QT118HA-ISG Datasheet QT118HA-ISG | P4-P6

1.3.5.2 Changing Cs, Cx

The values of Cs and Cx have a dramatic effect on

sensitivity, and Cs can be easily increased in value to

improve gain. Sensitivity is directly proportional to Cs and

inversely proportional to Cx:

S =

k$C

Where ‘k’ depends on a variety of factors including the gain

pin setting (see prior section), Vdd, etc.

Sensitivity plots are shown in Figures 4-1 and 4-2, page 10.

1.3.5.3 Electrode / Panel Adjustments

Sensitivity can often be increased by using a bigger

electrode, or reducing overlying panel thickness. Increasing

electrode size can have a diminishing effect on gain, as the

attendant higher values of Cx will start to reduce sensor gain.

Also, 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.

The panel or other intervening material can be made thinner,

but again there are diminishing rewards for doing so. Panel

material can also be changed to one having a higher

dielectric constant, which will help propagate the field through

to the front. Locally adding some conductive material to the

panel (conductive materials essentially have an infinite

dielectric constant) will also help; for example, adding carbon

or metal fibers to a plastic panel will greatly increase frontal

field strength, even if the fiber density is too low to make the

plastic bulk-conductive.

1.3.5.3 Ground Planes

Grounds 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

2 - QT118HA SPECIFICS

2.1 SIGNAL PROCESSING

The QT118HA digitally processes all signals

using a number of algorithms pioneered by

Quantum. The algorithms are specifically

designed to provide for high survivability in the

face of all kinds of adverse environmental

changes.

2.1.1 DRIFT COMPENSATION ALGORITHM

Signal drift can occur because of changes in Cx

and Cs over time. It is crucial that drift be

compensated for, otherwise false detections,

non-detections, and sensitivity shifts will follow.

Drift compensation (Figure 2-1) is performed by making the

reference level track the raw signal at a slow rate, but only

while there is no detection in effect. The rate of adjustment

must be performed slowly, otherwise legitimate detections

could be ignored. The QT118HA drift compensates using a

slew-rate limited change to the reference level; the threshold

and hysteresis values are slaved to this reference.

Once an object is sensed, the drift compensation mechanism

ceases since the signal is legitimately high, and therefore

should not cause the reference level to change.

The QT118HA's drift compensation is 'asymmetric': the

reference level drift-compensates in one direction faster than

it does in the other. Specifically, it compensates faster for

decreasing signals than for increasing signals. Increasing

signals should not be compensated for quickly, since an

approaching finger could be compensated for partially or

entirely before even touching the sense pad. However, an

obstruction over the sense pad, for which the sensor has

already made full allowance for, could suddenly be removed

leaving the sensor with an artificially elevated reference level

and thus become insensitive to touch. In this latter case, the

sensor will compensate for the object's removal very quickly,

usually in only a few seconds.

2.1.2 THRESHOLD AND HYSTERESIS

The internal signal threshold level can be set to one of three

settings (Table 1-1). These are fixed with respect to the

internal reference level, which in turn moves in accordance

with the drift compensation mechanism.

The QT118HA employs a hysteresis dropout below the

threshold level of 17% of the delta between the reference and

threshold levels.

2.1.3 MAX ON-DURATION

If an object or material obstructs the sense pad the signal

may rise enough to create a detection, preventing further

operation. To prevent this, the sensor includes a timer which

monitors detections. If a detection exceeds the timer setting,

the timer causes the sensor to perform a full recalibration.

This is known as the Max On-Duration feature.

After the Max On-Duration interval, the sensor will once again

function normally, even if partially or fully obstructed, to the

best of its ability given electrode conditions. There are two

timeout durations available via strap option: 10 and 60

seconds.

2.1.4 DETECTION INTEGRATOR

It is desirable to suppress detections generated by electrical

noise or from quick brushes with an object. To accomplish

q 4 QT118HA_AR1.02_0408

Pin 7

Low

Pin 6

Medium

Leave open

High

Tie Pin 5 to:Gain

Table 1-1 Gain Strap Options

Figure 2-1 Drift Compensation

Threshold

Signal

Hysteresis

Reference

Output

this, the QT118HA incorporates a detect integration counter

that 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. The required count is 4.

