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Determining The CF and RP Problem, LMP91300

Mehmet TASPINAR
Intellectual 355 points
Other Parts Discussed in Thread: LMP91300

Hi,

I couldn't configure CF and Rp_max / Rp_min section 8.2.2.2 because I don't have any impedance analyser.

Q1: From the section 8.2.2.2 and part 4th and 5th it described how to adjust Rp min and Rp max. What is the desired value of Rp?

Q2: For CF (section 8.2.2.4)  I never get the wave form as figure 21 and 22, although I have used variety of capacitor  between 150pF to 10n,

Could you please give me some advise?

(My circuit is PNP mode)

Best regards,

Mehmet

  • 0
    TI__Genius 14270 points
    Hi Mehmet,

    Let us look into this.

    Regards,

    ChrisO
  • 0
    TI__Genius 16890 points
    HI Mehmet,

    In section 8.2.2.2 you are trying to set the RP min and max values of the LMP91300 to match the LC tank that you have. The LC tank has a Rp value that varies with the distance of the coil from the metal target.

    Set you LC tank the distance from the target that you are interested in having as the switching distance. Start with the default values for RP min and max. Then using the instructions in section 8.2.2.2 adjust RP min and max. Just to make sure, the INA signal that you have looks like the signal in FIgure 21 of the datasheet?

    Then use section 8.2.2.4 to adjust the CF value. Do not have the metal target near the LC tank.

    Then recheck you RP min and max value and then recheck your CF value, if you changed the Rp min or max values.

    Mike
  • 0 in reply to Michael Stout
    Intellectual 355 points

    Hi Mike,
    I took three photo.
    Pic.1: with target as 4mm (1us/DIV)--Coupling DC
    Pic.2: No target (1us/DIV)--Coupling DC
    Pic.3: 50us/DIV, no target --Coupling DC

    yadi.sk/.../VQnvyyYEjFBN2

    Ind_Pic.rar

    as you can see from the picture, the switch just can open If the target is too close.
    Is there any problem?
    Because my oscilloscope result is not the same like figure 21 its datasheet.

    Note: I saw switch was on/off when the target was close. But I said before the switch open just the target is too close as under 1mm.
    If the target distance is bigger than 1mm, sensor never see the target.

    Please give some solutions

    thank you
    Best regards,
    Mehmet

  • 0 in reply to Mehmet TASPINAR
    TI__Genius 16890 points
    HI Mehmet,

    I was not able to access the pictures. Can you attach them to your entry in this forum?

    Thanks,

    Mike
  • 0 in reply to Michael Stout
    Intellectual 355 points

    Hi Mike,

    I have attached my photo in this forum.

    Could you please check it?

    3348.Ind_Pic.rar

    Pic.1: with target as 4mm (1us/DIV)--Coupling DC
    Pic.2: No target (1us/DIV)--Coupling DC
    Pic.3: 50us/DIV, no target --Coupling DC

    Thank you so much,

    Mehmet

  • 0 in reply to Mehmet TASPINAR
    TI__Genius 16890 points

    Hi Mehmet,

    I think the the waveforms in the pictures look ok.  They may look different from the datasheet picture because of the way that the scopes are sampling.

    To make the switch trigger at a certain distance you need to set the Threshold registers.  Set the target at the distance you want the switch to turn on.  Read the Rp value 0x70 and 0x71.  It is best to read multiple values and use the average.  You can use the continuous read function in the software and read several hundred values.  Put this Rp value in registers 0x66 and 0x67.  Then move the target slightly away from the LC tank (for example if the switch is to trigger at 4mm, move it to 4.5mm).  Again read the Rp value multiple times.  Put this value in registers 0x68 and 0x69.

    Now the switch will turn on when the target is closer than 4mm and turn off when the target is farther than 4.5mm away.

    Mike

  • 0 in reply to Michael Stout
    Intellectual 355 points
    Hi Mike,

    It has almost done my project. Thank you so much for your advice.

    About shield, how can I add shied on my circuit to avoid noise?
    or is there any pin to support shield on the LM91300?

    Best regards,
    Mehmet
  • 0 in reply to Mehmet TASPINAR
    TI__Genius 16890 points
    Hi Mehmet,

    To reduce the susceptibility to RF noise you can put a common mode choke between the LMP91300 and the LC tank. We have tried some tests using the common mode inductor SRF2012-670YA. You may also need to put some small capacitors (10-20pF) between the common mode coil and the LMP91300 connected from INA to ground and INB to ground. You may need to characterize the LC tank at this point including the common mode choke and the small capacitors.

    Mike