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LMT01: LMT01 / extra pulses

Thomas Frey
Prodigy 30 points
Part Number: LMT01

Hi there,

with the following set-up (signal generator VG1 supplies 3.3V square wave)

we get the following output. The first diagram looks as expected...

But looking at the position of power-up, we get this:

There is an extra pulse that can not be found in the data sheet. There is also a second, smaller pulse, which sometimes makes it through the comparator in the actual application circuit. In turn, this causes trouble in the application software. Is this to be expected? Are we dealing with faulty sensor elements? Are there any recommendations to ignore initial pulses over a specific time?

Initially, we suspected a problem with out circuit, so we took these measurements with just a signal generator and resistor. The diagrams here are the result.

  • 0
    TI__Guru* 91086 points

    Dear Thomas - 

    It seems that the images you meant to insert or attach did not make it. 

    would you please try inserting them again?

  • 0
    Prodigy 30 points
    Dear Josh,
     
    thanks for pointing that out. I pasted the images from a powerpoint presentation. Perhaps that didn’t work out.
     
    Anyway, I tried again.
  • 0 in reply to Thomas Frey
    TI__Guru* 91086 points

    Dear Thomas - 

    looks like that worked out now  - i can see the scope shots - thanks - 

    in this case, the tconv time (which includes power up time or device turn on time that is typically 3 ms after POR threshold of 1.2 V is exceeded.) is typically 50mSec. 

    if you have a look at Figure 1 on page 8 of the datasheet http://www.ti.com/lit/ds/symlink/lmt01.pdf, you can see that this time period is before the time period in which 88kHz data would be expected from the part. 

  • 0 in reply to Josh Wyatt
    Prodigy 30 points

    Yes, I did expect a tconv of 50 ms. But what we observe is a pulse right at power-up. This is nowhere shown in the data sheet.

    Does that mean that our sensor sample parts show an erroneous behavior?

    My understanding was that after power-up the sensor remains quiet for about 50 ms, then outputs its pulses and, after some additional time starts again.

    Our software discards the first pulse. But sporadically, the second little spike (as can be seen in the second scope diagram) gets through the comparator and causes a second pulse to be seen by the software. That in turn starts pulse counting and then timeout, leading to a measurement result of one pulse.

    I did not find any recommendation on how to deal with those extra pulses. So that makes me wondering whether there is anything wrong here.

  • 0 in reply to Thomas Frey
    TI__Mastermind 35895 points

    We haven't seen this issue, but you might resolve it with a filter. I recommend some R and/or C on the VP to limit the ramp rate of the power-on voltage. 

    ren