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QT1+T+G Datasheet(PDF) 2 Page - Quantum Research Group |
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QT1+T+G Datasheet(HTML) 2 Page - Quantum Research Group |
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2 / 14 page  1 - OVERVIEW The QT118H is a digital burst mode charge-transfer (QT) sensor designed specifically for touch controls; it includes all hardware and signal processing functions necessary to provide stable sensing under a wide variety of changing conditions. Only a few low cost, non-critical discrete external parts are required for operation. Figure 1-1 shows the basic QT118H circuit using the device, with a conventional output drive and power supply connections. Figure 1-2 shows a second configuration using a common power/signal rail which can be a long twisted pair from a controller; this configuration uses the built-in pulse mode to transmit the output state to the host controller. 1.1 BASIC OPERATION The QT118H employs short, low duty cycle bursts of QT cycles to acquire capacitance. Burst mode permits power consumption in the low microamp range, dramatically reduces RF emissions, lowers susceptibility to EMI, and yet permits excellent response time. Internally the signals are digitally processed to reject impulse noise, using a 'consensus' filter which requires four consecutive confirmations of a detection before the output is activated. The QT switches and charge measurement hardware functions are all internal to the QT118H (Figure 1-3). A single-slope switched capacitor ADC includes both the required QT charge and transfer switches in a configuration that provides direct ADC conversion. The sensitivity depends on the values of Cs, Cx, and to a smaller degree, Vdd. Vdd is used as the charge reference voltage. Higher values of Cs increase gain; higher values of Cx load reduce it. The value of Cs can thus be increased to allow larger values of Cx to be tolerated (Figures 4-1 and 4-2, page 10). Piezo sounder drive: The QT118H can drive a piezo sounder after a detection for feedback. The piezo sounder replaces or augments the Cs capacitor; this works since piezo sounders are also capacitors, albeit with a large thermal drift coefficient. If Cpiezo is in the proper range, no additional capacitor. If Cpiezo is too small, it can simply be ‘topped up’ with a ceramic capacitor in parallel. The QT118H drives a ~4kHz signal across SNS1 and SNS2 to make the piezo (if installed) sound a short tone for 75ms immediately after detection, to act as an audible confirmation. Option pins allow the selection or alteration of several special features and sensitivity. 1.2 ELECTRODE DRIVE The internal ADC treats Cs as a floating transfer capacitor; as a direct result, the sense electrode can in theory be connected to either SNS1 or SNS2 with no performance difference. However, the noise immunity of the device is improved by connecting the electrode to SNS2, preferably via a series resistor Re (Figure 1-1) to roll off higher harmonic frequencies, both outbound and inbound. In order to reduce power consumption and to assist in discharging Cs between acquisition bursts, a 470K series resistor Rs should always be connected across Cs (Figure 1-1). The rule Cs >> Cx must be observed for proper operation. Normally Cx is on the order of 10pF or so, while Cs might be 10nF (10,000pF), or a ratio of about 1:1000. It is important to minimize the amount of unnecessary stray capacitance Cx, for example by minimizing trace lengths and widths and backing off adjacent ground traces and planes so as keep gain high for a given value of Cs, and to allow for a larger sensing electrode size if so desired. The PCB traces, wiring, and any components associated with or in contact with SNS1 and SNS2 will become touch sensitive and should be treated with caution to limit the touch area to the desired location. lq 2 QT118H R1.08 / 0405 Figure 1-1 Standard mode options SENSING ELECTRODE Cs Rs 2nF - 500nF 3 46 5 1 +2.5 ~ +5 7 2 OUT OPT1 OPT2 GAIN SNS1 SNS2 Vss Vdd OUTPUT = DC TIMEOUT = 10 Secs TOGGLE = OFF GAIN = HIGH Cx 8 RE Figure 1-2 2-wire operation, self-powered + 10µF 1N4148 n-ch Mosfet CMOS LOGIC 3.5 - 5.5V 1K Twisted pair Cs 8 OUT OPT1 OPT2 GAIN SNS1 SNS2 Vss Vdd 3 46 5 1 7 2 Rs SENSING ELECTRODE Cx RE |
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