Capacitive sensing

Human interface devices based on capacitive sensing, such as touchpads,[1] can replace the computer mouse.

Digital audio players, mobile phones, and tablet computers will sometimes use capacitive sensing touchscreens as input devices.

A capacitive touchscreen typically consists of a capacitive touch sensor along with at least two complementary metal–oxide–semiconductor (CMOS) integrated circuit (IC) chips, an application-specific integrated circuit (ASIC) controller and a digital signal processor (DSP).

Capacitive sensing is commonly used for mobile multi-touch displays, popularized by Apple's iPhone in 2007.

[3][4] Capacitive sensors are constructed from many different media, such as copper, indium tin oxide (ITO) and printed ink.

Copper capacitive sensors can be implemented on standard FR4 PCBs as well as on flexible material.

ITO allows the capacitive sensor to be up to 90% transparent (for one layer solutions, such as touch phone screens).

Since the parasitic capacitance of the sensor is related to the electric field's (E-field) path to ground, it is important to choose a ground plane that limits the concentration of E-field lines with no conductive object present.

Tools such as CapExt,[5] ANSYS Q3D Extractor,[6] and solutions from FastFieldSolvers[7] can be employed to optimize designs by enhancing sensitivity, accurately modeling electromagnetic fields, and improving performance across varying environmental conditions.

A small voltage is applied to this layer, resulting in a uniform electrostatic field.

[10] When a conductor, such as a human finger, touches the uncoated surface, a capacitor is dynamically formed.

Because of the sheet resistance of the surface, each corner is measured to have a different effective capacitance.

With no moving parts, it is moderately durable, but has low resolution, is prone to false signals from parasitic capacitive coupling, and needs calibration during manufacture.

Such conductive smudges come mostly from sticky or sweaty finger tips, especially in high humidity environments.

Bringing a finger or conductive stylus near the surface of the sensor changes the local electric field which reduces the mutual capacitance.

Mutual capacitance allows multi-touch operation where multiple fingers, palms or styli can be accurately tracked at the same time.

With self-capacitance, current senses the capacitive load of a finger on each column or row.

[14] Capacitance is typically measured indirectly, by using it to control the frequency of an oscillator, or to vary the level of coupling (or attenuation) of an AC signal.

Basically the technique works by charging the unknown capacitance with a known current, since rearranging the current–voltage relation for a capacitor,

allows determining the capacitance from the instantaneous current divided by the rate of change of voltage across the capacitor:

is 0 V, then the capacitance is simply the value of that constant current multiplied by the charging time duration

For instance, if measuring after a constant amount of time, then the capacitance can be determined using only the final voltage.

For an example capacitive sense IC, Texas Instruments's FDC1004 applies a 25-kHz step waveform to charge up an electrode, and after a defined amount of time, converts the analog voltage representing that charge into a digital value of capacitance using a built-in analog-to-digital converter (ADC).

[16] The capacitance bridge helps to compensate for any variability that may exist in the applied signal.

Also, to minimize the unwanted effects of stray capacitance, it is good practice to locate the sensing electronics as near the sensor electrodes as possible.

However, projective capacitance improves a touchscreen's accuracy as it forms a triangulated grid around the point of touch.

Some cannot be used with gloves and can fail to sense correctly with even a small amount of water on the screen.

Mutual capacitive sensors can provide a two-dimensional image of the changes in the electric field.

Power supplies with a high level of electronic noise can reduce accuracy.

Many stylus designs for resistive touchscreens will not register on capacitive sensors because they are not conductive.