1.3 ELECTRODE DESIGN

There is no restriction on the shape of

the electrode; in most cases common

sense and a little experimentation can

result in a good electrode design. The

QT118H will operate equally well with

long, thin electrodes as with round or

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

composition, and the ground coupling

quality of both the sensor circuit and

the sensed object.

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'. 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.

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 1-4), 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 1-4) 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:

- A nearby piece of metal or metallized housing;

- A floating conductive ground plane;

- Another electronic device (to which its might be

connected already).

Free-floating ground planes such as metal foils should

maximize exposed surface area in a flat plane if possible. A

square of metal foil will have little effect if it is rolled up or

crumpled into a ball. Virtual ground planes are more effective

and can be made smaller if they are physically bonded to

other surfaces, for example a wall or floor.

‘Ground’ as applied to capacitive fields can also mean power

wiring or signal lines. The capacitive sensor, being an AC

device, needs only an AC ground return.

1.3.5.1 Gain Pin

The QT118H can be set for one of 3 gain levels using option

pin 5 (Table 1-1). This sensitivity change is made by altering

the internal numerical threshold level required for a detection.

Note that sensitivity is also a function of other things: like the

values of Cs and Cx, electrode size, shape, and orientation,

the composition and aspect of the object to be sensed, the

thickness and composition of any overlaying panel material,

and the degree of ground coupling of both sensor and object.

The Gain input should never be connected to a pullup or

pulldown resistor or tied to anything other than SNS1 or

SNS2, or left unconnected (for high gain setting).

3

QT118H R1.08 / 0405

Figure 1-3 Internal Switching & Timing

SNS2

SNS1

ELECTRODE

C ha rge

Amp

R esult

Do ne

Start

Figure 1-4 Kirchoff's Current Law

S e nse E lectrode

S u rrou n d in g en viro n m e n t

SENSO R