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DAC8740H: question about carrier detection

Atsushi Hirai
Expert 4880 points
Part Number: DAC8740H

Hi team,

I received several questions about DAC8740H's carrier detection. Please support.

1) Is the starting point for carrier detection ON the first point where the MOD_IN voltage exceeds the 80-120mVpp threshold?

2) What is the starting point for carrier detection OFF? I cannot understand it bacause it's AC signal so, the voltage swing crosses the threshold voltage every cycle.

3) The maximum CD output delay is "6 Bit-times". Is it 5ms because the UART is 1200 baud rate?(1/1200 * 6 = 5ms)

4) Why the unit of "detect on" is Bit-times but unit of "detect off" is ms?

Best Regards,

  • +1
    TI__Guru* 97465 points

    Atsushi-san,


    1. Carrier detection for the DAC8740H starts when the device detects that the HART signal has exceeded the amplitude threshold. However, I would note that the timing in the datasheet is described such that the time to carrier detect starts at the start of the sinusoid.

    2. For the timing purposes, you would consider the starting point for the carrier detect off to be the point where the signal stopped and returned to the nominal "zero" position in the sinusoid. For both carrier detection on and off, there is added time to ensure the proper detection of the signal start and stop.

    3. You are correct in that 6 bit times is the equivalent to 5ms. The HART transmits at 1200 baud.

    4. I'm not sure why the detect on is specified in bit-times, and the detect off is specified in milliseconds. My guess is that we care about the communication detected by the modem and may want to know how many equivalent bits may have been transmitted on the loop. This timing may be important for reading the device and sequencing the HART data. At the end of the transmission, where the carrier detect turns off, there may be less need for coordinating the HART data. Regardless, 3 ms is 3.6 bit-times.


    Joseph Wu