8.1.3 Selecting a Filter Capacitor (CFA and CFB Pins)

The filter capacitor is critical to the operation of the LDC1001-Q1 device. The capacitor should be low leakage, temperature stable, and it must not generate any piezoelectric noise (the dielectrics of many capacitors exhibit piezoelectric characteristics and any such noise is coupled directly through R_P into the converter). The optimal capacitance values range from 20 pF to 100 nF. The value of the capacitor is based on the time constant and resonating frequency of the LC tank.

If a ceramic capacitor is used, then a C0G (or NP0) grade dielectric is recommended. The voltage rating should be 10 V or higher. The traces connecting the CFA and CFB pins to the capacitor should be as short as possible to minimize any parasitics.

For optimal performance, the selected filter capacitor connected between the CFA and CFB pins must be as small as possible, but large enough such that the active filter does not saturate. The size of this capacitor depends on the time constant of the sense coil, which is given by L / r_s (L = inductance, r_s = series resistance of the inductor at oscillation frequency). The larger this time constant, the larger filter capacitor is required. Therefore the time constant reaches the maximum when there is no target present in front of the sensing coil.

Use the following procedure to find the optimal filter capacitance:

1. Use with a large filter capacitor. For a ferrite core coil, a value of 10 nF is generally large enough. For an air coil or PCB coil, a value of 100 pF is generally large enough.
2. Power on the LDC and set the desired register values.
3. Minimize the eddy currents losses by ensuring maximum clearance between the target and the sensing coil.
4. Observe the signal on the CFB pin using a scope. Because this node is very sensitive to capacitive loading, the use of an active probe is recommended. As an alternative, a passive probe with a 1-kΩ series resistance between the tip and the CFB pin can be used.
5. Vary the values of the filter capacitor until the signal observed on the CFB pin has an amplitude of approximately 1 V_PP. This signal scales linearly with the reciprocal of the filter capacitance. For example, if a 100-pF filter capacitor is applied and the signal observed on the CFB pin has a peak-to-peak value of 200 mV, the desired 1-V_PP value is obtained using a filter capacitor value that is calculated in Equation 16.