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ADS1263 Chop mode

Julien Erdogan
Intellectual 420 points
Other Parts Discussed in Thread: ADS1263

Hi,

I'm currently designing a device using the ADS1263 and I have a few questions concerning the chop mode funcitonality and the data rate.

My settings are the following:

- Data rate: 400 sps
- Filter: sinc3
- Chop mode: ON
- Programmed delay: 0ms

I understand that the chop mode performs 2 acquisition to cancel the offset voltage but it is not mentioned how it works internally.
More importantly in the specification you can read "As a result of the delay required by the digital filter to settle after reversing the inputs, the chop-mode data rate is less than the nominal data rate, depending on the digital filter order and programmed settling delay.".

Also, I get that the chop mode multiplies by 2 the setting time of the filter as stated here "Chop mode: latency = 2 · td(STDR)"

Can you tell me more about it ? I don't understand how the digital filter order impacts the data rate. Will the ADC performs its acquisition at 400 sps or will the data rate change ?

Does the chop mode requires to wait the whole time for the filter setting ? (for information the setting time for sinc3 filter at 400sps is 8ms)

Thank you in advance.

Julien

  • 0
    TI__Mastermind 44680 points

    Hi Julien,

    Section 9.4.11 in the datasheet discuss the chop mode. However before clarifying how chop mode works, I need to discuss the digital filter...

     

    SINC3 Filter
    The digital filter behaves like a moving average filter. As you change the order the order of the digital filter you are essentially changing the number of averages performed by the filter. You can compare the SINC3 filter to a moving average filter that averages the last 3 conversion results. Therefore, at least three conversions need to be completed before the SINC3 filter's output result is valid. (You don't need to keep track of this because the /DRDY signal does not go low until the SINC3 filter has averaged 3 conversion results.) After the third conversion, /DRDY starts toggling with every new conversion result (with a period of 1/Date Rate). Therefore, it is only when you start or re-start the ADC conversions that you have to wait ~3 periods to get data.

     

    Chop Mode
    The chopping control block comes after the digital filter (as shown in Figure 102). When chopping is enabled, the input MUX swaps the positive and negative inputs and subtracts and averages subsequent conversion results to remove any internal offset voltage. The internal offset voltage is removed because the polarity of the offset voltage remains the same between conversion results, while the polarity of the input voltage is changing - hence subtracting and dividing by 2 removes the offset - refer to the following document for additional details on chopping:

    Now, in order for the chopping control block to receive valid data, it too must wait for the SINC filter data to be settled. So when you use chopping with the SINC3 filter the ADC will perform the following operations:

    1. Collect 3 conversion results (SINC3)
    2. Average these results and send the averaged value to the chopping control block
    3. Switch the positive and negative MUX inputs and re-start ADC conversions
    4. Collect 3 conversion results (SINC3)
    5. Average these results and send the averaged value to the chopping control block
    6. The chopping control block takes the two results and subtracts and divides by two to produce a final result (/DRDY goes low)
    7. Repeat steps 3-7.

    So when chopping is enabled the settling time for the first valid output takes twice as long (2*td(STDR)) and every other conversion result thereafter takes the usual settling time (td(STDR)) because the ADC must restart conversions each time the input MUX switches the positive and negative inputs.

    Typically, the first conversion result will always be slow (to allow for filter settling + any post-processing), but thereafter data comes out at regular intervals... When chopping is off, the regular interval will be the data rate period (1/Data Rate), but when chopping is enabled the regular interval will be much slower depending on the filter selection. The SINC1 and FIR filters are the only single-cycle settling filters, so chopping won't slow down the regular data period intervals as much when used with these filters.

    I hope that clarifies things for you!

    Best Regards,
    Chris