The Detection Integrator can also be viewed as a 'consensus'

filter, that requires four detections in four successive bursts to

create an output. As the basic burst spacing is 95ms, if this

spacing was maintained through 4 consecutive bursts the

sensor would be very slow to respond. In the QT118HA, after

an initial detection is sensed, the remaining three bursts are

spaced only about 2ms apart, so that the slowest reaction

time possible is the fastest possible.

2.1.5 FORCED SENSOR RECALIBRATION

The QT118HA has no recalibration pin; a forced recalibration

is accomplished only when the device is powered up.

However, the supply drain is so low it is a simple matter to

treat the entire IC as a controllable load; simply driving the

QT118HA's Vdd pin directly from another logic gate or a

microprocessor port (Figure 2-2) will serve as both power and

'forced recal'. The source resistance of most CMOS gates

and microprocessors is low enough to provide direct power

without any problems. Almost any CMOS logic gate can

directly power the QT118HA.

A 0.01uF minimum bypass capacitor close to the device is

essential; without it the device can break into high frequency

oscillation.

Option strap configurations are read by the QT118HA only on

powerup. Configurations can only be changed by powering

the QT118HA down and back up again; a microcontroller can

directly alter most of the configurations and cycle power to

put them in effect.

2.2 OUTPUT FEATURES

The QT118HA is designed for maximum flexibility and can

accommodate most popular sensing requirements. These

are selectable using strap options on pins OPT1 and OPT2.

All options are shown in Table 2-1.

OPT1 and OPT2 should never be left floating. If they are

floated, the device will draw excess power and the options

will not be properly read on powerup. Intentionally, there are

no pullup resistors on these lines, since pullup resistors add

to power drain if the pin(s) are tied low.

2.2.1 DC MODE OUTPUT

The output of the device can respond in a ‘DC mode’, where

the output is active-high upon detection. The output will

remain active for the duration of the detection, or until the

Max On-Duration expires, whichever occurs first. If the latter

occurs first, the sensor performs a full recalibration and the

output becomes inactive until the next detection.

In this mode, two nominal Max On-Duration timeouts are

available: 10 and 60 seconds.

2.2.2 TOGGLE MODE OUTPUT

This makes the sensor respond in an on/off mode like a flip

flop. It is most useful for controlling power loads, for example

in kitchen appliances, power tools, light switches, etc.

Max On-Duration in Toggle mode is fixed at 10 seconds.

When a timeout occurs, the sensor recalibrates but leaves

the output state unchanged.

10sVddGnd

Pulse

10sGndGnd

Toggle

60sGndVdd

DC Out

10sVddVdd

DC Out

Max On-

Duration

Tie

Pin 4 to:

Tie

Pin 3 to:

Table 2-1 Output Mode Strap Options

2.2.3 PULSE MODE OUTPUT

This generates a positive pulse of 95ms duration with every

new detection. It is most useful for 2-wire operation (see

Figure 1-2), but can also be used when bussing together

several devices onto a common output line with the help of

steering diodes or logic gates, in order to control a common

load from several places.

Max On-Duration is fixed at 10 seconds if in Pulse output

mode.

The piezo beeper drive does not operate in Pulse mode.

2.2.4 HEARTBEAT™ OUTPUT

The output has a full-time HeartBeat™ ‘health’ indicator

superimposed on it. This operates by taking 'Out' into a

tri-state mode for 350µs once before every QT burst. This

output state can be used to determine that the sensor is

operating properly, or, it can be ignored using one of several

simple methods.

Since Out is normally low, a pullup resistor will create positive

HeartBeat pulses (Figure 2-3) when the sensor is not

detecting an object; when detecting an object, the output will

remain active for the duration of the detection, and no

HeartBeat pulse will be evident.

