QT118H-S View Datasheet(PDF) - Quantum Research Group

Part Name

Description

Manufacturer

QT118H-S

CHARGE-TRANSFER TOUCH SENSOR

Quantum Research Group 

object, however it is impossible for the

sensor to distinguish between the two

touch areas.

1.3 ELECTRODE DESIGN

R esult

Figure 1-3 Internal Switching & Timing

SNS2

E LE C TRO DE

1.3.1 ELECTRODE GEOMETRY AND SIZE

There is no restriction on the shape of

the electrode; in most cases common

sense and a little experimentation can

S tart

result in a good electrode design. The

QT118H will operate equally well with

long, thin electrodes as with round or

Do ne

Cs

Cx

SNS1

square ones; even random shapes are

acceptable. The electrode can also be

a 3-dimensional surface or object.

Sensitivity is related to electrode

surface area, orientation with respect

to the object being sensed, object

C ha rge

Amp

composition, and the ground coupling

quality of both the sensor circuit and

the sensed object.

If a relatively large electrode surface is desired, and if tests

show that the electrode has more capacitance than the

QT118H can tolerate, the electrode can be made into a

Even when battery powered, just the physical size of the

PCB and the object into which the electronics is embedded

will generally be enough to couple a few picofarads back to

local earth.

1.3.3 VIRTUAL CAPACITIVE GROUNDS

When detecting human contact (e.g. a fingertip), grounding

of the person is never required. The human body naturally

has several hundred picofarads of ‘free space’ capacitance

to the local environment (Cx3 in Figure 1-5), which is more

than two orders of magnitude greater than that required to

create a return path to the QT118H via earth. The QT118H's

PCB however can be physically quite small, so there may be

little ‘free space’ coupling (Cx1 in Figure 1-5) between it and

the environment to complete the return path. If the QT118H

circuit ground cannot be earth grounded by wire, for example

via the supply connections, then a ‘virtual capacitive ground’

may be required to increase return coupling.

A ‘virtual capacitive ground’ can be created by connecting

the QT118H’s own circuit ground to:

sparse mesh (Figure 1-4) having lower Cx than a solid plane.

Sensitivity may even remain the same, as the sensor will be

operating in a lower region of the gain curves.

1.3.2 KIRCHOFF’S CURRENT LAW

Like all capacitance sensors, the QT118H relies on Kirchoff’s

Current Law (Figure 1-5) to detect the change in capacitance

of the electrode. This law as applied to capacitive sensing

requires that the sensor’s field current must complete a loop,

returning back to its source in order for capacitance to be

sensed. Although most designers relate to Kirchoff’s law with

regard to hardwired circuits, it applies equally to capacitive

field flows. By implication it requires that the signal ground

and the target object must both be coupled together in some

manner for a capacitive sensor to operate properly. Note that

there is no need to provide actual hardwired ground

connections; capacitive coupling to ground (Cx1) is always

sufficient, even if the coupling might seem very tenuous. For

example, powering the sensor via an isolated transformer

will provide ample ground coupling, since there is

capacitance between the windings and/or the transformer

core, and from the power wiring itself directly to 'local earth'.

(1) A nearby piece of metal or metallized housing;

Figure 1-5 Kirchoff's Current Law

CX2

S e nse E le ctro de

SENSOR

CX 1

Su rro und ing e nv iro nm e nt

CX3

lq

2

Share Link: