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ADS1292R Sampling Frequency

Vala
Expert 1110 points

Hi,

I have some doubts about the real data rate of each channel of ADS12929R, can someone please confirm my understanding about this issue. When the data rate is set to e.g. 125 SPS in the device, this means 125SPS for each channel, right? If so, the bandwidth of the signal would be 62.5Hz. As stated in the datasheet, in this case the cut-off frequency of the digital filter is 0.262 * fDR which would result in 32.75, meaning that the bandwidth of the final output signal would be 32.75 Hz, right? Am I making mistake about anything here?

Actually, I fed a couple of sinusoidal signals to the device, took the signal to MATLAB to see the frequency characteristics of them, and noticed that 125 is the correct data rate for one channel (frequency peaks were at the correct locations). I also tried to see if there is any attenuation after 32.75Hz in the signal but I didn't notice any.

I would appreciate if anyone can clarify this for me.

I also took a brief look at the TI's two documents about delta-sigma ADCs, but I didn't find the answer of this question.

Regards,

-Vala

  • 0
    TI__Mastermind 38745 points
    Hey Vala,

    Your understanding is correct; the -3 dB bandwidth for the 125 SPS data rate is 32.75 Hz. This means that the magnitude of signals with frequency less than 32.75 Hz will no be attenuated by more than 3 dB from their input magnitude and that signals with frequency greater than the cutoff will have attenuation greater than 3 dB. The filter is not a "brick wall" filter and so signals with frequencies greater than the cutoff frequency will appear at the output, albeit attenuated. Keep in mind that -3 dB corresponds to a multiplicative factor of 1/sqrt(2), so it may not be obvious unless you inspect it closely. The response of the filter is specified in a few handy charts (as well as by a Z-domain transfer function, if you're into the math behind it) beginning on page 23 of the datasheet and continuing on until page 25.

    Regards,
    Brian Pisani
  • 0 in reply to Brian Pisani
    Expert 1110 points
    Thanks. Now it's completely clear for me.