensure all of the measurement averages reflect only the present input signal!

    Another thing to take note of is the analog filter settling time. If you have a large RC circuit placed after the multiplexer, it will take some time for the capacitor to charge up to the input voltage. Since the ADC would be sampling the voltage present on this capacitor, then this circuit need to settle BEFORE counting the next 6 conversion cycles.

    I would recommend placing any large filtering before the multiplexer (so that it settled before the multiplexer switches), and then using a smaller differential filter after the multiplexer that settles quickly.

    Sorry, I'm afraid there is no simulation model from the ADS1252. If there was, it would be found on the product webpage.

    Please let me know if you have any additional questions!

    Best Regards,
    Chris

  • 0 in reply to Christopher Hall
    Intellectual 285 points

    Thanks Chris. I am clear now.