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ADS1158: Converter Timing with External Clock

Joe Martin
Prodigy 20 points

Part Number: ADS1158

I wanted to find out what the timing and process is when using the ADS1158 with the external clock. When the supplies come up (AVdd and DVdd) along with the CLKIO signal, when does conversions begin? Is there some wait period like when using the internal oscillator?

  • 0
    TI__Guru 59541 points

    Hi Joe Martin,

    The power up timing is described in detail for both internal and external clocks in the "Power Up Timing" section. This is on page 22 in the ADS1158 datasheet

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 20 points

    Thanks for the reply, Bryan. So, I just have some confusion on how to verify the device is actually ready. Fig. 38 of the datasheet says after PWDN is high, the wake up time is 2/fclk then  the device is ready. will the device begin outputting data at this point? What pin can I monitor to ensure this is happening on time? I've included a logic analyzer timing plot that captured using the same signals as fig. 38 plus Dout to see when data is being sent out. Can you comment on this?

  • 0 in reply to Joe Martin
    TI__Guru 59541 points

    Hi Joe Martin,

    Are you controlling conversions using the START pin, or by using the PULSE command?

    If the START pin is high initially i.e. using a pull-up, the DRDY pin will toggle at approximately 1/data rate, which indicates that conversions have started and data is ready to be clocked out of the ADC. However, the device should not output data on DOUT if there are no SCLKs

    You analyzer plot does not show the state of START or SCLK, so it's hard to determine what is going on

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 20 points

    Hi Bryan,

    We are using the PULSE command and not the START pin to control conversions. 

    I added the SCK and START signals to the logic analyzer. This only added more confusion, as the SCK comes in 40 ms later with the external clock implementation as opposed to the internal clock implementation. The SPI clock is supplied from our uC and software is the same for both implementations.

    Internal oscillator

    External Clock

  • 0 in reply to Joe Martin
    TI__Guru 59541 points

    Hi Joe Martin,

    How are you currently determining when to send the PULSE command then? With the START pin low, the ADC won't do anything until you issue the PULSE command (other than output the clock signal on CLKIO assuming you are using the internal oscillator). Are you unable to issue the commands any sooner?

    -Bryan