1 - OVERVIEW

The QT110 is a digital burst mode charge-transfer (QT) sensor

designed specifically for touch controls; it includes all hardware

and signal processing functions necessary to provide stable

sensing under a wide variety of changing conditions. Only a

few low cost, non-critical discrete external parts are required for

operation.

Figure 1-1 shows the basic QT110 circuit using the device,

with a conventional output drive and power supply

connections. Figure 1-2 shows a second configuration using a

common power/signal rail which can be a long twisted pair from

a controller; this configuration uses the built-in pulse mode to

transmit output state to the host controller (QT110 only).

1.1 BASIC OPERATION

The QT110 employs low duty cycle bursts of charge-transfer

cycles to acquire its signal. Burst mode permits power

consumption in the low microamp range, dramatically reduces

EMC problems, and yet permits excellent response time.

Internally the signals are digitally processed to reject impulse

noise, using a 'consensus' filter which requires four

consecutive confirmations of a detection before the output is

activated.

The QT switches and charge measurement hardware functions

are all internal to the QT110 (Figure 1-3). A single-slope

switched capacitor ADC includes both the required QT charge

and transfer switches in a configuration that provides direct

ADC conversion. Vdd is used as the charge reference voltage.

Larger values of Cx cause the charge transferred into Cs to

rise more rapidly, reducing available resolution; as a minimum

resolution is required for proper operation, this can result in

dramatically reduced apparent gain.

The IC is highly tolerant of changes in Cs since it computes the

signal threshold level ratiometrically. Cs is thus non-critical and

can be an X7R type. As Cs changes with temperature, the

internal drift compensation mechanism also adjusts for the drift

automatically.

Piezo sounder drive: The QT110 can drive a piezo sounder

after a detection for feedback. The piezo sounder replaces or

augments the Cs capacitor; this works since piezo sounders

are also capacitors, albeit with a large thermal drift coefficient.

ceramic capacitor in parallel. The QT110 drives a ~4kHz signal

across SNS1 and SNS2 to make the piezo (if installed) sound a

short tone for 75ms immediately after detection, to act as an

audible confirmation.

Option pins allow the selection or alteration of several other

special features and sensitivity.

1.2 ELECTRODE DRIVE

The internal ADC treats Cs as a floating transfer capacitor; as a

direct result, the sense electrode can in theory be connected to

either SNS1 or SNS2 with no performance difference.

However, the noise immunity of the device is improved by

connecting the electrode to SNS2, preferably via a series

resistor Re (Figure 1-1) to roll off higher harmonic frequencies,

both outbound and inbound.

In order to reduce power consumption and to assist in

discharging Cs between acquisition bursts, a 470K series

resistor Rs should be connected across Cs (Figure 1-1).

The rule Cs >> Cx must be observed for proper operation.

Normally Cx is on the order of 10pF or so, while Cs might be

10nF (10,000pF), or a ratio of about 1:1000.

It is important to minimize the amount of unnecessary stray

capacitance Cx, for example by minimizing trace lengths and

widths and backing off adjacent ground traces and planes so

as keep gain high for a given value of Cs, and to allow for a

larger sensing electrode size if so desired.

The PCB traces, wiring, and any components associated with

or in contact with SNS1 and SNS2 will become touch sensitive

and should be treated with caution to limit the touch area to the

desired location.

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 QT110 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 QT110

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

2

QT110 R1.04/0405

Figure 1-1 Standard mode options

SENSING

ELECTRODE

Cs

Rs

2nF - 500nF

3

46

5

1

+2.5 ~ +5

7

2

OUT

OPT1

OPT2

GAIN

SNS1

SNS2

Vss

Vdd

OUTPUT = DC

TIMEOUT = 10 Secs

TOGGLE = OFF

GAIN = HIGH

8

Figure 1-2 2-wire operation, self-powered

+

10µF

1N4148

n-ch Mosfet

CMOS

LOGIC

3.5 - 5.5V

1K

Twisted

pair

8

OUT

OPT1

OPT2

GAIN

SNS1

SNS2

Vss

Vdd

3

46

5

1

7

2

SENSING

ELECTRODE