If the sensor is wired to a microcontroller as shown in Figure

2-4, the controller can reconfigure the load resistor to either

ground or Vcc depending on the output state of the device,

so that the pulses are evident in either state.

Electromechanical devices will usually ignore this short

pulse. The pulse also has too low a duty cycle to visibly

activate 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 (Figure 2-5).

q 5 QT118HA_AR1.02_0408

Figure 2-2 Powering From a CMOS Port Pin

0.01µF

CMO S

mic rocon troller

OU T

PORT X.m

PORT X.n

Vdd

Vss

QT118

QT118HA

2.2.5 PIEZO ACOUSTIC DRIVE

A piezo drive signal is generated for use with a piezo sounder

immediately after a detection is made; the tone lasts for a

nominal 95ms to create a ‘tactile feedback’ sound.

The sensor drives the piezo using an H-bridge configuration

for the highest possible sound level. The piezo is connected

across pins SNS1 and SNS2 in place of Cs or in addition to a

parallel Cs capacitor. The piezo sounder should be selected

to have a peak acoustic output in the 3.5kHz to 4.5kHz

region.

Since piezo sounders are merely high-K ceramic capacitors,

the sounder will double as the Cs capacitor, and the piezo's

metal disc can even act as the sensing electrode. Piezo

transducer capacitances typically range from 6nF to 30nF in

value; at the lower end of this range an additional capacitor

should be added to bring the total Cs across SNS1 and

SNS2 to at least 10nF, or possibly more if Cx is above 5pF.

Piezo sounders have very high, uncharacterized thermal

coefficients and should not be used if fast temperature

swings are anticipated, especially at high gains. They are

also generally unstable at high gains; even if the total value

of Cs is largely from an added capacitor the piezo can cause

periodic false detections.

The burst acquisition process induces a small but audible

voltage step across the piezo resonator, which occurs when

SNS1 and SNS2 rapidly discharge residual voltage stored on

the resonator. The resulting slight clicking sound can be

greatly reduced by placing a 470K resistor Rs in parallel with

the resonator; this acts to slowly discharge the resonator,

attenuating of the harmonic-rich audible step (Figure 2-6).

Note that the piezo drive does not operate in Pulse mode.

2.2.6 OUTPUT DRIVE

The QT118HA’s output is active high and it can source or

sink 1mA 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

regulated. The device derives its internal references from the

power supply, and sensitivity shifts can occur with changes in

Vdd, as happens when loads are switched on. This can

induce detection ‘cycling’, whereby an object is detected, the

load is turned on, the supply sags, the detection is no longer

sensed, the load is turned off, the supply rises and the object

is reacquired, ad infinitum. To prevent this occurrence, the

output should only be lightly loaded if the device is operated

from an unregulated supply, e.g. batteries. Detection

‘stiction’, the opposite effect, can occur if a load is shed when

Out is active.

3 - CIRCUIT GUIDELINES

3.1 SAMPLE CAPACITOR

When used for most applications, the charge sampler Cs can

be virtually any plastic film or good quality ceramic capacitor.

The type should be relatively stable in the anticipated

q 6 QT118HA_AR1.02_0408

Figure 2-4

Using a micro to obtain HB pulses in either output state

Figure 2-3

Getting HB pulses with a pullup resistor when not active

+2 ~ +5

Figure 2-5 Eliminating HB Pulses

4 6

O UT

O PT1

O PT2

GAIN

SNS 1

SNS 2CMO S

100p F

GATE OR

MICR O IN PUT

Figure 2-6 Piezo Sounder Circuit

Piezo Sounder

10-30nF

4 6

OUT

OPT2

GAIN

SNS2

SNS1

Vss

Vdd

OPT1

SENSING

ELECTRODE

+2 ~ +5

P1-P3 P4-P6 P7-P9 P10-P12

QT118HA-ISG

Mfr. #:

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Manufacturer:

Microchip Technology / Atmel

Description:

Interface - Specialized Qtouch IC

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QT118HA-ISG