Guide for PTC Driven Shield Design for Atmel START QTouch® Library

Last modified by Microchip on 2023/11/09 08:55


Capacitive sensors in close proximity to 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.

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Active Shield

Driven Shield

  • Drives ‘shield’ electrode with a sequence of DC levels synchronized to the sensor measurement
  • Requires a dedicated shield electrode
  • Reduces or eliminates loading of sensors due to capacitance with neighbors
  • Rear shield prevents touch from behind
  • Improved water tolerance

Any ground-referenced trace near a sensor will load that sensor, reduce its sensitivity, and may even produce false touches in certain environmental conditions, such as specifically wet or very humid conditions.

Circuit diagram of driven shield system

Driven Shield Circuit

Two classes of driven shield are available on Microchip touch sensor devices, three-level shield and two-level shield.

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Three-Level Shield

The shield is driven through a sequence of voltages matching the electrode potential at each stage in the measurement. This effectively decouples the touch sensor from the ground, reducing the capacitive loading, and provides an electrical shield to EMI improving the Signal to Noise Ratio (SNR) of the sensor. By placing the shield between the sensor and other circuit components, the operation in the presence of moisture is greatly improved.

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Sensor to Shield Separation – Three-Level Shield

0.2 mm0.5 mm3 mm

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Two-Level Shield

Drives a charge pulse during the sensor measurement which shields the sensor from outside influence while additionally boosting the sensitivity of the sensor.

The shield electrode is driven with pulses synchronized to the measurements. These pulses have the effect of boosting the self-capacitance measurement by injection of additional charge to the sensor capacitance. Greater touch sensitivity is achieved as a user touch contact interacts with the electric field between shield and sensor, as well as the electric field between sensor and shield and the electric field between sensor and ground.

Sensor load capacitance is reduced as the shield isolates the sensor from nearby ground referenced circuit components.

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Separation Between Sensor and Shield Electrodes

1 mm2 mm3 mm

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Driven Shield Examples

Driven shield layout example

Driven Shield Layout

Alternatively, a ring shield may be used to isolate each of the sensor electrodes from each other and the ground plane. The ring shield consists of a shield electrode wrapped around each touch sensor. The electrode should be at least 2 mm wide and separated from the touch sensor by approximately 2 mm.

The shield should not form a complete ring around the sensor electrode as this may lead to problems with RF noise. Breaking the ring also allows simplified routing and enables a single-layer sensor design.

Example of ring shield layout

Ring Shield Layout

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Driven Shield+

Some devices have the facility to drive the ‘shield’ signal–three-level or two-level–not only to a dedicated shield electrode but also to other touch sensor electrodes on the UI.

Even in the case where all pins are used as touch sensors and there are no pins available for a shield, Driven Shield+ can be used to drive the other sensors as shield. In the application examples shown in the figure below, Y0 is the active sensor and all other electrodes are driven as shield.

Driven shield plus examples

Driven Shield + Examples

Illustrating the effect of ground in close proximity to the touch sensor

Sensors with Ground in Close Proximity

In the figure above, sensor Y0 is measured while all other sensors are held static at VDD. There is also a ground flood or signal near the sensors. In this scenario, additional capacitance exists between Y0 and ground. Charge driven into Y0 will be shared with ground, reducing the electric field at the touch surface, and so reducing touch sensitivity. This may be mitigated by increasing the space between the sensor and the ground shield but this is not always possible in UI design with high sensor density.

Sensor and driven shield plus stack up drawing

Sensor with Driven Shield+

With Driven Shield+ there is little capacitive loading between Y0 and the other electrodes as they are driven to the same potential. There is a stronger electric field between the sensor and the user, which increases sensitivity and SNR.

This effect of using Driven Shield+ allows greater field projection and improved performance in proximity sensor applications.

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Moisture Tolerance

With Driven Shield+, water coupling between a sensor and the shield does not create a touch delta because the shield and sensor are driven to the same potential. Where a driven shield is used but adjacent keys are not shielded, water can potentially cause a false touch detection due to coupling to neighboring keys.

Care should be taken when designing systems where the touch sensor may be exposed to 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 touch detection.

Illustration showing the effect of water on touch sensors

Effect of Water on Touch Sensors with Driven Shield+

Layout drawing of driven shield plus and sensors

Driven Shield + Layout Example

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Radiated Emissions

Depending on the application and its environment, the use of Active Shield may cause excessive radio frequency emissions. This is caused by high-speed switching of large area electrodes and can lead to products failing to achieve required RFI standards.

High emissions are particularly prevalent, not at the switching frequency of the touch sensors, but at higher frequencies dependent on the MCU core speed and the I/O pin slew rate.

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Add or increase the series resistor to the shield electrode:

  • By increasing the series resistance, the time constant of the RC shield is increased and the amount of energy available at high frequencies is reduced.

The resistor package has a parasitic capacitance which at RF frequencies may be lower impedance than the resistor itself.

Reduce the area of active shield:

  • Instead of a full flood, consider using patches of shield electrodes behind each touch sensor, extending only 2-5 mm beyond the edge of each sensor.
  • The patches have to be joined together at a single physical point and connected to the resistor in a ‘star’ formation.
Minimum driven shield area

Minimum Driven Shield Area

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Configuring Driven Shield in START

Atmel 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 in the following figure. Atmel 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.

Screenshot of enabling driven shield in START

Enabling Driven Shield in START

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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.

Code for configuring the driven shield

Configuring Driven Shield in Firmware

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