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Employ glove touch in capacitive touch UI

01 Jul 2015  | Joshan Abraham, Vibheesh Bharathan

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However, the problem here is that it produces a condition known as "unwanted hover", where a bare finger in close proximity to the sensor (hovering above the sensor) produces an equivalent capacitance change as introduced by a glove touch. An erroneous touch could be registered as a glove touch even though the finger neither touched the sensor nor did it have a glove on. This condition is mostly undesirable and can adversely affect the user experience of the product. Figure 2 indicates the signal produced by a glove touch, finger touch and a hovering finger.

Figure 2: The signal produced by a glove touch, finger touch and a hovering finger.

A designer hence faces the following problem: A system tuned for regular touch-sensing doesn't pick up touches from a gloved hand and a system tuned for glove touch produces false touches due to "unwanted hover".

An easy, non-elegant solution would be for the design to add a user-triggered interrupt or physical switch to indicate if they are wearing a glove or not. This diminishes the user experience, especially in consumer products that need to have "one action less" and in medical products which need to work the same in all conditions.

Improving signal strength
There are three important design parameters that need to be considered to improve the signal strength of glove touches:

Sensitivity: Sensitivity is a measure of the ability of a capacitive touch-sensing circuit to produce a signal—a more sensitive circuit produces a larger signal. Sensitivity is typically measured in counts per capacitance. In the context of capacitive touch-sensing, the magnitude of change introduced by a touch is in the order of 100s of femto-farads (fF). A touch by a gloved finger typically introduces a capacitance of 100fF. A circuit with a sensitivity of 500 counts/pF can produce 50 counts of signal for a 100fF touch, while a circuit with sensitivity of 50 counts / pF can only produce 5 counts of signal for the same touch. Therefore, a circuit with a higher sensitivity can detect a glove touch more reliably. Parasitic

Capacitance: Parasitic capacitance is the intrinsic capacitance of the sensor, which is introduced due to its proximity to other conductive objects. The change in capacitance due to a touch interaction, the signal, is perceived relative to the parasitic capacitance of the sensor. Higher the ratio between the change in capacitance and the parasitic capacitance, the higher the sensitivity the sensor can be tuned for.

Avoiding challenges
For example, a capacitance touch-sensing circuit that uses a 12bit A-to-D converter can have a maximum output of 4096. A sensor having a parasitic capacitance of 16pF can be tuned to achieve a maximum sensitivity of 256 counts/pF, beyond which the A-to-D converter would get saturated. However by decreasing the parasitic capacitance to 8pF, the sensor can be tuned to a maximum sensitivity of 512counts/pF. A change of 100fF capacitance due to a touch, would produce a signal of approximately 25 counts in the first case and 50 counts in the second case.

The parasitic capacitance of a sensor is dominated by design specifications of sensor stack up and layout, such as trace thickness, distance between traces and distance between PCB layers. A careful sensor layout design and sensor stack-up is required to maintain low parasitic capacitance.

To improve performance and to provide flexibility to the designer, some touch-sensing controllers integrate the following two features to reduce the impact of excess parasitic capacitance on sensitivity:

 • Faux differential measurement capability.
 • Support for shield electrodes.
Faux differential measurement capability: A typical touch-sensing controller measures capacitance from 0 to a maximum measurable value (e.g. from 0pF to 8pF). A touch-sensing controller that can implement faux-differential measurement (i.e. faux differential A-to-D conversion) can be set to measure a specific range of capacitance (e.g. from 8pF to 16pF) and achieve a higher sensitivity. Using this method, a capacitance touch-sensing circuit with a 12bit A-to-D converter, can be tuned to achieve a sensitivity of 512 counts/pF, even with a sensor having a parasitic capacitance of 16pF.

Support for shield electrodes: Shielding the sensor from other conductive objects around the sensors will minimise the extra capacitance added and therefore minimise the parasitic capacitance of the sensor.

Controllers supporting both the faux differential measurement capability and shield electrodes, typically double the sensitivity and subsequently better glove touch performance.

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