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QT1+T+G Datasheet(PDF) 8 Page - Quantum Research Group |
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QT1+T+G Datasheet(HTML) 8 Page - Quantum Research Group |
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8 / 14 page  3.4 EMC AND RELATED NOISE ISSUES External AC fields (EMI) due to RF transmitters or electrical noise sources can cause false detections or unexplained shifts in sensitivity. The influence of external fields on the sensor is reduced by means of the Rseries described in Section 3.2. The Cs capacitor and Rseries (see Figure 1-1) form a natural low-pass filter for incoming RF signals; the roll-off frequency of this network is defined by - FR = 1 2✜RseriesCs If for example Cs = 22nF, and Rseries = 10K ohms, the rolloff frequency to EMI is 723Hz, vastly lower than any credible external noise source (except for mains frequencies i.e. 50 / 60 Hz). However, Rseries and Cs must both be placed very close to the body of the IC so that the lead lengths between them and the IC do not form an unfiltered antenna at very high frequencies. PCB layout, grounding, and the structure of the input circuitry have a great bearing on the success of a design to withstand electromagnetic fields and be relatively noise-free. In brief summary, the following design rules should be adhered to for best ESD and EMC results: 1. Use only SMT components. 2. Keep Cs, Rs, Re and Vdd bypass cap close to the IC. 3. Maximize Re to the limit where sensitivity is not noticeably affected. 4. Do not place the electrode or its connecting trace near other traces, or near a ground plane. 5. Do use a ground plane under and around the QT118 itself, back to the regulator and power connector (but not beyond the Cs capacitor). 6. Do not place an electrode (or its wiring) of one QT device near the electrode or wiring of another device, to prevent cross interference. 7. Keep the electrode (and its wiring) away from other traces carrying AC or switched signals. 8. If there are LEDs or LED wiring near the electrode or its wiring (ie for backlighting of the key), bypass the LED wiring to ground on both its ends. 9. Use a voltage regulator just for the QT118 to eliminate power noise coupling from other switching sources. Make sure the regulator’s transient load stability provides for stable voltage just before each burst commences. For further tips on construction, PCB design, and EMC issues browse the application notes and faq at www.qprox.com lq 8 QT118H R1.08 / 0405 |
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