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ADS1248: When exactly SDATAC command stops updating output shift register?

Part Number: ADS1248

We have ADS1248 with START pin connected to high level (so continuous conversion) and we don't observe DRDY output pin. The ADC default state is RDATAC (read data continuous). To read ADC register without risk of being updated by new conversion result we need to switch to SDATAC mode (stop reading data continuous). 

My questions are:

1) when exactly SDATAC command gets effective?

2) without observing DRDY pin (and not knowing sample rate selected) what is secure procedure with applying SDATAC command?

Notes to Q1:

Datasheet says: "The SDATAC command takes effect after the next DRDY.". I also went through forum threads and found @Joseph Wu comment saying "If you use the SDATAC, it'll take place after the current conversion completes." In other thread the same guy also mentioned "We were talking about it here and we had thought that it the updates would stop after the next /DRDY indication after the command, but we're not sure."

So it rather looks like SDATAC is not effective immediately after command is written but end of conversion (DRDY pule) is this moment when it gets effective. Can somebody confirm?

Notes to Q2:

Is it the case that not knowing sample rate before issuing SDATAC and not observing DRDY the only safe procedure to be sure that SDATAC got effective and new data from conversion cannot spoil register readout is to issue SDATAC command and wait max time conversion can take which is 200.3ms max? Quite long procedure, any other possibilities?

Regards

Wojtek

  • Hi Wojtek Jodlowski,

    The datasheet (and Joseph) is correct: the SDATAC command takes effect after the next DRDY, which is equivalent to the current conversion completing (DRDY will toggle high to low when the current conversion completes)

    For question #2: if you don't know what the data rate is and you cannot monitor DRDY, then you must wait the maximum conversion time plus some margin to be sure. I would strongly encourage you to instead monitor DRDY. I am not sure why this pin cannot be monitored in your system, but it seems like it would help you out a lot if you could.

    -Bryan