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QT114-S Datasheet(PDF) 8 Page - Quantum Research Group |
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QT114-S Datasheet(HTML) 8 Page - Quantum Research Group |
8 / 12 page 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 Re1 Re2 C 10 F 1 ✙ CS 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 R1 R2 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 Co GATE OR MICRO INPUT 4 CMOS Co |
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