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ADS8694: Some question on Alarm Feature

Lin_Charles
Expert 4260 points

Part Number: ADS8694

Hi all, I have some question about the alarm feature of ADS8694. In the datasheet 8.3.11,  the alarm signal will be triggered when an alarm condition is encountered for a particular channel. But the resolution of the ADC is 18 bit, and the data of threshold register is 16 bit. How these two signal compared in the internal circuit?

And the state will be changed on the 16th falling edge of the SCLK signal. It starts from when the CS is low?

  • 0
    TI__Expert 4260 points
    Hi all,

    Any update message on this issue? Thanks a lot!
  • 0
    TI__Mastermind 25070 points
    Hello Charles,

    Yes, you need to have CS low, this initiates when you should start counting. I do not believe there is a idrect corelation between the 16 clock count and 18 bit, but I if I had to guess , I would say that this might be related to the effective number of bits.
    Regards
    Cynthia
  • 0 in reply to Cynthia
    TI__Expert 4260 points
    Hi, Cynthia

    Thank you for your reply. I am not clear about how the internal circuit work. Can you give me more explanation on the alarm feature? Does they work on the same clock, right? Maybe the circuit convert the digital to analog first, and then compare each other, right?
  • 0 in reply to Lin_Charles
    TI__Mastermind 25070 points
    Charles,
    The device does not convert the digital threshold of the alarm to analog. What happens is the analog input signal is converted to its digital value and then compared to the digital alarm threshold. They both work on SCLK.
    I looked more into the correlation between the 18 bit resolution of the device and the 16 clock count. The alarm register has a 16 bit threshold, thus the digital threshold you can program into the device is 16 bits long. This 16 bit threshold is then compared to the 16 MSB’s of the device input. Because the alarm is only 16 bits long, the alarm only needs 16 SCLK counts.
    The device still has an 18 bit resolution and data output.

    Hope this helps,
    Cynthia