1 - OVERVIEW

The QT113 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 single low cost, non-critical capacitor is

required for operation.

Figure 1-1 shows the basic QT113 circuit using the device,

with

a

conventional

output

drive

and

power

supply

connections.

1.1 BASIC OPERATION

The QT113 employs bursts of charge-transfer cycles to

acquire its signal. Burst mode permits power consumption in

the microamp range, dramatically reduces RF emissions,

lowers susceptibility to EMI, and yet permits excellent

response time. Internally the signals are digitally processed

to reject impulse noise, using a 'consensus' filter which

requires three consecutive confirmations of a detection

before the output is activated.

The QT switches and charge measurement

hardware

functions are all internal to the QT113 (Figure 1-2). A 14-bit

single-slope switched capacitor ADC includes both the

required QT charge and transfer switches in a configuration

that provides direct ADC conversion. The ADC is designed to

dynamically optimize the QT burst length according to the

rate of charge buildup on Cs, which in turn depends on the

values of Cs, Cx, and Vdd. 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. Conversely, larger values of Cs reduce the rise of

differential voltage across it, increasing available resolution

by permitting longer QT bursts. The value of Cs can thus be

increased to allow larger values of Cx to be tolerated (Figures

4-1, 4-2, 4-3 in Specifications, rear).

The IC is responsive to both Cx and Cs, and changes in Cs

can result in substantial changes in sensor gain.

Option pins allow the selection or alteration of several 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 be connected to

either SNS1 or SNS2 with no performance difference. In both

cases the rule Cs >> Cx must be observed for proper

operation. The polarity of the charge buildup across Cs

during a burst is the same in either case.

It is possible to connect separate Cx and Cx’ loads to SNS1

and SNS2 simultaneously, although the result is no different

than if the loads were connected together at SNS1 (or

SNS2). It is important to limit the amount of stray capacitance

on both terminals, especially if the load Cx is already large,

for example by minimizing trace lengths and widths so as not

to exceed the Cx load specification 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. Multiple touch electrodes can be

used, for example to create a control button on both sides of

an object, however it is impossible for the sensor to

distinguish between the two touch areas.

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

QT113 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.

If a relatively large electrode surface is

desired, and if tests show that the

electrode has more capacitance than

the QT113 can tolerate, the electrode

- 2 -

Figure 1-1 Standard mode options

SENSING

ELECTRODE

10nF

3

46

5

1

+2.5 to 5

7

2

OUT

OPT1

OPT2

GAIN

SNS1

SNS2

Vss

Vdd

OUTPUT=DC

TIMEOUT=10 Secs

TOGGLE=OFF

GAIN=HIGH

8

Figure 1-2 Internal Switching & Timing

SNS2

SNS1

ELE C TRO DE

Charge

Am p

Result

Do ne

Start