Getting the most from touch sensors

Capacitive touch sensor devices detect a finger touch by measuring the capacitive changes on a touchpad. From there they provide various slider, button and LED driver functions, however to get the most out of such systems it is important to design and layout the associated printed circuit board correctly.

Even though the capacitance can, in theory, be calculated using a simple parallel plate capacitance equation, in practice it is far more complex as the total system comprises capacitors, resistors and even inductors as it takes into account the PCB, overlay and human body. Thus, any calculations will need adjusting to suit the real-world conditions.

Layout

Capacitive touch sensors in the RightTouch range from Microchip Technology’s SMSC subsidiary are designed for a base capacitance (untouched sensor) operating range of 5 to 50pF with a sensitivity for a touch of less than 0.1pF. The device circuitry and logic automatically compensate the measurement to provide a consistent ΔC value over the entire range of base capacitance. This simplifies the system design work and provides the most flexibility for PCB layout, overlay selection and environmental requirements.

To get the best out of these sensors, it is important to keep ∆C large relative to noise, to reduce the overlay thickness and avoid conductive overlay material. The LED output traces must be isolated from the capacitive sensor (CS) pads on different layers by a ground plane between them. The same isolation must be observed for the CS pads and traces and any other switching signals on the PCB, including signals generated by sources other than the touch sensor device. It is, however, acceptable to route CS traces on adjacent layers to CS pads without an isolation ground plane between them.

The CS traces cannot be parallel with LED output traces on the same layer or on adjacent layers. Try to keep the distance between the LED and CS trace with ground guard trace to at least three times the trace width. The minimum distance between the LED and CS trace without ground guard trace should be 1.27mm. The CS minimum trace width is 0.1 to 0.2mm and the CS minimum trace-to-trace distance should be 0.1mm.

If a CS trace must cross an LED output signal on adjacent layers due to PCB routing constraints, they must cross at a 90˚ angle. Try to reduce the number of vias and layer changes in the CS traces as these add parasitic capacitance.

Unused CS pins and LED pins should be terminated, either with a pull-down resistor or tied directly to ground. Ensure unused LED and GPIO pins shorted to ground are not driven by controlling firmware. Always cover unused areas with a 15% cross-hatch fill ground plane around and below CS traces. Add vias to ground planes to ensure there are no islands.

Design

These capacitive touch sensors work well with any CS pad shape, including the most commonly used square, rectangular, round and oval. When designing a rectangular or oval CS pad, a length and breadth ratio of less than 4:1 is recommended.

In general, a larger CS pad will have a bigger ΔC and can provide a more repeatable touch detection. However, since the average adult’s fingertip is about 10mm in diameter, oversize CS pads will only increase detection sensitivity up to a point.

The CS pad size also depends on the overlay thickness. For thicker overlays, larger CS pads are needed. These sensors can detect a finger touch with a CS pad as small as 16mm2 (4 by 4 mm) in a system with 2mm plastic overlay.

For typical applications with a normal thickness (1 to 3mm) of generic plastic overlay, the recommended pad size is equal or greater than 29mm2. If it is allowed by the board dimensions and CS pad location, a larger pad area is always better.

The minimum spacing between two CS pads is about 1.3mm from edge to edge. However, if two CS pads are placed too close, touching one pad could cause a capacitance change on the other. To avoid such an unwanted detection, the suggested minimum distance is 10mm from edge to edge, but other factors, such as pad size and the user’s fingertip size, must be considered.

Some RightTouch capacitive sensing devices, such as the CAP1114, have the capability to use several of the CS inputs as a single slider group. Typical slider shapes are shown in Figure 2. These sliders are similar to the individual CS pad design and the pad area needs to be greater than 29mm² with the distance between the pads about 1.3mm.

Theoretically, any pad shape used for a button pad can also be used for a slider pad. However, the arrow type slider shape will provide smoother responses when a finger crosses from one pad to the next. It will also provide a clear direction indicator for schematic design, PCB layout and assembling processes. During typical operation, SMSC capacitive touch sensors will scan each channel for capacitance change and connect all other channels to ground. Therefore, for the pad to be measured, the two nearby slider pads can be considered as ground planes.

The seven-pad slider will provide good sensitivity as well as enough accuracy for most applications, but many applications can also use a slider with fewer than seven pads. In most applications, the width of each pad and the distance between pads will usually be limited by the total length of the slider, and the height of the slider will also be limited by the physical dimensions of the machine. If the pad’s size cannot meet the minimum requirement and reduced accuracy is acceptable, the number of pads in the slider can be reduced to five, three or even two for just up and down control.

ESD and RFI

These capacitive touch sensors can withstand high levels of ElectroStatic Discharge (ESD) without physical damage. In addition, operational immunity from ElectroMagnetic Interference (EMI) and ESD environments is reduced through proprietary techniques. However, excessive environmental conditions can produce false touches, activate internal ESD protective clamps, or affect VDD and ground resulting in a device reset. As such, it is important for ElectroMagnetic Compatibility (EMC) to be considered as early as possible in the design process.

Typically, there are two points of entry for ESD into a system; transient charge entering through a board-to-board connection, and transient charge coupling to the PCB. Figure 3 illustrates several approaches for dealing with the former.

The first method is to increase the impedance to high frequencies using a series resistor, a ferrite bead or a common mode choke on the VCC and ground lines. Adding Transient Voltage Suppression (TVS) diodes, also known as avalanche breakdown diodes, between VCC and ground can shunt the ESD current. And ESD protection devices, such as series resistors or ferrite beads, can be added to the communications lines themselves.

There are several ways to deal with transient charge coupling to the PCB. The ESD points of entry can be visible air gaps in the covering material, areas where the two pieces of covering material come together, around the edges of the covering material and so on.

Some of the metal on the PCB can be designed to conduct the ESD charge to ground. This should be exposed as a metal ring around the outer edge of the PCB to conduct ESD current to the chassis. It is also possible to route the signal ground between this and all other traces on the PCB.

Some of these sensors can sense and respond to RF fields. When an RF signal is detected, the response is to disable active sensors to avoid false touches. Once the RF field is removed, the sensors are re-activated. However, there are some environments that require additional work to provide immunity to RF signals. Microchip’s RightTouch capacitive sensor family provides a wide variety of slider, button and LED driver functionality. However, to optimise system performance it is important to employ the correct design and PCB layout techniques.