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Capacitive touchscreens and finger capacitance

Views: 208     Author: Reshine Display     Publish Time: 2023-09-08      Origin: Site


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Capacitive touchscreens and finger capacitance

1. Finger Capacitance Overview

The term "finger capacitance" refers to the electrical charge that is applied to the surface of a capacitance touch screen in response to a touch command. When a capacitance touch screen is touched, it absorbs some electrical charges from the user's body. Instead, there is only a small amount of electrical discharge that a capacitance touch screen can detect. This additional electrical charge, however, is known as finger capacitance because it originates from the user's finger.

Finger Capacitance Principle of Operation

To understand how finger capacitance works, one must first be familiar with the fundamental characteristics of capacitance touch screens. Capacitance devices are touch screens that detect user commands by sensing capacitance. When activated, they will project a consistent electrostatic field across the display interface. The electrostatic field will then be measured by capacitance touch screens.

Overview of Finger Capacitance

Because the human body is electrically conductive, the electrostatic field of a capacitance touch screen will change when touched with a bare finger. As a result, the device's display interface will receive a small electrical charge from the user's finger. As a result, the electrostatic field of the capacitance touch screen will become stronger in the vicinity of a touch command. Simply put, finger capacitance is the additional electrical charge added to the display interface by a finger.

The capacitance of the Finger

This electrical phenomenon, known as "finger capacitance," is not limited to a single finger. A capacitance touch screen can be operated with any conductive object. As long as the object conducts electricity, the device's electrostatic field will be distorted.

A conductive stylus is a common example. Conductive styluses have a consistent appearance. The only distinction is that they are made of conductive material. When a capacitance touch screen is touched with a conductive pen, the capacitance of a finger is added to the display interface. As a result, the device will recognize and record the order when that location is touched.

Finger capacitance is created when a finger or other conductive object adds an electrical charge to a capacitance touch screen. It allows capacitance touch screens to recognize touch commands. When finger capacitance is applied, the device recognizes it as a touch command.

The Capacitor Made of PCBs

Capacitors are classified into several types. Although surface-mount packages and LED components are typically associated with capacitance, all that is required is two conductors separated by an insulating layer (i.e., the dielectric). As a result, making a capacitor with the conducting layers built into a printed circuit board is relatively simple. Consider the following top and side views of a PCB capacitor used as a touch-sensitive button as an example.

A capacitor is formed by the insulating space between the touch-sensitive button and the surrounding copper. Because the surrounding copper is wired to the ground node, the touch-sensitive button can be thought of as a capacitor between the ground and the touch-sensitive signal.

The Influence of a Finger

Because the solder mask on the PCB and typically a plastic layer that isolates the device's electronics from the environment act as barriers between the finger and the capacitor, no direct conduction occurs. As a result, the finger does not discharge the capacitor. Furthermore, the quantity of interest is the capacitance at that same time rather than the charge remaining in the capacitor.

Why does capacitance change when there is a finger present? There are two reasons for this: The first is related to the conductive properties of the finger, and the second is related to its dielectric properties.

Dielectric Finger

Because the solder mask on the PCB and typically a plastic layer that isolates the device's electronics from the environment act as barriers between the finger and the capacitor, no direct conduction occurs. As a result, the finger is not discharging the capacitor, and the quantity of interest is the capacitance at that same moment rather than the charge remaining in the capacitor.

Because the capacitor's electric field extends outside, the finger can affect the dielectric properties without coming into contact with the plates.

Because our bodies are mostly made of water, human flesh is an excellent dielectric material. The dielectric constant of air is slightly greater than that of a vacuum, which is one (about 1.0006 at sea level and room temperature). Water, on the other hand, has a dielectric constant of approximately 80, which is significantly higher. As a result, the interaction of the finger with the capacitor's electric field raises the dielectric constant, which raises the capacitance.

Conductor's Finger

Anyone who has ever received an electric shock is well aware that human skin conducts electricity. As previously stated, there is no direct conduction between the finger and the touch-sensitive button, so the finger cannot discharge the PCB capacitor. This lack of direct conduction does not, however, imply that the finger's conductivity is unimportant. On the contrary, the finger serves as the second conductive plate of an additional capacitor, making it extremely important.

