What are you sensing? Active shielding for capacitive sensing, part 2

Thanks for tuning into part 2 of this series on active shielding. In my last post, I talked about the benefits of shielding and how it helps mitigate parasitic-capacitance interference from your capacitance measurements. Today, I’ll discuss shield sensor designs and how the size and placement of the shield in relation to the sensor electrode affects sensor performance.

The shape and position of the shield relative to the sensor is an important factor in capacitive-sensor design. The sensing angle without a shield, as shown in Figure 1, picks up any stray interference within the field-line vicinity. The sensing angle with a shield depends on both how large the shield is compared to the sensor and how close the shield is to the sensor. Although the shield helps mitigate the effects of parasitic-capacitance interference from the surrounding area, it does reduce the sensitivity and overall dynamic range of the system.

Figure 1: Direct/focusing the sensing area

I performed an experiment with four different shielding configurations to determine what kind of relationship shielding has with directivity, sensitivity and parasitic-capacitance interference mitigation. The isolated sensor topology employed here is mainly used for proximity and gesture-recognition applications such as system wakeup detection and infotainment display interaction. The target object is the human hand (grounded target). The four configurations were:

  1. CIN1 electrode only.
  2. Shield1 the same size as CIN1 and directly underneath.
  3. Shield1 200% larger than CIN1 and directly underneath.
  4. Shield1 ring added on the same plane as CIN1 with Shield1 underneath (same as configuration 3).

Figure 2: Sensor layouts

Figure 2 shows the top and side profiles of the sensor layout stack-up. Shielding the sensor electrode will help block any external interference and noise. The experimental results show that even though shielding does not totally eliminate all of the interference, it does significantly reduce it. The top side of the sensors is the intended target area for the human hand (in proximity and gesture-recognition applications). The top side is the most common direction for proximity detection, whereas the proximity from the side and the bottom is treated as the unwanted interference.

Figure 3: Interference data comparison from the side

Figure 3 shows the change in capacitance as the parasitic capacitance (human hand) approaches from the side of the sensors. It is apparent that as the shield size increases, the effects of the interference are reduced.

Figure 4: Sensitivity data comparison from the top

Figure 4 displays the sensitivity of the sensors from the intended target direction (from the top). Note that increasing the area of the shield decreases sensitivity and dynamic range to some extent in the target zone. This occurs because the shield decreases the amount of electric field lines that terminate to the nearest ground source. Various applications will require a certain proximity range and margin for interference; the shield will need to be sized appropriately for each case since it does not have a linear relationship to range and interference. Table 1 shows that either using a shield the same size as the sensor, or one that is 200% larger in area, has about the same impact on target-zone sensitivity. But using a larger shield can reduce the vulnerability to interference from the side.

Measurements with bottom-side interference show a significant reduction in capacitance change at a fixed distance away from the sensor. All of the interference cannot be eliminated unless the shield is much larger (by an order of magnitude) than the sensor due to fringing fields near the edges of the electrodes.

Table 1: Error-reduction comparison

Overall, shielding is a very effective method for protecting the signal integrity of the system. The placement and configuration of the shield depends on the application and amount of acceptable parasitic capacitance. Up to 77% of parasitic-capacitance interference can be eliminated at an expense of up to 74% decreased sensitivity, depending on the desired sensing range and shield configuration. You will need to characterize each system properly to determine the optimal shield parameters.

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  • Hi David,

    Can the 4th configuration (with shield ring) be used for water level sensing? I made a similar design to what you have, but instead of a square, I made a  long strip that I placed on a side of a container. I am using the FDC1004EVM with the provided sensors detached. I also included a water reference sensor in addition to the level sensing. My thinking was that using the shield ring, I can limit the sensing angle of the strip so it just registers the liquid data and neglects outside interference. Any help would be appreciated.