sense through a completely shield housing.

Capacitive sensor works by detecting the change of capacitance

due to the influence of external object. Roughly speaking,

capacitive sensor comprises of electronic circuits that measure capacitance

across electrodes

also known as an antennas.

When the capacitance changes,

the circuits and the algorithm infer the presence of the external object.

But exactly where is the capacitor, and how does the human finger change the capacitance?

A common model is that the electrode forms one plate the capacitor

and the grounded finger forms the other plate and changes to overall capacitance

of the sensor

But our experience is that user need not be electrically connected to the

circuits

or put their feet on the ground to operate the sensor.

So grounded finger as a requirement for capacitive sensor

is a misconception.

so let's look at the physics on how a ungrounded finger influence

capacitance.

when conductors are connected to the source

the electric field from the source

pushes the charges out to the conductors.

The positive charge the negative charge are attractive, so they move to each other

as close as possible.

charges on the same plate are repulsive,

so they push each other away to the edges.

As more charges join the plate

they build up electric field to oppose others from joining in.

Eventually, the nett force along the conductor is zero

and the charges stay in equilibrium.

This is when the plates have the same potential difference as the voltage source.

and the capacitance is defined as the ability to store charges on a per-volt basis

When an external conductor is put nearby, it cuts into the electric field.

The electric field polarize the conductor, and energy is transferred to

the poloarized charges.

The polarized charges get energy from the plates

and the plates now have a lower potential.

If the source is still connected, more charges will join in the plates until

the plate potential is back again.

The extenal object allows more charges to store in the plates,

and therefore it increases the capacitance.

So even when the extennal object is not physically connected to any of the plates

it is still capable of influencing the capacitance.

Any charge in the system

is subject to attactive forces from the unlike charges

and repulsive charges from the like charges. This complex tug of war happens to

every single charge in the system.

Based on this,

a mathematical model can be developed

to solve for the charge distribution

which would satisfy the equilibrium condition as mentioned previously.

The charge of distribution

shows the physics in the system

and can be used to calculate the capacitance.

For a coplanar capacitor,

numerical solution shows that the capacitance increases when a third plate is placed nearby.

and the third plate is indeed polarized by the electric field.

Two plates are required to detect a nearby object,

this dismiss the idea that cap-sense uses one plate

to detect the object's capacitance with respect to infinity.

We can also do numerical experiments

to study the effects of antenna geometry

on the sensitivity to extenal the influence. For a human touching the antenna

configuration as shown,

the capacitance increased by 0.056pF

amounts to 30% increase

If the antennas are enlarged,

they can pick up more influence. The capacitance change is more,

and the percentage change is more, too.

so larger antennas are easier for the measuring circuit.

If the distance between the antennas is shortened,

the electric field is more confined.

Although the capacitance is higher,

the delta change is actually less, and make it a less effective sensor.

In most designs, one antenna is made as the touch focus point and the other integrated

to the ground plane.

Without special care,

the ground plane can pick up the object's influence as well.

So how can we increase the sensitivity of one plate and reduce the sensitivity

of the other?

one method is to make one antenna more accessible than the other.

Another method is to control the size of the antenna and will be explained here.

In a simulation of a finger 2mm above a coplanar structure

the capacitance increased by 12%.

When the human body is included in the calculation, the capacitance

increased by 22%.

the human body itself

double the influence on the capacitance.

The body by its huge surface area and the finger by its close proximity, couple

themselves to the antennas.

If one of the plates is enlarged to provide good and constant coupling to the

human body, then the finger approaching the smaller plate

becomes the deciding factor for the change of capacitance.

Using numerical simulation for the above configuration

we see capacitance change by more than 100%.

If the finger approaches the big plates

it doesn't close the loop and the capacitance change is minimal.

So by having a large ground plane,

which is one of the antennas, the sensitivity automatically goes to the

smaller antenna,

which is meant to be the touch focus point.

Even if the ground is made smaller,

and the touch antenna made bigger, the senstivity still follows the smaller

plates.

The smaller antenna gets more sensitivity because of the higher percentage of

change compared to the bigger antenna in the same configuration.

but the smaller antenna do not necessarily get more absolute delta C

then the bigger but less sensitive antenna in another configuration.

So to enjoy higher sensitivity from a higher percentage change of a smaller

antenna

the sensing IC must have a good S/N to begin with.

If a capacitive sensor is connected to the mains wiring,

effectively one of the plate area is increased significantly.

The absolute delta C and therefore the signal level is higher. The touch

antenna because of its relatively small area

will get all the sensitivity.

The mains wire

because of its relatively big coupling area

will become insensitive.

Without the inverse relationship

the mains wire will get a lot of false triggers.

Thank you for watching and I hope you enjoy the video