LDC1000

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SNOSCX2 – SEPTEMBER 2013

Choosing Filter Capacitor (CFA and CFB Pins)

The Filter capacitor is critical to the operation of the LDC1000. 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 Rp into the converter). The optimal capacitance

values range from 20pF to 100nF. 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

≥10V. The traces connecting CFA and CFB to the capacitor should be as short as possible to minimize any

parasitics.

For optimal performance, the chosen filter capacitor, connected between pins CFA and CFB, needs to 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/Rs, (L=inductance, Rs=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 sensing coil.

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

1. Start with a large filter capacitor. For a ferrite core coil, 10nF is usually large enough. For an air coil or PCB

coil, 100pF is usually large enough.

2. Power on the LDC and set the desired register values. Minimize the eddy currents losses by ensuring there

is maximum clearance between the target and the sensing coil.

3. Observe the signal on the CFB pin using a scope. Since this node is very sensitive to capacitive loading, it is

recommended to use an active probe. As an alternative, a passive probe with a 1k

Ω series resistance

between the tip and the CFB pin can be used.

4. Vary the values of the filter capacitor until that the signal observed on the CFB pin has an amplitude of

approximate 1V peak-to-peak. This signal scales linearly with the reciprocal of the filter capacitance. For

example, if a 100pF filter capacitor is applied and the signal observed on the CFB pin has a peak-to-peak

value of 200mV, the desired 1V peak-to-peak value is obtained using a 200mV / 1V * 100pF = 20pF filter

capacitor.

The waveforms below were taken on the CFB pin with a 14mm, 2 layer PCB coil(23 turns,4 mil trace width, 4mil

spacing between trace,1oz Cu thickness,FR4):

Figure 15. Waveform on CFB with 100pF

Figure 16. Waveform on CFB with 20pF

Copyright © 2013, Texas Instruments Incorporated

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