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Confusion on RP Min/Max settings on LDC1000evm

Leon Snyman
Prodigy 45 points
Other Parts Discussed in Thread: LDC1000EVM

I have an LDC1000EVM module and am a little confused. Sorry if my question is really basic. I want to use my own wound coils so not PCB coils. I do not have a $20K impedance tester. I can measure the inductance of the wound coil say 50uH.

How do I go about determining the correct Rp min and Rp Max settings for this coil? I want to experiment with different sizes of wound coils and come up with a set of parameters that make the best use of the coil.

Any help would be appreciated.

  • 0
    TI__Prodigy 160 points

    Hi Leon,

    To get you jump started, please refer to the "EVM Quick Start Guide" under the "More literature" section here:

    http://www.ti.com/tool/ldc1000evm#Technical Documents

    It was exactly for your kind of applications that this document was created. Please let us know if the steps here help you through the setup. 

    Thanks!

  • 0
    TI__Genius 9300 points

    Hello Leon,

    To measure RpMIN you can follow this procedure:

    1) Set Rpmin to 3F, RpMAX to 12.

    2) Expose the coil to the maximum metal coverage for your application (closest target position, thickest part etc)

    3) Take measurement of Rp by LDC and calculate back the Rp value (please refer to DS for instructions)

    4) Now that you know impedance, you can determine optimal RpMIN value as described in DS

    5) Set RpMIN to the value from 4) , set RpMAX to RpMIN value minus x28 (hex notation)

    6) Move your target to a position where it's exposed the least (farthest position, thinnest part) etc.

    7) Take measurement of Rp by LDC and calculate back the Rp value (please refer to DS for instructions)

    8) Now that you know impedance, you can determine optimal RpMAX value as described in DS

    There is an empirical method to do the same:

    1) Set Rpmin to 3F, RpMAX to 0.

    2) Expose the coil to the maximum metal coverage for your application (closest target position, thickest part etc)

    3) Start reducing RpMIN setting 1 code at a time, and take Rp Measurements (they will go up with each change).

    4) When Rp gets in the range of 20,000 - 30,000 codes, that is your optimal RpMIN setting.

    5) Move your target to a position where it's exposed the least (farthest position, thinnest part) etc.

    6) Start increasing RpMAX setting 1 code at a time, and take Rp Measurements (they will go down with each change).

    7) When Rp gets in the range of 2,000 - 3,000 codes, OR the difference between RpMIN and RpMAX reaches x25 - x26, that is your optimal RpMAX setting. It may get too noisy, then back off 1 RpMAX code.

    Hope this helps.

  • 0 in reply to Evgeny Fomin
    Prodigy 45 points

    Thank you Evgeny.

    . I used the empirical method you gave. Seemed to work smoother than the quick start guide method, but probably just in my mind. I got RpMax at 349.066K and RpMin at 27704K with my inductor which indicates a range of 800uH to 7470uH for 15mm with an infinity of 7800uH. I am using 15pf right now but still have 2vp-p on the CFB pin so i need to get even lower?

    So the values of 800uH to 7800uH will be available as 24-bit data, am I reading that correctly?

    When can we get the source code to start playing with the available data?

    Thanks

    Leon

  • 0 in reply to Leon Snyman
    TI__Genius 9300 points

    2vpp on CFB is ok, if it does not rail when you change inductance all the way.

    For inductance measurements, please refer to p.12 in DS. In brief, 24 bits are achieved when the oscillator frequency is at the minimum, i.e. 5kHz. As the frequency increases, ENOB goes down.

  • 0 in reply to Leon Snyman
    TI__Intellectual 2790 points

    Hi Leon,

    The documentation for the API will be released soon.  There will be a forum announcement thread for the release.  In the meantime, I would suggest interfacing the LDC1000 data through a microcontroller via SPI.

    Thanks

  • 0 in reply to Evgeny Fomin
    Prodigy 240 points

    Hi Evgeny,
    when you write "take Rp masurements" do you mean the proximity data, Regs 0x21/0x22 directly or are performing a conversion between this value and anything else?

    Thanks & regards

    Gerardo

  • 0 in reply to Gerardo Vanoni
    TI__Intellectual 2790 points

    Hi Gerardo,

    I assume you are referring to step#7 : Actual Rp.  The DS describes how to obtain the actual Rp from the raw proximity data.

    Thanks,

  • 0 in reply to Charles Cheung*
    Prodigy 115 points

    Sorry to hijack the thread but I have a similar question. 

    I want to use my LDC100EVM to measure an inductance to a high level of precision. I already know roughly what value the inductance is. These are the steps I've followed, 

    1. Snap off PCB, attach my own LC circuit

    2. Pick a C such that it is within range of both equations in the user guide

    3. Configure the GUI settings as per the user guide, although as I'm not using it for 'inductive sensing' the instructions on selecting RP_min and RP_max don't seem relevant. I would have thought that in my case Rp_max = Rp_min = (1/Rs) * (L/C) but this doesn't seem to work. 

    If I leave Rp_min and Rp_max at the default values I get a value close to what I know L to be, and for some values I've magically gotten the inductance correct, but there doesn't seem to be a reliable heuristic to follow. 

    Any help appreciated.

  • 0 in reply to James Randjelovic
    TI__Genius 9300 points

    RpMIN and MAX settings are important for proper operation.

    Please refer to FAQ on how to tune them.