SLYA048B March 2020 – June 2021 FDC1004 , FDC1004-Q1 , FDC2112 , FDC2112-Q1 , FDC2114 , FDC2114-Q1 , FDC2212 , FDC2212-Q1 , FDC2214 , FDC2214-Q1 , LDC0851 , LDC1001 , LDC1041 , LDC1051 , LDC1101 , LDC1312 , LDC1312-Q1 , LDC1314 , LDC1314-Q1 , LDC1612 , LDC1612-Q1 , LDC1614 , LDC1614-Q1 , LDC2112 , LDC2114 , LDC3114 , LDC3114-Q1
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This section reviews the basic theory of operation of an inductive and capacitive sensing system.
LDC devices operate on a resonant sensing principle. The sensor connected to the LDC is essentially a fixed capacitor in parallel with an inductor, which is typically a coil printed on a PCB as shown in Figure 1-1.
The fixed capacitor and inductive coil form the external LC tank circuit required for LDC operation. Use Equation 1 to calculate the inherent resonant frequency of this LC tank.
As a conductive target approaches the inductive coil, eddy currents form on the surface of the conductive target. The magnetic field of these eddy currents resist the current of the inductive coil, which reduces the inductance of the system and increases the resonant sensing frequency. The LDC devices convert this resonant sensing frequency to a digital value for the user to see. The frequency shift upon a metal target is shown in Figure 1-2.
For a more detailed explanation of the LDC theory of operation, along with the differences between the LDC devices, see the LDC Device Selection Guide.
TI's FDC1004 uses a traditional switched-cap approach to capacitive sensing. It includes an active shield driver that helps concentrate fields to a desired direction. This is useful in liquid sensing applications if the engineer only wants to monitor the direction of the liquid instead of the added effects of human hands or other parasitic capacitances in the area.
The sensor electrode for a liquid level sensing system consists of both a sense and ground electrode. The device measures the fringe capacitance between the two electrodes, which can vary as the dielectric (or level of liquid) varies.
Capacitive level sensing allows for a contactless method of level sensing, enabling a more robust solution compared to mechanical level sensors that can be prone to rust. TI’s capacitive level sensing solution incorporates an active shield driver that can reduce the external interference from foreign objects, such as a human hand. The engineer can use shield drivers using an out-of-phase technique to further stabilize the measurements and maximize the SNR.
Common applications include: wiper fluid and coolant level detection, coffee makers, refrigerators, small fuel tanks, and 3D printers.