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ADS8598S: Additional group delay for Oversample

Raman Sridahran
Prodigy 130 points
Part Number: ADS8598S
Other Parts Discussed in Thread: ADS8598H

This part supports Oversampling modes through pins OS0 OS1 and OS2.  The datasheet states that the oversample is accomplished by taking multiple samples and averaging them together.

But, the datasheet block diagram on page 1 also shows a "Digital filter" connected to the OS[2:0] pins.

Does the oversample mode add any additional delays; atop the group delay of the 3rd order analog AA filters?  

Latency is important in my application and I am deciding whether or not to use oversample mode and a lower sample frequency.

  • 0
    TI__Mastermind 37955 points

    Hi Raman,

    The over-sampling increases the conversion time which leads to a higher cycle time and lower sampling rate. Please see the following table and also the timing figure 71 in the datasheet. For instance, the maximum sampling rate with over-sampling 4 is approximate 52ksps, and also approximate 3.45ksps for over-sampling 64.

    If a higher sampling is expected, the ADS8598H can be considered which is fully compatible with ADS8598S, the sampling rate 500Ksps on ADS8598H vs. 200Ksps on ADS8598S without over-sampling. The corresponding sampling rate with over-sampling is increased too.

    Thanks&regards,

    Dale

  • 0 in reply to Dale Li
    Prodigy 130 points

    Hi Dale,

    Yes; I know the time to conversion will scale proportionally with the oversample.  But is there any additional filter delay that is added?  ie; is the digital stream Low Pass filtered at the full sample rate before downsampling; to avoid aliasing? 

    My application is relatively low frequency; ~10kHz sample rate max; but latency is critical.  For example the full sample rate has a third order lowpass filter that adds a group delay.  Is there a higher group delay in oversampling mode?

  • 0 in reply to Raman Sridahran
    TI__Mastermind 37955 points

    Hi Raman,

    No additional delay. The LPF filter requires a settling time to settle your input signal, which needs approximate 80us for +/-10V range and 120us for +/-5 range, this requirement limits your maximum input signal frequency and it's not a group delay. The group delay is specified in the datasheet. 13us for +/-10V range and 19us for +/-5V range.

    Regards,

    Dale