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Original question:

LDC1614EVM: calculation of total inductance from target interaction

LDC1614EVM: LDC1614EVM: Relation between register values and the real inductance.

abderraouf Lalla
Prodigy 10 points
Part Number: LDC1614EVM
Other Parts Discussed in Thread: LDC1614, LDC3114

Tool/software:

Hello,

I was able to read the values in the registers of the LDC1614 (Registers values in hexadecimal) in the different channels. I would like to know what is the relation of the registers values (DATA_CH00, DATA_CH01) and the real value of the planar coil inductance (also the frequency)? 

  • 0
    TI__Mastermind 35885 points

    Abderraouf,

    The inductance can be calculated from the data register value by first using equation (6) in the data sheet to determine the sensors oscillator frequency:

    Once you know the sensor frequency, you can calculate the sensor inductance via equation (3):

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 10 points

    Hello John, 

    Thank you for your response.

    Question1: Since CHx_FIN_DIVIDER is only mentioned in the revision of 2014 reference data sheet, Please confirm the registers mapping for better understanding the LDC1614:

    1. CH0_OFFSET = 0x0C

    2. CH1_OFFSET = 0x0D

    3. CH0_FIN_DIVIDER = 0x14

    4. CH1_FIN_DIVIDER = 0x15

    5. Frefx = Which register holds the reference frequency? Or better speaking which register holds the clock FCLK since: FREFx = FCLK / F_DIVIDERx

    Question2: Since you are dividing DATAx over base of 2. I guess you have combined MSB and LSB and transformed them into decimal number correct?

    Thank you in advance.

  • 0 in reply to abderraouf Lalla
    TI__Genius 9290 points

    Hello Abderraouf,

    John is currently out of office today and will be back after Labor Day weekend. He should be able to take a look at your question then.

    Thank you for your patience.

    Best,

    ~Alicia

  • 0 in reply to abderraouf Lalla
    TI__Mastermind 35885 points

    Abderraouf,

    Items 1-4 are correct.

    If I understand your question correctly, Frefx is not stored in a register. It is simply the clock that drives some internal counters. 
    The clock dividers simply divide down the clock in real time.

    For the purposes of  calculations per equation (6), DATAx can be considered a base10 value.

    Monday is holiday in the US. We will be back in the office on Tuesday September 3rd.

    Regards,

    John

  • 0
    Prodigy 10 points

    Hello John,

    The equation for f_SENSOR_x involves the term { CHx_FIN_DIVIDER × f_REFx}, which always yields a constant reference clock source (43.4 MHz in the case of the LDC1614). Why is this referred to as a variable? So we can say that the f_SENSORx value is just a percentage of {DATAx / 2^28 } multiplied by the Reference Clock Source (43.4 MHz in the case of the LDC1614).

  • 0 in reply to abderraouf Lalla
    TI__Mastermind 35885 points

    Abderraouf,

    I believe you are correct. 
    The main idea of all of our LDCs is to normalize LC sensor oscillator frequency by the ref clock over the specified interval (RCOUNT for LDC161x/131x, and W for the LDC3114). Scaling can also be accomplished by the clock sensor dividers.

    Regards,

    John