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ADS1261: Continuous conversion mode timing

Dennis McGlumphy
Expert 1390 points

Part Number: ADS1261
Other Parts Discussed in Thread: ADS124S08

Dear Sirs

I'm using the ADS1261BIRHBR ADC in one of my designs.  I need some clarification on the continuous conversion mode timing.  If I hold the start pin permanently high for continuous conversions, will the !DRDY go low at the end of each conversion?  If so, how long will it stay low?  If I use the AC excitation mode, will the same !DRDY sequence happen with a different timing?  Thank you.

  • 0
    TI__Guru 58711 points

    Hi Dennis,

    I think your first set of questions is answered in section 9.5.2 in the ADS1261 datasheet. I have copied this information below:

    AC excitation operation will be similar to normal operation, you will just have to wait longer between conversion results due to the input switching behavior. This causes the digital filter to reset each time. The behavior is similar to the image below, which is actually taken from the ADS124S08 datasheet. Note that this image specifically references the sinc3 filter.

    -Bryan

  • 0 in reply to Bryan Lizon86
    Expert 1390 points

    Dear Bryan

    I'm sorry for being such a dolt, but due to control timing issues, I need to thoroughly understand the data acquisition/data read timing.  As to understand the continuous mode correctly,  if for example, the sampling rate is 2400SPS and a SINC3 filter is used, (standard sampling, not AC excitation) the first data will be available in 1.675mS and the subsequent data will be available every .417mS there after.  For the same setup but using AC excitation, the first data will be available in 3.35mS and the subsequent data will be available every 1.675mS there after.  Therefore, in the continuous mode, the next conversion starts mediately after the previous conversion.  Do I understand this correctly?  

    In addition, so I understand the AC excitation averaging correctly, the +1 data and the -1 data are averaged together, then the -1 data and the +2 data are averaged together, then the +2 data and the -2 data, and so on.  Is this correct?  And this happens in the ADS1261?  

  • 0 in reply to Dennis McGlumphy
    TI__Guru 58711 points

    Hi Dennis,

    Your thought process and examples are correct. Note that the values given in Table 8 in the ADS1261 datasheet assume the nominal clock frequency is used, so if you change this value, the conversion latency values will scale. Similarly if you change the conversion-start delay. This also assumes your input signal is settled i.e. there are not step inputs occurring during the conversion process.

    The next conversion starts immediately after the previous conversion in continuous conversion mode.

    The AC excitation mode operates as you described (the first result is an average of conversions 1 and 2, the second result is an average of conversions 2 and 3, and so on). This is why there is 2x latency for the first conversion but only 1x latency for the second conversion (in continuous mode). This behavior is described in the Chop Mode section, 9.4.2, in the ADS1261 datasheet.

    -Bryan

  • 0 in reply to Bryan Lizon86
    Expert 1390 points

    Thank you for your conformation and help.