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What are you sensing? Capacitive sensing for proximity detection and more

David Wang11

Capacitive sensing is becoming popular to replace optical detection methods for applications like proximity and gesture detection, material analysis, and liquid-level sensing. Capacitive sensing technology measures the capacitance resulting from change in dielectric material between two conducting plates.

Capacitance measurement basics

Capacitance is the ability of a capacitor to store an electrical charge. The capacitance (measured in Farads) of the common form―a parallel plate capacitor―consists of two conductor plates and is calculated by:

  • The parallel plate equation ignores the fringing effect due to the complexity of modeling the behavior but is a good approximation if the distance (d) between the plates is small compared to the other dimensions of the plates.
  • The fringing effect occurs near the edges of the plates, and depending on the application, can affect the accuracy of measurements from the system.
  • The density of the field lines in the fringe region is less than the density directly underneath the plates since the field strength is proportional to the density of the equipotential lines. This results in weaker field strength in the fringe region and a much smaller contribution to the total measured capacitance.

Figure 1 displays the electric fields’ path of a parallel plate capacitor.

Figure 1: Electric fields of a parallel plate capacitor

Applications

There are three common sensor topologies for capacitive sensing applications that utilize different working principles and target use cases. 

Figure 2: The three common sensor topologies

System-level integration with a capacitance-to-digital converter like the FDC1004 is fairly straightforward. The sensor, ground (GND) and shield electrodes can be any metal plate or foil. The FDC1004 is directly connected to a microcontroller or host processor via the I2C bus lines, as shown in Figure 3.

Figure 3: System-level block diagram

Theory of operation

The FDC1004’s basic operation of capacitive sensing implements a switched capacitor circuit to transfer charge from the sensor electrode to the sigma-delta ADC, as shown in Figure 4. A 25 kHz step waveform is driven on the sensor line for a particular duration of time to charge up the electrode. After a certain amount of time, the charge on the sensor is transferred to a sample-hold circuit for the sigma-delta ADC to convert the analog voltage into a digital signal. Once the ADC completes its conversion, the result is digitally filtered and corrected based on gain and offset calibrations.

Figure 4: FDC1004 capacitive sensing theory of operation

Use cases

There are several use cases where the FDC1004 provides significant advantages due to the device specifications and design features. These include:

  1. Independent channels
    1. Allows measurements from each channel to be unaffected by the other channels’ parasitic capacitance and noise.
    2. Enables the system to compensate for capacitance variances and offsets individually.
  2. Differential or ratiometric measurements
    1. Allows the system to track environmental changes to obtain accurate capacitance measurements from the sensor (minus the environmental factors).
    2. One channel (reference/calibration sensor) monitors changes in dielectric due to factors like temperature, humidity, material type, container stress, etc.
  3. Remote sensing
    1. Allows parasitic capacitance along the signal path from sensor to input of the FDC1004 to be compensated up to 100 pF.
  4. Time-varying offset measurements
    1. Similar to the differential concept but can track offsets that vary over time.
    2. Environmental factors that are not constant can be tracked and utilized to have a more accurate capacitance measurement.

Active shielding

Capacitive sensing with active shield drivers allows the capacitance measurements to be unaffected by any capacitance-to-ground interference along the signal path between the sensing device pins to the electrode sensor. The shield driver is an active signal output that is driven at the same voltage potential of the sensor input so there is no potential difference between the shield and sensor input.

There are several benefits to using a shield in capacitive sensing applications:

  1. Directs and focuses the sensing to a particular area
    1. Blocks fringing parasitic capacitances.
  2. Reduces environmental interferers
    1. Uses a shield wrapped around the sensor signal path.
    2. Environmental interferers include the human hand, radiated electromagnetic signals and noise from other electronic devices.
  3. Reduces and eliminates parasitic capacitances
    1. Helps mitigate fringing effects to PCB ground plane.
  4. Eliminates temperature variation effects on the ground plane
    1. PCB contracts and expands with temperature variations, creating capacitance variations that are not a constant offset.

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