2.9.2 HeartBeat™ Output

The QT100 output has a HeartBeat™ ‘health’ indicator

superimposed on it in both LP and SYNC modes. This

operates by taking the output pin into a three-state mode for

15µs 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.

The HeartBeat indicator can be sampled by using a pull-up

resistor on the OUT pin, and feeding the resulting

positive-going pulse into a counter, flip flop, one-shot, or other

circuit. The pulses will only be visible when the chip is not

detecting a touch.

If the sensor is wired to a microcontroller as shown in

Figure 2.7, the microcontroller can reconfigure the load

the QT100, so that the pulses are evident in either state.

Electromechanical devices like relays will usually ignore the

short Heartbeat pulse. The pulse also has too low a duty cycle

to visibly affect LEDs. It can be filtered completely if desired,

by adding an RC filter to the output, or if interfacing directly

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

or at most adding a small noncritical capacitor from OUT to

2.9.3 Output Drive

The OUT pin is active high and can sink or source up to 2mA.

When a large value of Cs (>20nF) is used the OUT current

should be limited to <1mA to prevent gain-shifting side effects,

which happen when the load current creates voltage drops on

the die and bonding wires; these small shifts can materially

influence the signal level to cause detection instability.

3 Circuit Guidelines

3.1 Application Note

Refer to Application Note AN-KD02, downloadable from the

Quantum website for more information on construction and

design methods. Go to http://www.qprox.com, click the

Support tab and then Application Notes.

3.2 Sample Capacitor

Charge sampler capacitor Cs should be a stable type, such as

X7R ceramic or PPS film. The normal Cs range is from 2nF to

50nF depending on the sensitivity required; larger values of

Cs demand higher stability to ensure reliable sensing.

For more consistent sensing from unit to unit, 5 percent

tolerance capacitors are recommended. X7R ceramic types

can be obtained in 5 percent tolerance at little or no extra cost.

3.3 Power Supply, PCB Layout

The power supply can range between 2.0V and 5.0V. At 3V

current drain averages less than 500µA in Fast mode.

If the power supply is shared with another electronic system,

care should be taken to ensure that the supply is free of digital

spikes, sags, and surges which can adversely affect the

be badly affected by rapid voltage fluctuations. It is highly

recommended that a separate voltage regulator be used just

for the QT100 to isolate it from power supply shifts caused by

other components.

If desired, the supply can be regulated using a Low Dropout

(LDO) regulator, although such regulators often have poor

transient line and load stability. See Application Note

AN-KD02 (see Section 3.1) for further information on power

supply considerations.

Parts placement: The chip should be placed to minimize the

SNSK trace length to reduce low frequency pickup, and to

reduce stray Cx which degrades gain. The Cs and Rs

resistors (see Figure 1.1) should be placed as close to the

body of the chip as possible so that the trace between Rs and

the SNSK pin is very short, thereby reducing the antenna-like

ability of this trace to pick up high frequency signals and feed

them directly into the chip. A ground plane can be used under

the chip and the associated discretes, but the trace from the

Rs resistor and the electrode should not run near ground to

reduce loading.

For best EMC performance the circuit should be made entirely

with SMT components.

Electrode trace routing: Keep the electrode trace (and the

electrode itself) away from other signal, power, and ground

traces including over or next to ground planes. Adjacent

switching signals can induce noise onto the sensing signal;

any adjacent trace or ground plane next to, or under, the

electrode trace will cause an increase in Cx load and

desensitize the device.

Important Note: for proper operation a 100nF (0.1µF)

ceramic bypass capacitor must be used directly between

bypass capacitor should be placed very close to the

device’s power pins.

6

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