3.2.2 Sensor Frequency Selection

The choice of sensor capacitor value and type is important to maintain a stable oscillation of the resonant circuit and is critical for optimum signal-to-noise ratio. The capacitor characteristics can affect the resonant behavior, so a high quality dielectric is recommended. A NP0/C0G capacitor is chosen because it does not exhibit common non-idealities such as piezo-electric effects, dC/dV, or a significant temperature coefficient. The combination of inductance and sensor capacitance determines the sensor frequency of the LC tank determined by Equation 1.

Equation 1.

The optimal choice of sensor frequency depends on the selection of metal material and thickness.  Metals with higher conductivity, such as aluminum have a shallower skin depth which moves the induced eddy currents to surface of the material. This dense concentration of eddy currents produces a greater shift in the AC magnetic field of the LC sensor, making the metal deflection easier to detect. This also enables use of a wider sensor frequency range and thinner metal surfaces. Alloys with a lower conductivity such as stainless steel do not produce as much inductance shift at low sensor frequencies and therefore require an increased sensor frequency to produce an equivalent response. As a rule of thumb, it is better to operate at a high sensor frequency to provide the most flexibility in material selection, especially when a small metal thickness is used.  The LDC1612 has a maximum sensor frequency of 10 MHz, but to allow manufacturing tolerance, a value of 100 pF is chosen to achieve a sensor frequency of 9.1 MHz and provide some system margin.