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ADS1258-EP: Questions on Data Rate

Errol Leon6
Expert 5830 points
Part Number: ADS1258-EP

Hello team,

My customer is designing with the ADS1258-EP. They were reading the datasheet and had a few questions. Could you please provide info on following points:

  1. Data Rate for Auto Scan Mode & Fixed Channel Mode: I can see that in datasheet data rates are provided for both of these modes. As per my understanding, auto scan mode provides data rate per channel and fixed channel mode provides data rate for total of 16 channels, so to calculate per channel it would be (fixed channel rate/16). I want to make sure that my understanding is correct here for data rates of these modes. Please confirm?

  2. Switch-Time Delay– Do we need to add switch time delay in both the modes? And how do I know how much time delay should I add to calculate my output data rate?

  3. With Chopping – If I enable the chopping then as per datasheet equations, auto-scan mode and fixed-channel mode data rates are same. So I am not sure here what is the data rate for each channel in both of these modes. This question connects to the first one.

I am attaching the screen shot of datasheet, which shows equations to calculate data rates.

Thank you in advance for your support!

Errol

 

  • 0
    TI__Mastermind 35081 points

    Hi Errol,

    Here are the answers to your questions:

    1. The ADS1258-EP uses a sinc 5 filter, which requires 5 samples for first settled data. Second and subsequent settled data on the same channel will be available at 1/(Data Rate), assuming you are not using global chop, a delay, and there is not an input step. Since auto-scan mode is intended to loop through multiple channels, the ADC assumes it must wait these 5 samples (plus some minor overhead) for each conversion. That is why the effective data for 1 channel in auto-scan mode when DR = 11b is 23.7 kSPS, which is ~125kSPS / 5 (assuming no chop or delay). If you want to convert multiple channels in auto-scan mode, then the data rate reduces by the number of channels. For example if you wanted to scan 4 channels, your effective data rate would be 23.7/4 = 5.925 kSPS / ch. In fixed-channel mode, the ADC makes no assumptions about the external data and relies on the user to ensure there is settled data at the input, wait 5 conversions for the digital filter to settle, etc. You could consider this more like a manual mode. In this mode, if you were only converting 1x channel at DR = 11b, the first conversion would be available at a data rate of ~25 kSPS, while the second and subsequent conversions would be available at 125 kSPS (assuming no delay, chop, or step inputs)
    2. The programmable delay is generally used to allow external conditioning circuitry to settle so that the ADC does not sample unsettled data. For example, an  external RC filter would require some time to settle if you applied an input step, and this time could be greater than the time it takes the switch the mux to the next channel. This is where the delay would be useful. Table 6 in the datasheet summarizes the number of time constants required to reach a certain resolution. The time delay can be used in both modes
    3. Since chop takes one reading, flips polarity, takes another reading, and then averages them together, this is similar to the situation where the ADC is reading two different channels. Per #1 above, the ADC will have to wait 5 samples for settled data each conversion, so whether this is auto-scan or fixed-channel mode, the data rate is the same

    Let me know if anything is unclear

    -Bryan

  • 0 in reply to Bryan Lizon86
    TI__Expert 5830 points

    Thank You Bryan for detailed explanation. I have one more question regarding this: 

    1. For auto scan mode, as per datasheet ADC scans all the channels one by one, and there are total 16 channels. So if chopping is disabled, shouldn’t  the effective data rate (EDR) for each channel be 23.7k/16. And in fixed channel mode, we set the channels that need to be read. For example if we set 8 channels to be read in fixed channel mode then I would assume that in fixed channel mode, the EDR would be 23.7k/8 and in auto scan mode it would be 23.7k/16.

    Is the above assumptions in calculating these data rates correct?

    Thanks!

    Errol

  • 0 in reply to Errol Leon6
    TI__Mastermind 35081 points

    Hi Errol,

    If you want to sample the same # of channels with only one conversion per channel at the same data rate, then yes the auto-scan and fixed channel modes will provide approximately the same data rate per channel. But the ADC will not automatically wait for settled data in fixed mode, this is something the user must control manually.

    The fixed channel mode would be useful if you wanted to take multiple conversions per channel at a higher throughput rate, which would not be possible with auto-scan mode (even if you selected only 1x channel).

    -Bryan