For practical purposes, the finger-created capacitor, referred to as the finger cap, is assumed to be connected in parallel to the capacitor already present on the PCB. Because the person using the touch-sensitive device is not electrically connected to the PCB's ground node, the two capacitors are not "in parallel" in the sense of a standard circuit analysis, which complicates matters.

The human body, on the other hand, is thought to act as a virtual ground due to its relatively high capacity to absorb electric charge. As a result, the precise electrical connection between the finger cap and the PCB cap is unimportant. What matters is that the finger will increase the total capacitance because capacitors add in parallel due to the two capacitors' pseudo-parallel design.

As a result of both systems controlling how the finger interacts with the capacitive touch sensor, the capacitance increases.

Contact vs. Proximity

The preceding discussion highlights an intriguing feature of capacitive "touch" sensing. In addition to physical contact, mere proximity to the sensor can result in detectable capacitance changes. Touch-sensitive devices are frequently misidentified. Capacitive sensing technology adds a new level of functionality to mechanical switches or buttons by allowing systems to calculate the separation between a sensor and a finger.

The effects of the above-mentioned capacitance-altering methods are inversely proportional to distance. When the finger approaches the conductive areas of the PCB capacitor for the dielectric-constant-based method, more fleshy dielectric interacts with the capacitor's electric field. As a result, the capacitance of the finger cap for the conductivity-based mechanism is inversely proportional to the separation between the conducting plates, just like any other cap.

Remember that this is not a method for determining the exact distance between the sensor and the finger; capacitive sensing does not provide the information required to perform accurate absolute distance computations. However, because capacitive sense circuitry is designed to detect changes in capacitance, this technology is suitable for detecting changes in distance, i.e., while a finger moves near or far from a sensor.

2. The Benefits of Capacitive Touch Screens


One of the primary advantages of a projected capacitive touch screen is its toughness. Touch displays have a wide range of applications in business. If the function is carefully chosen and created, the capacitive touch screen will not be harmed by common issues such as dust and moisture. After surface treatment with AG, AR, and AF, it may successfully reduce light reflection, avoid fingerprint stains, and prevent scratching. Furthermore, when carefully selected and created to meet application requirements, the projected capacitive touch screen lasts longer.


Furthermore, due to its durability, the projected capacitive touch screen is extremely unlikely to be scratched. Even if the surface is scratched as a result of an accident, the projected capacitive touch screen will continue to function normally unless the back-mounted conductive matrix is damaged. This functionality is provided because it will continue to measure changes in the generated electric field regardless of damage.

Touch precision

One of the key reasons why this technology is so popular in consumer electronics and is now so successful in commercial/industrial applications is that it is a highly sensitive touch technology that only responds to fingers or conductive pens (which means the risk of "wrong contact" is tiny). While inanimate objects can affect optical or acoustic touch displays, resistive touch screens require more stress than projected capacitive touch screens (rain, leaves, ties, cuffs, etc.).

Image sharpness

Because they are typically made of clear, uncoated glass with a matrix of micro-conductors on the back, projected capacitive touch displays often provide superior image quality when compared to most other touch technologies. Capacitive displays are ideal for the latest HD, UHD, and OLED displays.

To generate signals, capacitive touch displays only require a touch, not pressure. While resistive technology necessitates traditional calibration, capacitive touch panels necessitate only one calibration after manufacturing or none at all.

Because the components in a capacitive touch screen do not need to move, the capacitive solution has a longer life. On resistive touch screens, the upper ITO film must be thin and flexible to bend downward and make contact with the lower ITO film.

Capacitive technology outperforms resistive technology in terms of light loss and system power consumption. The item that makes contact with the screen determines whether capacitive or resistive technology is used. If it is touched with a finger, a capacitive touch screen is preferable. A resistive touch screen, whether made of plastic or metal, can function as a stylus. It is also possible to use a stylus with a capacitive touch screen; however, a compatible stylus is required.

The inductive capacitive type is commonly used for small and medium-sized touch screens and can recognize gestures. The surface capacitive type, on the other hand, can be used for large-size touch displays and has a low relative content, but it does not currently support gesture recognition.

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