Design capacitive touch systems for robustness and manufacturability
Using Active Parasitic Cancellation
Whilst the design guidelines will certainly improve the system sensitivity and robustness, a device which will auto tune to its environment for optimal sensitivity, will shorten design times with less PCB iterations. More importantly it will save costly delays in production where process variations, often in the mechanical construction, causes production delays due to the non-conformance of the touch sensing circuit.
From the sensor perspective, minute differences in process parameters may render the touch sensor unstable or un-usable. These include variations in the power supply stability, thickness of the overlay material, possible air gaps between sensor electrode and overlay material and in many cases, the nearness of the product’s housing. When housings are manufactured from a conductive material, the nearness of the housing introduces a large parasitic capacitance which has a significant impact on the sensor sensitivity.
Parasitic capacitance is an unwanted capacitance between sensor antenna and a nearby (normally grounded) potential. The aim of achieving a sensitive capacitive sensor is to have the sensor project electric field into a dielectric overlay material and further into free air (Figure 1 below). The user touching the designated touch sensor area would disturb this electric field.
Figure 1. The 'ideal' case has the field lines projecting freely into free air
In real life the electric field from the sensor would rather terminate to the nearby grounded potential, than be projected through a die-electric overlay and into free air (Figure 2, below). Parasitic capacitance can often account for up to 95% of the total capacitance as seen from the sensor. When 95% of the sensor capacitance is static, touching the antenna, can only impact the remaining 5% of variable capacitance. Once overlay materials exceed 1mm, the effect of the touch is as little as 5%, meaning the sensor only sees a 0.25% change between touch and non-touch. This may be very close the noise level of the system.
Figure 2 The real life case has the sensitivity reduced by filed lines terminating to surrounding components, tracks, ground planes and the housing
By using hardware compensation circuits, the effect of static parasitic capacitance could be greatly reduced. This would entail the ‘subtraction’ of the unwanted capacitance from the sensor sample, which in turn means the sensor only sees the variable capacitance, which is disturbed by the user touch. (In another analogy, this can be seen as the removal of a large DC component from a signal with a very small AC component). Once the parasitic capacitance is removed (or greatly reduced), the sensor sensitivity is recovered.
Azoteq’s range of ProxSense capacitive touch and proximity sensors has very effective compensation circuits implemented on-chip. The on-chip implementation means that the designer does not need to worry about the design of complex analog circuits and sensitivity variation, as these are all performed on-chip.
The Azoteq technology is aptly dubbed ATI, or Antenna Tuning Implementation. The on-chip circuits will recover most of the sensor sensitivity, yielding industry leading sensitivity even in environments with severe parasitic capacitance. This further allows for the use of much thicker overlay materials, few constraints on PCB design and much greater degrees of freedom in the nearness of the product housing.