QT114 Datasheet, PDF (8/12 Pages) Quantum Research Group – CHARGE-TRANSFER QLEVEL SENSOR IC

Figure 3-2
Getting HeartBeat pulses with a pull-down resistor
HeartBeat™ Pulses
2
OUT1
Ro
3
OUT2
Ro
4
FILT
7
SNS2
6
SNS1
5
POL
Figure 3-3
Using a micro to obtain HB pulses in either output state
PORT_M.1
R₁
PORT_M.2
Microprocessor PORT_M.3
R₂
PORT_M.4
2
OUT1
3
OUT2
4
FILT
7
SNS2
6
SNS1
5
POL
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
GATE OR
MICRO INPUT
CMOS
Co
100pF
CMOS
100pF
Co
2
OUT1
3
OUT2
4
FILT
7
SNS2
6
SNS1
5
POL
Figure 3-4 Eliminating HB Pulses
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.
Vcc
1
2 _{OUT1 SNS2} 7
3 _{OUT2 SNS1} 6
4 _FILT
POL ⁵
8 Gnd
Re2 C1 10✙F
Re1
To Electrodes
CS
Figure 3-5 ESD Protection Network
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