lQQT110

The QT110 charge-transfer (“QT’”) sensor IC is a self-contained digital IC used to implement near-proximity or touch sensors. It

projects sense fields through almost any dielectric, like glass, plastic, stone, ceramic, and wood. It can also turn small metal-bearing

objects into intrinsic sensors, making them respond to proximity or touch. This capability coupled with an ability to self-calibrate

continuously leads to entirely new product concepts.

The QT110 employs digital signal processing techniques pioneered by Quantum, designed to make it survive real-world challenges,

such as ‘stuck sensor’ conditions and signal drift. Sensitivity is digitally determined for the highest possible stability. No external active

components are required for operation.

switch replacement. Another makes the sensor output a pulse instead of a DC level, which allows the device to 'talk' over the power

to continuously monitor sensor health.

By using the charge transfer principle, the QT110 delivers a level of performance clearly superior to older technologies in a highly

cost-effective package.

lq

is in the proper range, no additional capacitor is

is too small, it can simply be ‘topped up’ with a

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).

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

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.

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

Like all capacitance sensors, the QT110

1-5) to detect the change in capacitance

of the electrode. This law as applied to

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

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

drop-off in sensitivity.

It is desirable to suppress detections generated by electrical

noise or from quick brushes with an object. To accomplish this,

increments with each detection until a limit is reached, after

which the output is activated. If no detection is sensed prior to

the final count, the counter is reset immediately to zero. In the

apart, so that the slowest reaction time possible is

75+20+20+20 or 135ms and the fastest possible is 60ms,

depending on where in the initial burst interval the contact first

occurred. The response time will thus average about 95ms.

The QT110 has no recalibration pin; a forced recalibration is

the supply drain is so low it is a simple matter to treat the entire

directly from another logic gate or a microprocessor port

LED’s. It can be filtered completely if desired, by adding an RC

timeconstant to filter the output, or if interfacing directly and

only to a high-impedance CMOS input, by doing nothing or at

most adding a small non-critical capacitor from Out to ground

The QT110’s output is active low ; it can source 1mA or sink

5mA of non-inductive current.

Care should be taken when the IC and the load are both

powered from the same supply, and the supply is minimally

1. Use a ground plane under the IC itself and Cs and Rs but

NOT under Re, or under or closely around the electrode or

its connecting trace. Keep ground away from these things

to reduce stray loading (which will dramatically reduce

that an increase of Re by 50% does not appreciably

decrease sensitivity; if it does, reduce Re until the 50%

The power supply can range from 2.5 to 5.0 volts. At 2.5 volts

current drain averages less than 10µA with Cs = 10nF,

provided a 470K Rs resistor is used (Figure 2-6). Idd curves

are shown in Figure 4-4.

Higher values of Cs will raise current drain. Higher Cx values

can actually decrease power drain. Operation can be from

batteries, but be cautious about loads causing supply droop

As battery voltage sags with use or fluctuates slowly with

affected by rapid voltage steps.

if desired, the supply can be regulated using a conventional

low current regulator, for example CMOS LDO regulators that

have nanoamp quiescent currents. Care should be taken that

the regulator does not have a minimum load specification,

which almost certainly will be violated by the QT110's low

current requirement. Furthermore, some LDO regulators are

unable to provide adequate transient regulation between the

quiescent and acquire states, creating Vdd disturbances that

will interfere with the acquisition process. This can usually be

other traces, or near a ground plane.

back to the regulator and power connector (but not beyond

the Cs capacitor).

device near the electrode or wiring of another device, to

prevent cross interference.

carrying AC or switched signals.

for a stable voltage just before each burst commences.

For further tips on construction, PCB design, and EMC issues

browse the application notes and faq at www.qprox.com

unless otherwise noted.