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FDC2214: The related issues about IDRIVE

Wenhan He
Prodigy 230 points
Part Number: FDC2214

1.As for IDrive current, how to get the formula (1) - (2) in this technical document?

https://www.ti.com/lit/an/snoa950/snoa950.pdf?ts=1596163453917

2.I saw someone answer that IDrive is half sine wave in INA signal channel and half sine wave after 180 ° phase shift in INB channel. Is this driving waveform correct?

3. How to determine the resonant frequency.

My understanding is that the configuration of IDRIVE only changes its valid value (or peak peak), and its frequency is constantly changing, only at the resonant frequency, the LC parallel resonance has the largest impedance and the voltage amplitude is the highest, so if a large output voltage is detected, it can be assumed that the resonance has occurred at this time. This frequency is then considered to be the resonant frequency. Is my understanding correct?

 

Only understand the principle I can correctly configure my resonant inductive harmony vibration capacitance value, thank you for your reply and help.

  • 0
    TI__Expert 3435 points

    Hi Wenhan,

    1. Eq 1 and 2 are needed for LDC devices where inductance is laid out on coil and it varies according to the position of the target. It is not required for FDC devices. Measure the amplitude of the oscillation to chose the best IDRIVE setting for the Vp is between 1.2 V to 1.8V as explained in the section 6 of the datasheet.

    2. Can you please point me to that post?

    3. Did you get a chance to work with the EVM? Please order an EVM and play with it to understand the relation between the capacitance and the oscillations. When you move your hand closer to the sensor, the capacitance changes that result in a change in frequency. The equivalent frequency is reported by the FDC2214 in the digital format. 

    Regards

    sharath.

  • 0 in reply to Sharath Vallamkonda
    Prodigy 230 points

    1. In the previous E2E answers, some of your colleagues said that the detection principle of LDC and FDC is similar, and attached this document. https://www.ti.com/lit/an/snoa950/snoa950.pdf?ts=1596163453917

    Why is your answer just the opposite? What are the mathematical expressions of Vsensor and iDrive? Please expalin it in detail.

    2. In this answer, it is said that the driving signal of iDrive is half sine wave, so what kind of waveform is iDrive?

    e2e.ti.com/.../572956

    3. I have EVM version now. What I want to know is whether the internal working principle I understand is correct.

    My understanding is that iDrive configuration will only change its effective value (or peak value?), and its frequency will change constantly. Only at resonance frequency, LC parallel resonance has the largest impedance and the highest voltage amplitude. Therefore, if a larger output voltage is detected, it can be assumed that resonance has occurred at this time. This frequency is then regarded as a resonant frequency. Is my understanding correct?

    Please answer my question clearly instead of vague answer.

  • 0 in reply to Wenhan He
    TI__Expert 3435 points

    Hi,

    1. Yes, the working principle of LDC and FDC is the same. L is variable in LDC devices and C is variable in FDC devices. Calculating Rp is important for LDC devices.

    2. IDRIVE is not a waveform. It is a DC current provided by the device to sustain oscillations and it will also change the amplitude of the oscillations.

    3. My understanding is that iDrive configuration will only change its effective value (or peak value?), and its frequency will change constantly. Only at resonance frequency, LC parallel resonance has the largest impedance and the highest voltage amplitude. Therefore, if a larger output voltage is detected, it can be assumed that resonance has occurred at this time. This frequency is then regarded as a resonant frequency. Is my understanding correct?

    For a constant L and constant C values, the frequency of oscillations is constant. The frequency changes when L or C changes. You can change the amplitude of oscillations with IDRIVE current.

    Regards

    Sharath.