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

will trap in 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.

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.

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 QT110 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.

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 (Figure 1-1) form a natural low-pass

filter for incoming RF signals; the roll-off frequency of this

network is defined by -

1

2

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.

These 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 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 QT110 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 QT11x

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 QT110 to eliminate

noise coupling from other switching sources via Vdd.

Make sure the regulator’s transient load stability provides

for a 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

7

QT110 R1.04/0405