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FDC2214: LC-Tank configuration

Mark Philip Felipe
Genius 10830 points
Part Number: FDC2214
Other Parts Discussed in Thread: FDC1004

Hi Team, seeking some support. 

The datasheet of the FDC2214 shows different configurations of the LC-Tank connected to the chip.

For example, Figure 12 seems to show a series LC-Tank, whereas Figure 11 seems to shows a parallel LC-Tank.

Are both configuratons possible ? Is there a preference of one over the other ?

Is the additional parallel C besides the sensor C required (e.g. Fig. 11) or may it be omitted ?

  • 0
    TI__Mastermind 40295 points

    Mark,

    The two figures do seem to emphasize different LC topologies.
    But in the end, they are both parallel LC circuits.

    Figure 11 shows a PCB capacitor (C) in parallel with the inductor, with the shaded capacitor representing the varying capacitance because of the sensor. 
    The function of C is to set a nominal sensor frequency, while the sensor capacitance moderates it.
    The C is obviously in parallel with the sensor, while the equivalent sensor capacitance is referenced to ground, which could be the ground of the PCB, or a remote ground like a large piece of metal or a human hand.

    Figure 12 simply leaves out the PCB capacitance and assumes the LC capacitance comes from the sensor. 

    We have an app note that covers some aspects of this topic: Ground Shifting in Capacitive Sensing Applications.
    I hope it helps.

    Just so you know, we advise using the FDC1004 over the FDC2xxx family for new designs.
    The FC1004 does not use a resonant for the sensor and has active shield drivers to manage EMI, which the FDC2xxx devices do not have.
    There is an  FDC1004 E2E FAQ page that provides a comprehensive list of app notes and other collateral for the FDC1004.

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    TI__Genius 10830 points

    Hi John, Thank you for supporting. 

    Some additional questions from the customer. 

    You say, that "the C is obviously in parallel with the sensor" in figure 11. It would be, if IN0B is on the same GND as the sensor-C. But this is not clear from the figure. Can I really simply add C and sensor-C to give the total capacitance for determining the LC oscillation freuqency ?

    You also say, that "Figure 12 simply leaves out the PCB capacitance and assumes the LC capacitance comes from the sensor". But looking at both figures, sensor-C is connected to a different terminal of L, respectively.

    Already checked the mentioned app note "Ground shifting in capacitive sensing applications" but I do have difficulties in understanding that as well. Figure 1 states, that sensor-C is not effective during phase 2. Why ?? And if this is the case, it seems to me, that C0 on PCB is a must-have, otherwise there is no LC-circuit during phase 2. Currently, I don't plan to use C0, just using the sensor-C, but I'm not sure if this is permissible.

    To clarify things: we want to use the FDC2214 to measure a variable sensor-capacitance as acurate as possible. It shares the same GND as our PCB. We don't use it as e.g. a proximity sensor or gesture recognition sensor. So when looking at figure 11, the GND of the sensor-C is the same as our PCB-GND. Does this mean, that IN0B is also the same GND ? This would mean, that sensor-C and the on-board LC-tank are really in parallel and I can simply add them fo calculation resonant frequency.

    Thank you.

  • 0 in reply to Mark Philip Felipe
    TI__Mastermind 40295 points

    Mark,

    You can add the PCB capacitance and the sensor capacitance to estimate the total capacitance.

    Just so you know, we recommend using the FDC1004 instead of the FDC2xxx parts.
    The FDC1004 does not use a resonant-circuit input and has a fixed sensor frequency of 25kHz. 
    The FDC1004 also has active shield drivers which helps with EMI management. T
    he FDC2xxx parts do not support active shields, and for that reason users have struggled with their designs.

    Our capacitive and inductive sensing products are best at measuring changes in the sensor capacitance/inductance.
    For this, they can provide high accuracy.

    If they want absolute capacitance/inductance they will need to be very mindful of sensor & system calibration was well as environment-induced changes to their sensor and system. This type of application can be extremely challenging. 

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    TI__Genius 10830 points

    Hi John, seeking for further support.

    The expected capacitance range we want to measure is about 2nF - 2.5nF and measurement results need to be very accurate. As far as I understand, the FDC1004, it is not appropriate for these values, is it ?

    You said, that "capacitive and inductive sensing products are best at measuring changes in the sensor capacitance". Does this mean, the FDC2214 is not recommended for measuring absolute capacitance values ? This is what I have initially assumed, because the datasheet states its application as a proximity sensor, gesture recognition sensor etc. These applications only require recognition of capacitance changes, not measuring absolute capacitances.

  • 0 in reply to Mark Philip Felipe
    TI__Mastermind 40295 points

    Mark,

    You are correct. Our FDCs are most effective at measuring changes in capacitance, not absolute capacitance.

    John