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ADC Sample Frequency in Burst Mode

Zack Bomsta
Intellectual 470 points
Other Parts Discussed in Thread: CC430F5137

Hi All,

I am doing a little digital signal processing on the CC430F5137 and came across something that has stumped me. I am trying to implement a FIR hamming window low pass filter. I have all the equations needed to figure out the windowing coefficients; however, I am not sure what to use for my sampling frequency. That is because my ADC is being used to sample a signal 4 times at 500KHz every 1 millisecond (burst mode). How do I determine the sample rate given that the ADC is operating this way? Would it be 500KHz, an average of 500KHz over 1 millisecond, or something different all together?

If anyone has some good documentation on this, I would appreciate it if you could send it my way.

Thanks! 

  • 0
    Genius 3155 points

    500kHz?

    Looking at datasheet (12-Bit ADC, Timing Parameters, pag. 65), specified performance linearity parameters require ADC12CLK < 5MHz, so I'd say this is infeasible, as minimum conversion time is 2.6 us, and you have to account for sampling time too.

    Regards,

    Peppe

  • 0 in reply to Piergiuseppe Tundo
    Intellectual 470 points

    You're right... I am sampling 4 times at 125KHz every 1 millisecond. Let me rephrase the question as a more general case so as to not get caught up in pointless technical details that do nothing to answer the real question...

    The ADC is being used to sample a signal 4 times at x KHz (x = any frequency between 0.001 and 1,000,000,000. You decide, it doesn't really matter) every 1 millisecond (burst mode). How do I determine the sample rate given that the ADC is operating this way? Would it be x KHz, an average of x KHz over 1 millisecond, or something different all together?

    Thanks,

  • 0 in reply to Zack Bomsta
    Genius 3155 points

    I'm not sure if I am understanding well... are you taking 4 samples spaced 1/125k s every 1 ms? Or what?

    Regards,

    Peppe

  • 0 in reply to Piergiuseppe Tundo
    Intellectual 470 points

    Peppe,

    That's correct. I am taking 4 samples each spaced 8 micro seconds apart and then waiting 1 millisecond before taking  the next four samples. I believe this is commonly known as burst mode.

  • 0 in reply to Zack Bomsta
    Genius 3155 points

    IMHO burst sampling makes sense to scan different channels (4 in your case) "simultaneously" at a given frequency (1 kHz in your case)... as e.g. reported here.

    Thus if you are sampling only one channel, I'd say your equivalent sampling frequency is 1 kHz and you'd have to input each n-th sample to your FIR filter, i.e. bursting is useless and you could take only one sample per ms.

    Regards,

    Peppe

  • 0
    Guru 227245 points

    A few general things:

    The ADC12 can only sample one channel at a time.
    Samplign takes 13 ADC12CLK cycles plus the sampling time. If you use minimum sampling time of 4 ADC12CLk cycles, with a clock of 6MHz, this means the sample is a low-pass (switch resistance and sampling capacitor size) of the 666ns sampling period. Each sample of your 4-sample burst is 2.8µs apart (which would be 353kHz sampling frequency) and of course you don't have a clue what your signal might be during the remaining 988µs of your 1ms interval.

    Not to mention that with 666ns sampling time, the sampling capacitor doesn't have a chance to fully charge, so the results likely are quite a bit off.

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