Guide for PTC Driven Shield Design

Introduction

Capacitive sensors in close proximity to a ground do not perform as well as those located far away from ground. A ground in close proximity to a sensor will load that sensor, reduce its sensitivity and may even produce false touches in certain environmental conditions, specifically wet or very humid conditions.
Unfortunately, ground is all around most electric devices and, as size shrinks, proximity to ground increases. Ground is also used as a shield for electrical noise. One solution to this problem is a hardware driven shield; the shield effectively decouples the touch sensor from ground, provides an electrical shield, and provides an increase in touch response, which in turn increases the Signal to Noise Ratio (SNR) of the sensor. In addition, operation in the presence of moisture is greatly improved.

Driven Shield

DrivenShieldCircuit.png

The ATtiny817 has the capability to drive a driven shield in self-capacitance mode; hardware shielding is not available when using the mutual capacitance mode of operation. Hardware shielding is achieved by driving the shield at the same potential as the sensor. Any Peripheral Touch Controller (PTC) pin can be used to drive the shield, there are no reserved pins used as shield drivers. In addition to driving a shield, any of the sensor electrodes can be driven by the shield signal; this allows for a large area to be driven.

ShieldExamples.png

In the case where all pins are used as touch sensors and there are no pins available for a shield, the sensors themselves can form a shield. In all diagrams above, Y0 is the active sensor and all other sensors, as well as the shield, are being driven by the PTC as a shield.

SNR and Sensor Responsiveness Improvement

Figure3.png

In Figure 3 above, sensor Y0 is being acquired, while all other sensors are static at VDD. There is also a ground flood or signal in close proximity to the sensors. In this scenario, a capacitance exists between Y0 and ground due to a potential difference and close proximity. Any charge driven into Y0 will be shared with ground leaving a reduced field for the user to interact with, resulting in reduced sensitivity.

Figure4.png

When the shield is used, there is virtually no capacitance between Y0 and the shield, as they are driven at the same potential. All available field generated by the sensor is coupled to the user, increasing the influence on the sensor, thereby increasing sensitivity and SNR. Another desirable effect of using a shield is that the electric field around the touch sensor can project further, allowing the design of close proximity sensors.

Performance in the Presence of Moisture

Figure5.png

With the driven shield in place, any water coupling between a sensor and the shield will not be a problem since the shield and sensor are driven to the same potential. Additionally, no false touch will be detected because the shield will not load the sensor. In the event that a driven shield is used but adjacent keys are not driven by the shield, a water splash can potentially cause a false detect as Y3 will load the sensor and possibly case a detection.

No touch sensor is 100% immune to water and care should be taken when designing systems where the touch sensor may come in contact with water. If water is to bridge across the shield signal and over a ground, then some field from the touch sensor will couple to ground through the water which may cause false key presses.

Hardware Design Guide

The PTC Driven Shield is similar to the GND shield which shields from noise and helps achieve better noise immunity. The design guidelines for a driven shield are very similar to those of GND.

  • Connect any one unused PTC Y line to shield
  • Ensure that the shield Y line has a series resistor – typically a 100 ohm
  • The shield sensor can be as big as 300 pF
  • The gap between sensor and shield should be half of the front panel thickness

The following figures show example layouts with and without driven shields:

Typical layout without shield:

WithoutShield.png
  • Hatched ground pattern is present on top and bottom layer
  • High SNR
  • Poor Sensitivity

Layout with Driven Shield:

Pattern 1: Hatched Shield on top and bottom layer:

HatchedShield.png
  • Top layer is fully flooded with hatched-shield
  • Bottom layer has shield only in sensor area
  • Good SNR
  • Good Moisture performance

Pattern 2: Driven shield in addition to GND shield:

DrivenShield.png
  • The top layer has driven and GND shields. This arrangement may be required for some designs where extreme noise immunity is required.
  • Ensure that the driven shield is big enough; such that a big water droplet does not connect sensor-shield-GND (as shown in the figure).
  • Ensure that the gap between the shield sensor and GND is half of the front panel's thickness.

Pattern 3: Driven shield on a four layer board:

Using GND as shield:

GroundAsShield.png
  • GND shields the noise from other components and signals present in the bottom
  • Significant drop in sensitivity

Using driven shield:

UsingDrivenShield.png
  • Driven shield shields the noise from other components and signals present in the bottom
  • No drop in sensitivity

Configuring Driven Shield in START

START allows you to configure the driven shield in supported devices. Driven shield can be enabled and configured in the PINS section page of the QTouch® configurator as shown below. START provides options to enable and use a dedicated driven shield pin. If enabled, you can configure the Y line for the shield; if not enabled, ONLY other touch channels are configured as driven shield.

DrivenShieldSTART.png

Configuring Driven Shield in Firmware

Using the PTC Library
Configuring a sensor requires the sensor type to be configured as Node_SelfCap_Shield. Then you'll need to list the shield pins in the X drive for each self-cap sensor.

DrivenShieldFirmware.png
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