Electromechanical devices will ignore this short pulse. The

pulse also has too low a duty cycle to visibly affect 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 each used

OUT line to ground (Figure 3-4).

3.4 ESD PROTECTION

In some installations the QT114 will be protected from direct

static discharge by the insulation of the electrode and the

fact that the probe may not be accessible to human contact.

However, even with probe insulation, transients can still flow

into the electrode via induction, or in extreme cases, via

dielectric breakdown. Some moving fluids (like oils) and

powders can build up a substantial triboelectric charge

directly on the probe surface.

The QT114 does have diode protection on its terminals

which can absorb and protect the device from most induced

discharges, up to 20mA; the usefulness of the internal

clamping will depending on the probe insulation's dielectric

properties, thickness, and the rise time of the transients.

ESD dissipation can be aided further with an added diode

protection network as shown in Figure 3-5. Because the

charge and transfer times of the QT114 are relatively long,

the circuit can tolerate very large values of Re1, as much as

50k ohms in most cases without affecting gain. The added

diodes shown (1N4150, BAV99 or equivalent low-C diodes)

will shunt the ESD transients away from the part, and Re1

will current-limit the rest into the QT110's own internal clamp

diodes. C1 should be around 10µF if it is to absorb positive

transients from a human body model standpoint without

rising in value by more than 1 volt. If desired C1 can be

replaced with an appropriate zener diode. Directly placing

semiconductor transient protection devices or MOV's on the

sense lead is not advised; these devices have extremely

large amounts of parasitic C which will swamp the sensor.

Re2 functions to isolate the transient from the QT110's Vcc

pin; values of around 1K ohms are reasonable.

As with all ESD protection networks, it is important that the

transients be led away from the circuit. PCB ground layout is

crucial; the ground connections to the diodes and C1 should

all go back to the power supply ground or preferably, if

available, a chassis ground connected to earth. The currents

should not be allowed to traverse the area directly under the

QT114.

If the QT114 is connected to an external circuit via a long

cable, it is possible for ground-bounce to cause damage to

the OUT pins; even though the transients are led away from

the QT114 itself, the connected signal or power ground line

will act as an inductor, causing a high differential voltage to

build up on the OUT wires with respect to ground. If this is a

possibility, the OUT pins should have a resistance in series

with them on the sensor PCB to limit current; this resistor

should be as large as can be tolerated by the load.

3.5 SAMPLE CAPACITOR

Charge sampler Cs should be a stable grade of capacitor,

like PPS film, NPO ceramic, or polycarbonate. The

acceptable Cs range is anywhere from 10nF to 100nF

(0.1uF) and its required value will depend on load Cx. In

some cases, to achieve the 'right' value, two or more

capacitors may need to be wired in parallel.

- 8 -

Figure 3-5 ESD Protection Network

To Electrodes

Gnd

OUT1

OUT2

FILT

Vcc

SNS2

SNS1

POL

1

2

3

45

6

7

8

C 10 F

1

✙

Figure 3-3

Using a micro to obtain HB pulses in either output state

Figure 3-2

Getting HeartBeat pulses with a pull-down resistor

3

45

6

7

2

OUT1

OUT2

FILT

SNS1

POL

SNS2

Ro

Ro

HeartBeat™ Pulses

Microprocessor

PORT_M.1

PORT_M.2

3

45

6

7

2

OUT1

OUT2

FILT

SNS1

POL

SNS2

PORT_M.3

PORT_M.4

Figure 3-4 Eliminating HB Pulses

3

45

6

7

2

OUT1

OUT2

FILT

SNS1

POL

SNS2

CMOS

100pF

100pF

GATE OR

MICRO INPUT

4

CMOS