FDC2214: interference of Wi-Fi and bluetooth

Hi,

The Wi-Fi and bluetooth signals will not interfere with the FDC2114. As you mentioned, the LC tank is inherently a narrow band-pass filter, so the sensor will reject signals with significantly different frequencies than the resonant frequency. The 2.4 GHz and 5GHz signals are significantly higher than the FDC2214's maximum sensor frequency of 10 MHz. Just ensure that the Wi-Fi and bluetooth frequencies are not integer multiples of your sensor frequency, otherwise you could experience a parasitic effect called resonance locking.

Regards,

Hi,

Here is an excerpt about resonance locking from an app note about resonant-based inductive sensing (which is also affected by this phenomenon). Although the excerpt discusses an external clock as the source, RF signals can also cause this behavior.

Injection Resonance Locking
In the same way that independent pendulum clocks can become synchronized (as first documented by
Christiaan Huygens), the LDC can exhibit resonance locking. With low sensor amplitudes, a small amount
of energy can couple from an external clock source into the sensor, and lock the sensor frequency to an
integer divider of the clock frequency. For example, if the sensor frequency should be 8.0003 MHz due to
the target interaction, the sensor frequency could remain at 8 MHz if the reference frequency was 40 MHz.
This effect is more pronounced with lower value odd dividers of the reference frequency (fCLKIN÷3, fCLKIN÷5,
fCLKIN÷7) than higher value dividers or even dividers. It is important to note that it is the external frequency
fCLKIN that couples into the oscillator, not the internal frequency fREF.
This effect can be reduced by:
• Shifting the sensor frequency away from a strong locking frequency (one way is to simply adjust the
sensor capacitance).
• Increasing the sensor current drive value.
• Using a different fCLKIN that does not have an odd integer divider in the sensor frequency variation.
• Ensuring the CLKIN line has well-controlled Z0 and does not exhibit significant overshoot/undershoot.
• Using a good ground plane for the LDC
• Maintaining separation between the sensor traces and the CLKIN line.


Regards,