QT118H-DG View Datasheet(PDF) - Quantum Research Group

Part Name

Description

Manufacturer

QT118H-DG

CHARGE-TRANSFER TOUCH SENSOR

Quantum Research Group 

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 -

F= 1

R 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.

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.

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.

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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