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 -
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
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
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
QT118H R1.08 / 0405