3.2 Power Supply, PCB Layout

The power supply can range from 3.9 to 5. 5 volts. If this

fluctuates slowly with temperature, the device will track and

compensate for these changes automatically with only minor

changes in sensitivity. If the supply voltage drifts or shift s

quickly, the drift compensation mechanism will not be able to

keep up, causing sensitivity anomalies or false detections.

The QT240 will track slow changes in Vdd, but can be

seriously affected by rapid voltage steps.

If the supply is shared with another electronic system, care

should be taken to assure that the supply is free of digital

spikes, sags, and surges which can cause adverse effects.

The supply is best locally regulated using almost any

three-terminal LDO device from 4.0V to 5V.

For proper operation a 0.1µF or greater bypass capacitor

must be used between Vdd and Vss; the bypass capacitor

should be placed very close to the device Vss and Vdd pins.

The PCB should, if possible, include a copper pour under

and around the IC, but not extensively under the SNS lines.

3.3 Oscillator; Spread Spectrum

The oscillator is an internal type using an external current

bias source. Normal operation occurs at ~100 kHz ±20% with

R=62K at Vdd = 4.0V.

In Fast mode the oscillator can be spread-spectrum

modulated with a simple external RC network to avoid

dwelling on any one frequency. The circuit works by current

modulating the oscillator bias to provide a chirp modulation

within each burst. This helps dramatically with both RF

emissions and susceptibility. The circuit for this is shown in

Figure 1.2. The SYNC/SS pin outputs a rectangular pulse

with a period of one burst. At the end of the burst, SYNC/SS

clamps to ground. This forms a sawtooth modulation

waveform to create the frequency chirp modulation

(Figure 3.1).

The detection integrator (DI) filters out false detections

caused by interference on up to any six of these acquisitions.

The DI is a consensus filter that throws out a pending

detection if even one of the six samples does not confirm a

detection. As a result of this feature, it is extremely difficult

for an external signal to interfere with the device.

The typical modulation band of this circuit is ±7% around the

center frequency.

The oscillator frequency can be measured by

observing the acquisition pulses on any sense

channel with an oscilloscope. The first two

positive pulses in each burst will be exactly

10µs from rising edge to rising edge (100kHz) if

the oscillator is set correctly ( no spread

spectrum). Some subsequent pulse pairs will

have wider spacings; this is normal.

If desired the response time of the device in

Fast mode can be modified by altering the

oscillator frequency. The device can be set to

any center frequency from 20kHz to 150kHz by

simply altering the value of the bias resistor.

Lower values of R will speed up the device.

Higher resistor values will slow down the device

and reduce power consumption.

Slow mode relies on an internal RC timer which

cannot be adjusted via the bias resistor. During

intervals between bursts in Slow mode, the

100kHz oscillator is disabled.

Spread Spectrum in Slow Mode: The QT240 does not

support spread spectrum in Slow mode directly via the

SYNC/SS pin. However, the designer can still implement

spread spectrum by modulating the oscillator through a

resistor to OSC with a triangle or sawtooth wave.

The modulation signal should be exactly synchronous with

each burst, so that the resulting acquired signal is modulated

in the same way. If this rule is not followed, the signals will

contain modulation noise and false detections may occur.

3.4 Unused Channels

Unused SNS pins should not be left open. They should have

a small-value noncritical dummy Cs capacitor plus a 2.2K

series-R connected to their SNS pins to allow the internal

circuit to continue to function properly. A nominal value of

1nF (1,000pF) X7R ceramic will suffice.

Unused channels should not have sense traces or

electrodes connected to them except for the required option

resistors, which must always be installed and connected to

Vdd or Vss.

3.5 ESD Protection

Normally, only a series resistor is required for ESD

suppression. A 22K Rsns resistor in series with the sense

trace is sufficient in most cases. The dielectric panel (glass

or plastic) usually provides a high degree of isolation to

prevent ESD discharge from reaching the circuit . Rsns

should be placed close to the chip.

If the Cx load is high, Rsns can prevent total charge and

transfer and as a result gain can deteriorate. If a reduction in

Rsns increases gain noticeably, the lower value should be

used. Conversely, increasing the Rsns can result in added

ESD and EMC benefits provided that the increase does not

decrease sensitivity.

3.6 Crosstalk Precautions

Adjacent sense traces might require intervening ground

traces in order to reduce capacitive cross bleed if high

sensitivity is required or high values of delta-Cx are

anticipated (for example, from direct human touch to an

electrode connection). In normal touch applications behind

plastic panels this is almost never a problem regardless of

how the electrodes are wired.

7

QT240R R1.11/1006

Figure 3.1 Spread-spectrum Waveforms