Guide for PTC Driven Shield Design

Introduction

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.

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.

DrivenShieldCircuit.png
Driven Shield Circuit

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

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.
Min Typical Max
0.2 mm 0.5 mm 3 mm
Sensor to Shield Separation – Three-Level Shield

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.
Min Typical Max
1 mm 2 mm 3 mm
Separation between Sensor and Shield Electrodes

Driven Shield Examples

DrivenShieldEx.png
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.

RingShield.png
Ring Shield Layout

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.

DrivenShieldPlus.png
Driven Shield + Examples
CloseProximity.png
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 aground 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.

DrivenShieldPlusSensor.png
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.

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.

Water.png
Effect of Water on Touch Sensors
DrivenShieldPlusLayout.png
Driven Shield + Layout Example

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.

Mitigation

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.
DrivenShieldArea.png
Minimum Driven Shield Area

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
Enabling Driven Shield in START

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
Configuring Driven Shield in Firmware
© 2019 Microchip Technology, Inc.
Notice: ARM and Cortex are the registered trademarks of ARM Limited in the EU and other countries.
Information contained on this site regarding device applications and the like is provided only for your convenience and may be superseded by updates. It is your responsibility to ensure that your application meets with your specifications. MICROCHIP MAKES NO REPRESENTATIONS OR WARRANTIES OF ANY KIND WHETHER EXPRESS OR IMPLIED, WRITTEN OR ORAL, STATUTORY OR OTHERWISE, RELATED TO THE INFORMATION, INCLUDING BUT NOT LIMITED TO ITS CONDITION, QUALITY, PERFORMANCE, MERCHANTABILITY OR FITNESS FOR PURPOSE. Microchip disclaims all liability arising from this information and its use. Use of Microchip devices in life support and/or safety applications is entirely at the buyer's risk, and the buyer agrees to defend, indemnify and hold harmless Microchip from any and all damages, claims, suits, or expenses resulting from such use. No licenses are conveyed, implicitly or otherwise, under any Microchip intellectual property rights.