Capacitive Sensing Electrode Design
A capacitive control panel measures the change in capacitance produced by a finger near an electrode. The change is a fraction of a picofarad against a background of several picofarads, so the electrode geometry, the routing and the surrounding ground all matter as much as the sensing circuit itself.
Self Capacitance and Mutual Capacitance
In self capacitance, sometimes called surface capacitance, each electrode is measured against the system ground and the finger adds to the capacitance of the electrode it touches. The measurement is simple and the electrode can be a simple pad.
In mutual capacitance the controller drives one electrode and measures the coupling to another. The finger reduces that coupling by diverting field lines to ground, so the sign of the change is opposite to the self capacitance case.
Mutual capacitance allows each crossing of a row and a column to be measured separately, which is what makes a matrix touch screen possible. A control panel with a handful of buttons is usually built with self capacitance because it needs fewer connections.
Electrode Geometry
The electrode is a copper pad under a dielectric overlay, and its capacitance to a finger depends on the area of the overlap and on the thickness and permittivity of the material between. A larger pad and a thinner overlay both increase the change, at the cost of the resolution and the mechanical robustness.
Electrode design has to keep the parasitic capacitance seen by the controller within its measurement range. A large pad on a thick board with a ground plane close underneath has a large capacitance to ground, and the wanted change becomes a small fraction of it.
Interdigitated shapes and hollow pads are used to reduce the area without reducing the coupling, and they also allow a smaller gap between adjacent keys. The detail is a matter of experiment, and the sensor manufacturer usually publishes a recommended pattern.
Choosing the Overlay
The overlay material and thickness are the largest mechanical influences on the sensitivity. Glass with a relative permittivity around seven and a thickness of three millimetres attenuates the field more than a plastic overlay half as thick.
The overlay should be bonded with no air gap. An air layer has a permittivity of one and adds a series capacitance that reduces the coupling, and an uneven glue layer produces a difference in sensitivity between keys that is visible to the user.
Printed graphics on the reverse of the overlay must not contain a conductive layer. A metallic or carbon loaded ink between the finger and the electrode shields the field, and the key stops working while looking perfectly normal.
Guard Ring and Grounding
A guard ring is a track driven at the same potential as the electrode that surrounds the sensing area. Because there is no potential difference, no current flows between the electrode and the ring, and the surface leakage and the field spreading are both reduced.
A partial ground plane under the electrodes is used to control the parasitic capacitance but it must not be continuous under the pads, or the field will be shorted to ground and the sensitivity will collapse. A hatch pattern of thirty to fifty percent copper is a common compromise.
Grounding near the connector is also important. A hand near the panel changes the potential of the whole product, and if the product is floating the measurement drifts. A defined connection to the environment is what makes the reading repeatable.
Routing and Crosstalk
Route the electrode traces on the opposite side of the board from the electrodes themselves, or on the same side with a ground guard between them. A long trace running beside another couples the two channels, and a touch on one key appears as a partial touch on its neighbour.
Traces to the controller should be as short as the mechanical layout allows and similar in length between channels. A long trace adds capacitance to one channel, which detunes it relative to the others and makes the sensitivity uneven.
Do not route the sensing traces under a connector, a display or a battery. Any conductor close to the trace changes the capacitance, and a component that moves with the assembly changes it over time, which appears as a drifting baseline.
Noise, Water and Environment
A capacitive sensor responds to water as well as to a finger, and a film of water on the panel can produce a false touch. Water has a high permittivity, and the change it produces is larger than the change produced by a finger.
Guard based and differential measurement schemes reduce the effect, and a firmware algorithm that requires a moving touch rather than a static one rejects a water film. A key that is wet is often reported as a large but static change, and that signature can be identified.
Switching supplies and displays are the other noise sources. A mains frequency ripple and a display refresh couple into the sensing node, and the measurement frequency should be chosen away from both. Spreading the measurement over several frequencies and averaging the results is a standard technique.
Calibration and Baseline Tracking
Every electrode has a different baseline capacitance and every unit differs slightly because of the assembly. The controller therefore calibrates at start up, storing the measured value for each channel as its reference.
The baseline then has to follow slow changes caused by temperature and humidity while ignoring a finger, which is stationary for seconds rather than for minutes. A slow filter with a time constant of a minute or so achieves this, and a touch is detected as a deviation from the drifting baseline.
A product that is powered on with a finger already on a key will treat that key as the baseline and it will never respond until the finger is removed and the baseline is recalibrated. Good firmware detects the condition at start up and asks the user to remove contact.
Mechanical Integration
The gap between the overlay and the panel, the type of adhesive and the thickness of the front panel all sit between the user and the electrode, and each of them affects the coupling. Keeping these mechanical variables inside a tolerance band is what makes production units behave the same.
An air gap that appears only after thermal cycling, or an adhesive that flows under pressure, changes the sensitivity of a key over the life of the product. Specifying the bonding process as part of the design, rather than leaving it to assembly, is the practical way to control it. The general approach to keeping a mixed signal board quiet, described in our guide to mixed signal board design, applies here as it does to any high impedance node.
Verification and Common Faults
Verify the sensitivity of each key through the finished overlay with a test finger of a defined size. A key that needs a heavier touch than its neighbours has a thicker glue layer or a longer trace, and the difference is measurable.
Check the response with the product held in the hand and with it standing free, because the grounding changes and so does the reading. A design that works on the bench and not in a handheld case usually has a grounding problem rather than a sensing problem.
Check the power consumption, because a high drive level and a high sample rate both increase it. A battery powered product is often limited by the sensing duty cycle rather than by the processor. The release checks that keep these boards consistent are collected in our PCB design release checklist, and the assembly points that affect sensitivity are listed in judging PCB quality.
FAQ
Why does my capacitive key trigger when water is on the panel? Water has a high permittivity and produces a larger change than a finger. Use a guard based measurement and require a changing touch in firmware.
How thick can the overlay be? It depends on the material. Glass up to about four millimetres works with a well designed electrode and a high sensitivity controller.
Why is one key less sensitive than the others? A longer trace, a thicker glue layer or a nearby conductor changes the baseline. Compare the measured capacitance of the channels at start up.
