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.
3.3.1 SUPPLY CURRENT
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.
3.3.2 ESD PROTECTION
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 QT110A 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 -
F
R
=
1
2R
series
C
s
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 QT110A
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 QT110A 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
LQ 7 QT110A_1R0.01_0408
