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PGA460-Q1: Rationale for the "zero" of the ultrasonic measurement result begining after the burst stage is complete

user3263001
Prodigy 60 points
Part Number: PGA460-Q1

According to section 7.3.6.2.3 of the datasheet of PGA460-Q1 (SLASEC8B –FEBRUARY 2017–REVISED JANUARY 2019), the "zero" of the ultrasonic measurement result for the BURST/LISTEN command begins after the burst stage is complete (see the attached picture (Figure 32 of the datasheet)).

Why is this so? It seems a little odd to me, because the time when the echo signal rises above the time-varying threshold should be (ideally)

   2 * (distance to the object)/(speed of sound) + time when the burst begins

The second term seems to be replaced by "time when the burst ends" in the model the actual implementation assumes. I'd like to know the rationale for this.

Thank you.

  • 0
    TI__Genius 15220 points

    We appreciate you for reaching out with your question. I do see how this can seem a little misleading. The total burst time is included in the end TOF data for distance calcualtions. This does result with a total distance calculation of 

    I believe this is represented in the diagram as a time starting at the end of the pulse as the device cannot listen at this time.  So we have a set decay time which is then added to the listen time for the indicated tobject.  The timing of the bust would also need to be added into this internally to produce the total TOF measurement which would be indicated by the equation above.

    Thank you,

    Scott Bryson  

  • 0
    Prodigy 60 points

    Thank you for the reply.

    So, for clarification, are the 2-byte data returned through UART the one given in the figure attached? 

    (Sorry I can't try myself because I don't have access to a high precision measurement result at this time.)

  • 0 in reply to user3263001
    TI__Genius 15220 points

    Thank you for following up again.  I was able to find more information and the figure is correct as shown in the datasheet. I apologize that I missed this when I first checked the datasheet

    That is, you must manually add the length of the burst to your measurement manually in post processing for the most accurate result.

  • +1 in reply to Scott Bryson
    Prodigy 60 points

    Oh, I've missed that, too. Thank you for the clarification.