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ADS1248 questions

Takumi Suzuki1
Mastermind 8610 points
Other Parts Discussed in Thread: ADS1248

Hi,

I would like to ask you some questions about ADS1248 as below.

Q1) I do not understand "Overload" (data sheet p34) well.

       I think "overload" means that internal circuit setting（MUX, PGA, etc) time is slower than command transmitter time from host CPU.

       Is my idea correct?

       If yes, I suppose that "NOP" command is inserted between hard setting commands is useful instead of 2 separate command issue.

       And could you tell me how long does it takes to change hard circuits condition from command is received?

Q2) Calibration

       Could you advise me how frequent we should calibrate better?

       We are planning to use 4-differential inputs to ADS1248. (0 - 2ch : PGA x32, 3ch : PGA x1)

 Thanks,

Best Regards,

  • 0
    TI__Guru* 98025 points
    Takumi-san,


    On page 34 of the datasheet, the overload condition refers to the PGA. Let's use an example. If the input is set at 1V and the PGA is 1, then the amplifier works and it is able to give a good reading. However if you then switch the PGA to 64, then the PGA becomes overloaded because the amplifier cannot reach the correct output voltage. Even if you set the input back to 0V, then there may be a significant amount of overload recovery time.

    The datasheet also refers to making the chopper unstable if you change data rates while in the overloaded state.

    I don't think that inserting NOP commands will help with this type of problem. I don't have any data on how long it takes to recover from an overload, however I will check to see if there are any recommendations on it.

    According to the datasheet, a calibration should be done after powering on, a change in temperature, a change in PGA or sometimes a change in channel. Basically, you want to run a calibration any time there is a change in the configuration or when you think the ambient conditions have changed enough that the drift is enough that you would want to remove the drift error.


    Joseph Wu
  • 0
    TI__Guru* 98025 points

    Takumi-san,


    I did check into this problem and found that it is sometimes seen in the 2kSPS data rate for a couple of reads. In this case the problem lasts ~1-2ms.

    This could happen if someone is reading data from multiple inputs and using multiple gains. The way to avoid this problem would be to set the register settings in two steps so that the PGA never over-ranges. If the measurement goes from a large signal in PGA=1 to a small signal using PGA=128. Set the mux first so that the input is small and then set the PGA.

    If the measurement goes from small signal to a large signal, set the PGA to the lower value and then set the mux. In each case, it avoids the over-ranged PGA.


    Joseph Wu

  • 0 in reply to Joseph Wu
    Mastermind 8610 points
    Hi Joseph-san,

    Thank you very much for your answer and kind support!!
    I understood well the mechanism.

    Best Regards,