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 1-3), which is more than two

orders of magnitude greater than that required to create a

return path to the QT110 via earth. The QT110's PCB however

can be physically quite small, so there may be little ‘free space’

coupling (Cx1 in Figure 1-3) between it and the environment to

complete the return path. If the QT110 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

QT110’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.

The QT110 can be set for one of 3 gain levels using option pin

5 (Table 1-1). If left open, the gain setting is high. The

sensitivity change is made by altering the numerical threshold

level required for a detection. It is also a function of other

things: 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 are all influences.

Gain plots of the device are shown on page 9.

The Gain input should never be tied to anything other than

SNS1 or SNS2, or left unconnected (for high gain setting).

In some cases it may be desirable to increase sensitivity

further, for example when using the sensor with very thick

panels having a low dielectric constant.

Sensitivity can often be increased by using a bigger electrode,

reducing panel thickness, or altering panel composition to one

having a higher dielectric constant. Increasing electrode size

can have diminishing returns, as high values of Cx will reduce

sensor gain.

Increasing the electrode's surface area will not substantially

increase touch sensitivity if its diameter is already much larger

in surface area than the object being detected. Metal areas

near the electrode will reduce the field strength and increase

Cx loading and are to be avoided for maximal gain.

Ground planes around and under the electrode and its SNS

trace will cause high Cx loading and destroy gain. The possible

signal-to-noise ratio benefits of ground area are more than

negated by the decreased gain from the circuit, and so ground

areas around electrodes are discouraged. Keep ground,

power, and other signals traces away from the electrodes and

SNS wiring.

The value of Cs has a minimal effect on sensitivity with these

devices, but if the Cs value is too low there can be a sharp

drop-off in sensitivity.

3

QT110 R1.04/0405

Figure 1-3 Internal Switching & Timing

SNS2

SNS1

ELECTRODE

Charge

Am p

Result

Done

Start

Figure 1-5 Kirchoff's Current Law

Sense E lectrode

Su rro un d ing e n v iro n m e n t

SENSOR

Pin 7

Low

Pin 6

Medium

Leave open

High

Tie Pin 5 to:

Gain

Table 1-1 Gain Strap Options