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.

The output of the QT113 can directly drive a resistively

limited LED. The LED should be connected with its cathode

to the output and its anode towards Vcc, so that it lights when

the sensor is active. If desired the LED can be connected

from Out to ground, and driven on when the sensor is

inactive.

3 - CIRCUIT GUIDELINES

3.1 SAMPLE CAPACITOR

Charge sampler Cs can be virtually any plastic film or

medium-K ceramic capacitor. The acceptable Cs range is

from 10nF to 500nF depending on the sensitivity required;

larger values of Cs demand higher stability to ensure reliable

sensing. Acceptable capacitor types include PPS film,

polypropylene film, NPO/C0G ceramic, and X7R ceramic.

3.2 OPTION STRAPPING

The option pins 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 tied low.

The Gain input should be connected to either Vdd or Gnd.

Tables 1-1 and 2-1 show the option strap configurations

available.

3.4 POWER SUPPLY, PCB LAYOUT

The power supply can range from 2.5 to 5.0 volts. At 3 volts

current drain averages less than 600µA in most cases, but

can be higher if Cs is large. Increasing Cx values will actually

decrease power drain. Operation can be from batteries, but

be cautious about loads causing supply droop (see Output

Drive, Section 2.2.4).

As battery voltage sags with use or fluctuates slowly with

temperature, the QT113 will track and compensate for these

changes automatically with only minor changes in sensitivity.

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

the QT113. The QT113 will track slow changes in Vdd, but it

can be affected by rapid voltage steps.

if desired, the supply can be regulated using a conventional

low current regulator, for example CMOS regulators that have

low quiescent currents. Bear in mind that such regulators

generally have very poor transient line and load stability; in

some cases, shunting Vdd to Vss with a 4.7K resistor to

induce a continuous current drain can have a very positive

effect on regulator performance.

Parts placement: The chip should be placed to minimize the

SNS2 trace length to reduce low frequency pickup, and to

reduce stray Cx which degrades gain. The Cs and Rseries

resistors (see Figure 1-1) should be placed as close to the

body of the chip as possible so that the SNS2 trace between

Rseries and the SNS2 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.

For best EMC performance the circuit should be made

entirely with SMT components.

SNS trace routing: Keep the SNS2 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;

6

R1.05/0405

Figure 2-4

Using a micro to obtain HB pulses in either output state

Figure 2-3

Getting HearBeat pulses with a pull-down resistor

3

46

5

1

+2.5 to 5

7

2

OUT

OPT1

OPT2

GA IN

SNS1

SNS2

Vss

Vdd

8

Ro

H eartBeat™ P u lses

Microcontroller

PORT_M.x

PORT_M.y

3

46

5

7

2

OUT

OPT1

OPT2

GAIN

SNS1

SNS2

Figure 2-5 Eliminating HB Pulses

3

46

5

7

2

OUT

OPT1

OPT2

GAIN

SNS1

SNS2

CM O S

100pF

GATE OR

MICRO INPU T