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LDC0851: Datasheet ambiguity

Part Number: LDC0851

Hi Team,

Good day! With the help of our customer, we notice some ambiguity from the datasheet of this device. 

In the LDC0851 data sheet "SNOSCZ7A –DECEMBER 2015–REVISED JANUARY 2016" there are two equations that to me seem to contradict each other. 

First we have equation (7) which is used to calculate the minimum inductance based on max current. In this equation a factor of 4.83 is used.

Then we have equation (4) which is basically the same equation but using the factor 17.1 instead. I understand (7) is to calculate a limit of sorts, but still it looks a bit strange that the limit would vary from the actual value calculated with (6) with such a big factor of 17.1/4.83?

Thank you for checking

Best regards,

Jonathan

  • Jonathan,

    Thank you for the inquiry and the great questions.

    Equation (7) places a lower limit on the sensor inductance based on the sensor drive capability of the device (Isensor_max = 6mA at 3.3V operation).
    If the sensor inductance is below the lower limit of (7), then the sensor waveform quality may suffer.

    Equation (4) relates the sensor's contribution to the power supply current at the VDD pin. 

    The two equations use current parameters specified at two different points on the device.
    I don't have the details of what led to the values of the numerical coefficients in the two equations, but my guess is their differences arise from that.

    The sensor design will need to consider equation (6) for the desired frequency, then confirm the inductance meets the lower limit specified in equation (7).
    Equation (4) will play a role when calculating the power supply current.

    These calculations are included on the worksheet LDC0851_calc in the Excel-based Inductive Sensing Design Calculator Tool.
    Please feel free to download & use the tool if you think it will help.

    We have some blogs dedicated to the LDC0851, and some others dedicated to other aspects of inductive sensing.
    You can see Inductive sensing FAQ - Frequently asked questions for additional info on inductive sensing.

    Regards,
    John