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LDC1101 LHR Mode optimal Rpmin setting

Jan Rosenfelder
Prodigy 90 points
Other Parts Discussed in Thread: LDC1614

Hello,


there is a description how to set the optimal RP in SNOSD01C:

"The sensor amplitude must remain between 0.25 Vpk and 1.25 Vpk for accurate L measurements."

But what is Vpk is this Vpp? with 1,2V? So that mean the optimal sensor amplitude is between 0,3 - 1,5V.

The optimal Amplitude for LDC1614 is between 1,2 and 1,8 V ... so thats a little bit higher.

Best Regards

Jan

  • 0
    TI__Genius 13335 points
    Hello Jan,
    Vpk means Vpeak, which is essentially Vpp / 2. This just means that if you are measuring on an oscilloscope you should tune the RP_SET.RPMIN value to achieve an oscillation voltage amplitude between 0.25Vpk and 1.25Vpk. Note, this is for the LHR mode of operation.
    The LDC1614 has a different set of requirements which is between 1.2V and 1.8V as you pointed out.
    Regards,
    Luke
  • 0 in reply to Luke LaPointe
    Prodigy 90 points

    Hello Luke,

    i am not sure about the RPmin.

    This was my steps to measure the Rp:

    1.) Mesure the Rp and L with an Impedance Analyzer as described in SNOA936.

    I Measured that without any capacity ... is that right?

    2.) I got this result:

    As you can see the resonance frequency of the coil is about 8,3 MHz and i want to operate the coil with 1MHz.

    3.) So i measured the Rs @ 1MHz (5,74998 Ohm)  and  Ls @ 1MHz (21,3158µH) and use the formula:

    and the result for Rp is 3,12k Ohm

    4.) Now i use a Capacity with 1,2nF for operating @ 1MHz
    5.) So I set the Rpmin to 3kOhm in LHR - Mode.

    6.) And measured the Voltage with an Oscilloscope and get following result:

    Distance to an aluminium target:

    Distance to target

    Vpp

    10mm

    1,3V

    8mm

    1,28V

    6mm

    1,24V

    4mm

    1,15V

    2mm

    0,86V

    1mm

    0,52V

    0,7mm

    0,4V

    0,6mm

    0,3V

    0,5mm

    0,25V

    Is the setting right for the Rpmin?

    Best Regards Jan

  • 0 in reply to Jan Rosenfelder
    TI__Genius 9300 points
    Hi Jan,

    When a conductive target approaches a coil, its inductance drops, and Rs increases.
    Can you measure coil parameters with the target at 0.5mm (if that's your intended minimum distance between the coil and target)?
    Then you can calculate RpMIN which will be able to drive the coil under full load (target at closest proximity)

    Please also note, that your oscillation frequency will increase as the target approaches the coil: the cap is fixed while L drops.!