1.4.2.1 Driven Shield
- Drives a ‘shield’ electrode with the same signal as the sensor being required on
- Requires a dedicated shield electrode
- Reduces or eliminates loading of sensors due to capacitance with neighbors since there is no potential difference, so there is no electric field between electrodes
- Rear shield prevents touch from behind
- Provides 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.
Two classes of driven shield are available on Microchip touch sensor devices: three-level shield and two-level shield.
Three-Level Shield
A three-level shield is driven through a sequence of voltages matching the electrode potential at each stage in measurement. This effectively decouples the touch sensor from the ground, reducing the capacitive loading and provides an electrical shield to the 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.
A rear flooded shield may be placed over the full board area. A coplanar shield placed with a separation of 0.5 mm will be effective and allow an easy layout of the sensor.
Two-Level Shield
A 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 an injection of additional charge to the sensor capacitance. Touch sensitivity is increased through the interaction between the touch contact and the sensor-shield electric field.
The sensor load capacitance is reduced as the shield isolates the sensor from nearby ground-referenced circuit components.
| Type | Min. | Typical | Max. |
|---|---|---|---|
| Sensor – Shield Separation (S) | 1 mm | 2 mm | 3 mm |
If the shield electrode is too close to the sensor electrode, then the shield-sensor capacitance may exceed the sensor-ground capacitance. This results in reduced SNR, nonlinear operation or in some cases calibration failure.
This is of particular concern when using the two-level driven shield+ (see Driven Shield+) as the sensor traces and electrodes all contribute to the shield-sensor capacitance.
Driven Shield Examples
A coplanar driven shield is implemented on the same layer as the touch sensors electrodes, with appropriate separation between the electrodes.
A rear shield is placed on the layer behind the electrodes. To reduce shield loading (two-level shield), the rear shield may be cut out or hatched with 10% to 50% fill behind the sensor electrodes.
Figure 1-22 shows a sensor implementation with a coplanar shield around the sensor area and a flooded rear shield with cutouts behind each sensor electrode.
Alternatively, a ‘ring shield’ (see Figure 1-23) may be used to isolate each of the sensor electrodes from each other and the ground plane. The ring shield consists of a coplanar shield electrode surrounding each touch sensor.
