Fundamentals and Principles of Capacitive Touch

Touch keys are now widely used. This article provides an introductory overview of capacitive touch sensing and its basic operating principles.

01 | Overview

Capacitive touch sensing detects the presence or proximity of a finger by measuring changes in capacitance. With capacitive sensing, mechanical switches and knobs can be replaced by aesthetically pleasing buttons, sliders, and wheels to address issues such as:

1. Wear and reduced reliability after prolonged use
2. Gaps between the front panel and keys that allow moisture ingress and cause faults
3. The need to apply physical force to actuate a switch
4. Added cost due to front-panel cutouts
5. Limited fixed key shapes

Capacitive touch supports five sensor types: keys, proximity sensing, wheels, sliders, and touch panels. It also supports multiple cover materials. Typical advantages include low power consumption, robust sensing algorithms, strong noise immunity, and support for waterproofing.

02 | Basic Principle

Typical capacitive touch sensors use copper areas on the PCB as electrodes. The top surface is covered by a nonconductive protective layer such as glass or plastic bonded to the PCB with adhesive. A guard or grid ground is often placed around the sensors.

Based on the measured capacitance type, capacitive touch sensing is classified as self-capacitance detection (measuring capacitance between a single electrode and ground) or mutual-capacitance detection (measuring capacitance between a pair of electrodes).

2.1 Self-capacitance Detection

For a simple single key, the self-capacitance detection model is shown below. Self-capacitance sensing uses a single copper electrode (receive electrode Rx) to detect the electrode-to-ground capacitance change. The key's initial capacitance to ground is Cp. When a human finger touches, the loop is influenced by Ct, Ch, and Cg, causing the electrode-to-ground capacitance to increase.

Note: Solid lines indicate actual wiring; dashed lines indicate non-physical connections. Grey components represent equivalent capacitances or resistances.

• Rh: Human body resistance
• Rs: Series resistor, recommended value 470 Ω
• Cp: Parasitic capacitance from the key and its traces to system ground
• Cg: Capacitance between system ground and earth. For battery-powered applications, approximately 1 pF; for earth-grounded applications, it may be effectively shorted.
• Ch: Series capacitance between the human body and earth
• Ct: Capacitance formed between the electrode and the fingertip (similar to a parallel-plate capacitor)
• Cd: Capacitance between the hand and system ground

For simplified analysis, Rh and Rs effects are ignored. The electrode-to-ground equivalent capacitance is given by Formula 1-1 (see image). Sensitivity can be defined as the ratio of the touch-induced capacitance change to the baseline capacitance (also shown in Formula 1-1). Since Ch is large compared with Cg and Ct, Ch can be neglected. When the ground plane is sparse, Cd is small and Cg + Cd can be approximated as Cg.

Where:

• A: Contact area between fingertip and the sensor cover
• d: Thickness of the cover layer
• ε0: Permittivity of free space
• εr: Relative permittivity of the cover layer

From Formulas 1-2 and 1-3, methods to improve sensitivity include:

1. Reduce the cover thickness and/or increase the cover material's εr to increase Ct
2. Reduce the density of the grid ground or increase the PCB thickness to lower Cp
3. Since Ct and Cg can be of similar magnitude, connecting system ground to earth in a reasonable way increases Cg
4. Increase electrode area sensibly — raising the fingertip contact area A to increase Ct

Note that electrode area cannot be increased indefinitely to raise sensitivity. The effective parallel-plate capacitance area is limited by the fingertip contact area, and excessively large electrodes increase Cp, which can reduce sensitivity.

2.2 Mutual-capacitance Detection

Mutual-capacitance sensing uses paired copper electrodes (transmit Tx and receive Rx) to detect capacitance changes between the two electrodes. A major advantage of mutual-capacitance detection is that the parasitic capacitance from the key to system ground (Cp) can be largely ignored. For a single-key example, the detection model is shown below. When a finger touches, CRT is effectively split into two capacitors (2·CRT) and additional terms CRTt, Ct, Ch, and Cg are introduced. Ultimately, the capacitance between the electrode pair decreases.

Note: Solid lines indicate actual wiring; dashed lines indicate non-physical connections. Grey components represent equivalent capacitances or resistances.

• CRTt: Parallel capacitance introduced between Rx and Tx by the touching finger
• CRT: Baseline capacitance between Rx and Tx; when touched, it becomes two capacitors of value 2·CRT in the equivalent model

The equivalent capacitance between Tx and Rx is given by Formula 1-4, and sensitivity is defined as the ratio of the touch-induced capacitance change to the baseline capacitance (Formula 1-5).

For mutual-capacitance sensing, main ways to increase sensitivity are:

1. Reduce the cover thickness
2. Increase the spacing between Tx and Rx. Note: Increasing Tx–Rx spacing reduces CRT and can increase detection distance (effectively increasing sensitivity), but if a fingertip cannot simultaneously overlap both Tx and Rx, sensitivity may decrease.

In general, finger touches produce capacitance changes on the order of about 1 pF for both self- and mutual-capacitance sensing. However, the baseline (pre-touch) capacitance for self-capacitance sensors is typically higher than that for mutual-capacitance sensors. Therefore, mutual-capacitance designs tend to offer higher sensitivity relative to their baseline but are also more susceptible to noise.

From an application standpoint, self-capacitance schemes are widely used due to simpler structure, while mutual-capacitance schemes are often used for matrix key arrays where the supported key count far exceeds the number of IO pins available for self-capacitance keys. A comparison between the two schemes is shown in the figure below.