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QT1+T+G Datasheet(PDF) 7 Page - Quantum Research Group |
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QT1+T+G Datasheet(HTML) 7 Page - Quantum Research Group |
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7 / 14 page  temperature range. If fast temperature swings are expected, especially with higher sensitivities, more stable capacitors be required, for example PPS film. In most moderate gain applications (ie in most cases), low-cost X7R types will work fine. 3.2 ELECTRODE WIRING See also Section 3.4. The wiring of the electrode and its connecting trace is important to achieving high signal levels and low noise. Certain design rules should be adhered to for best results: 1. Use a ground plane under the IC itself and Cs and Rs but NOT under Re, or under or closely around the electrode or its connecting trace. Keep ground away from these things to reduce stray loading (which will dramatically reduce sensitivity). 2. Keep Cs, Rs, and Re very close to the IC. 3. Make Re as large as possible. As a test, check to be sure that an increase of Re by 50% does not appreciably decrease sensitivity; if it does, reduce Re until the 50% test increase has a negligible effect on sensitivity. 4. Do not route the sense wire near other ‘live’ traces containing repetitive switching signals; the trace will pick up noise from external signals. 3.3 POWER SUPPLY, PCB LAYOUT The power supply can range from 2.5 to 5.0 volts. At 2.5 volts current drain averages less than 10µA with Cs = 10nF, provided a 470K Rs resistor is used (Figure 1-1). Sample Idd curves are shown in Figure 4-3. Higher values of Cs will raise current drain. Higher Cx values can actually decrease power drain. Operation can be from batteries, but be cautious about loads causing supply droop (see Output Drive, Section 2.2.6) if the batteries are unregulated. As battery voltage sags with use or fluctuates slowly with temperature, the IC 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 device. The IC 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 LDO regulators that have nanoamp quiescent currents. Care should be taken that the regulator does not have a minimum load specification, which almost certainly will be violated by the QT118's low current requirement. Furthermore, some LDO regulators are unable to provide adequate transient regulation between the quiescent and acquire states, creating Vdd disturbances that will interfere with the acquisition process. This can usually be solved by adding a small extra load from Vdd to ground, such as 10K ohms, to provide a minimum load on the regulator. Conventional non-LDO type regulators are usually more stable than slow, low power CMOS LDO types. Consult the regulator manufacturer for recommendations. For proper operation a 100nF (0.1uF) ceramic bypass capacitor must be used between Vdd and Vss; the bypass cap should be placed very close to the device’s power pins. Without this capacitor the part can break into high frequency oscillation, get physically hot, stop working, or become damaged. PCB Cleanliness: All capacitive sensors should be treated as highly sensitive circuits which can be influenced by stray conductive leakage paths. QT devices have a basic resolution in the femtofarad range; in this region, there is no such thing as ‘no clean flux’. Flux absorbs moisture and becomes conductive between solder joints, causing signal drift and resultant false detections or temporary loss of sensitivity. Conformal coatings can trap existing amounts of moisture which will then become highly temperature sensitive. The designer should strongly consider ultrasonic cleaning as part of the manufacturing process, and in more extreme cases, the use of conformal coatings after cleaning and baking. 3.3.1 SUPPLY CURRENT Measuring average power consumption is a challenging task due to the burst nature of the device’s operation. Even a good quality RMS DMM will have difficulty tracking the relatively slow burst rate, and will show erratic readings. The easiest way to measure Idd is to put a very large capacitor, such as 2,700µF across the power pins, and put a 220 ohm resistor from there back to the power source. Measure the voltage across the 220 resistor with a DMM and compute the current based on Ohm’s law. This circuit will average out current to provide a much smoother reading. To reduce the current consumption the most, use high or low gain pin settings only, the smallest value of Cs possible that works, and a 470K resistor (Rs) across Cs (Figure 1-1). Rs acts to help discharge capacitor Cs between bursts, and its presence substantially reduces power consumption. 3.3.2 ESD PROTECTION In cases where the electrode is placed behind a dielectric panel, the IC will be protected from direct static discharge. However even with a panel transients can still flow into the electrode via induction, or in extreme cases via dielectric breakdown. Porous materials may allow a spark to tunnel right through the material. Testing is required to reveal any problems. The device has diode protection on its terminals which will absorb and protect the device from most ESD events; the usefulness of the internal clamping will depending on the dielectric properties, panel thickness, and rise time of the ESD transients. The best method available to suppress ESD and RFI is to insert a series resistor Re in series with the electrode as shown in Figure 1-1. The value should be the largest that does not affect sensing performance. If Re is too high, the gain of the sensor will decrease. Because the charge and transfer times of the QT118 are relatively long (~2µs), the circuit can tolerate a large value of Re, often more than 10k ohms in most cases. Diodes or semiconductor transient protection devices or MOV's on the electrode trace are not advised; these devices have extremely large amounts of nonlinear parasitic capacitance which will swamp the capacitance of the electrode and cause false detections and other forms of instability. Diodes also act as RF detectors and will cause serious RF immunity problems. lq 7 QT118H R1.08 / 0405 |
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