8.1.3 Choosing Filter Capacitor (CFA and CFB Pins)

The filter capacitor is critical to the operation of the LDC1001. 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 sensor.

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

For optimal performance, the selected filter capacitor connected between pins CFA and CFB 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. Hence, this time constant reaches its maximum when there is no target present in front of the sensor.

The following procedure can be used to find the optimal filter capacitance:

1. Start with a large filter capacitor. For a ferrite core coil, 10 nF is usually large enough. For an air coil or PCB coil, a value of 100 pF is usually large enough.
2. Power on the LDC1001 and set the desired register values. Minimize the eddy currents losses by minimizing the amount of conductive target covering the sensor. For an axial sensing application, the target should be at the farthest distance from coil. For a lateral or angular position sensing application, the target coverage of the coil should be minimized.
3. Observe the signal on the CFB pin using a scope. Because this node is very sensitive to capacitive loading, the use 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. The time scale of the scope should be set so that 10-100 cycles of the sensor oscillation frequency are visible. For example, if the sensor frequency is 1 MHz, the timescale per division of the oscilloscope should be set to 0.1 ms.
4. Vary the values of the filter capacitor until that the signal observed on the CFB pin has an amplitude of approximate 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 7.
Equation 7. 200 mV / 1 V × 100 pF = 20 pF