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Original question:

ADS124S06: Differential measurement and ADC counts output

ADS124S06: Conversion time with channel switching

Surya Mohan
Prodigy 160 points

Part Number: ADS124S06

Hi there,

We are using ADS124S06 ADC with sinc3 filter.

We are switching between two ADC channels by changing the value of INPMUX register at run time. Is the first conversion time applicable after every switching between 2 channels or it takes the subsequent conversion time?

Will the timing remain same if we use single shot or continuous conversion mode in the above switching scenario?

Regards,

Surya Mohan

  • 0
    TI__Guru** 113765 points
    Hi Surya,

    For the Sinc3 filter, the first valid data occurs 3 conversion periods after the start of conversion. Start of conversion would occur when the mux input is switched. This is illustrated in Figure 72 of the ADS124S06 datasheet on page 46. It is not necessary to count conversions as the DRDY signals valid data are available to be read from the device.

    So after the mux switch there will be approximately 3 times the nominal data rate before the first valid data. The specific times are shown in Table 15 on page 45 of the datasheet. If the Programmable Conversion Delay is used, you will need to add this time as well as the start of conversion will be delayed by this amount.

    In the contents of Table 15 you will notice that there is one column that says "FIRST DATA FOR CONTINUOUS CONVERSION MODE OR SINGLE-SHOT CONVERSION MODE". This means that the first conversion, no matter which mode is used, will always be longer due to the Sinc3 filter settling and only valid data are given as a result. If you are running in continuous mode, then the result following the initial result will be output much faster and will follow more closely the nominal data rate.

    Whether you use continuous mode or single-shot mode the first conversion result will take the same amount of time, with the only real difference is you must allow time for the Start command to be issued for single-shot mode. The advantage of single-shot mode is that it is easier to keep track of which conversion is taking place. If you take repeated measurements for each mux input (what I mean here as an example is switch the mux, take 3 measurements, switch the mux take 3 measurements, etc.), then continuous is much faster. Repeated measurements of this type will require the Programmable Conversion Delay to be used in each single-shot Start of conversion.

    Best regards,
    Bob B
  • 0 in reply to Bob Benjamin
    Prodigy 160 points
    Hi there,

    In the case of sinc3 filter at 5 sps in continuous conversion mode, where I am taking 3 measurements for the first signal, then switch the mux again take 3 measurements for the second signal. In such situation, when I again switch back the mux to measure first signal, what would be the time taken for collecting 3 samples? Would it be 600.254 ms or 200 ms?

    Thanks and Regards,
    Surya Mohan
  • 0 in reply to Surya Mohan
    TI__Guru** 113765 points

    Hi Surya,

    We will assume that the internal clock for the ADS124S06 is being used.  Looking at Table 15 in the 5sps row, the Sinc3 filter takes 600.254ms for the first conversion and 200ms for each additional conversion.  So it really doesn't matter which signal is being processed as they will be the same.

    Programmable Conversion Delay + Time for first valid conversion + Time for 2nd valid conversion + Time for 3rd valid conversion

    If we assume the default setting for programmable conversion delay and 5sps using the Sinc3 filter the time for 3 samples following a mux channel change would be as follows:

     0.055ms + 600.254ms + 200ms + 200ms = 1000.309ms

    This results in both signals measured every 2000.618ms using the Sinc3 filter at 5sps.

    As a comparison, if we take the low-latency information from Table 13, the first conversion completes in 206.504 ms and each following completes in 200ms.  The calculation for the low-latency filter is:

    0.055ms + 206.504ms + 200ms + 200ms = 606.559ms

    This results in both signals measured every 1213.118ms using the low-latency filter at 5sps.

    Best regards,

    Bob B