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 th e

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

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 vol tage

drops on the die and bonding wi res; 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 sensi tivity required; larger

values of Cs demand higher stability to ensure reliab le

sensing.

For more consistent sensing from unit to unit, 5% tolerance

capacitors are recommended. X7R ceramic types can be

obtained in 5% tolerance at little or no extra cost.

Values of Cs above 100nF will only be required for large

values of Cx. Sensing may become unstable if Cx is small

and Cs is large; for example, in attempting to implement

proximity fields.

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 assure that the supply is free of

digital spikes, sags, and surges which can adversely affect

can 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 cause d 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 R s

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

should be placed very close to the device’s power pins.

5

QT100-ISG R3.06/0606