QT1100A-ISG View Datasheet(PDF) - Quantum Research Group
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QT1100A-ISG Datasheet PDF : 42 Pages
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2 Device Control & Wiring
2.1 Oscillator
The QT1100A uses an external 12MHz resonator as its
frequency reference. This frequency can be lowered for
lower average power, however all functions will also slow
down including response time and communications
parameters. It is not advised to change the operating
frequency without a good reason.
The oscillator source can be from an external circuit, so that
two or more circuits can share the same oscillator. If an
external frequency source is used, it should be fed to OSC1,
pin 37. OSC2 should be left open-circuit.
2.2 Spread Spectrum Modulation
The device features spread spectrum modulation of its
acquisition bursts to dramatically reduce both RF emissions
and susceptibility to external AC fields. This feature cannot
be disabled or modified.
Spread spectrum modulation works together with the
detection integrator (‘DI’) process to eliminate external
interference in almost all cases.
2.3 Cs Sample Capacitors
The Cs sample capacitors accumulate the charge from the
key electrodes and determine sensitivity . (See Section 2.4)
The Cs capacitors can be virtually any plastic film or low to
medium-K ceramic capacitor. The ‘normal’ Cs range is 2.2nF
to 100nF depending on the sensitivity required; larger values
of Cs require higher stability to ensure reliable sensing.
Acceptable capacitor types for most uses include PPS film,
polypropylene film, and NP0 and X7R ceramics. Lower
grades than X7R are not advised.
The Cs capacitors and all associated wiring should be
placed and wired very tight to the body of the IC for noise
immunity to very high frequency RF fields. See Section 2.7.
2.4 Sensitivity
Sensitivity can be altered to suit various applications and
situations on a key-by-key basis. One way to impact
sensitivity is to alter the value of each Cs when the device is
in NTM = 0 mode (see page 25); higher values of Cs will
yield higher sensitivity; each key has its own Cs value and
so can be adjusted independently. The Setups block can
also be used to alter sensitivity, using an external EEPROM,
serial communications, or both (Section 4.1).
Sensitivity can also be increased by using bigger electrode
areas, reducing panel thickness, or using a panel material
with a higher dielectric constant (e.g. glass instead of
plastic).
In some cases the keys may be too sensitive. Gain ca n be
lowered by:
a) making the electrode smaller, or,
b) making the electrode into a sparse mesh using a high
space-to-conductor ratio, or,
c) by decreasing the Cs capacitors (if NTM = 0).
Sensitivity trimming is usually done through a process of trial
and error, using a range of ‘standard fingers’ made of
earthed conductive rubber on the end of a plastic rod.
2.5 Sensitivity Balance
A number of factors can cause sensitivity imbalances among
the keys. Notably, SNS wiring to electrodes can have
differing stray amounts of capacitance to ground, perhaps
due to trace length differences or the presence of ground,
power, or other signal wiring near the SNS traces. Increasing
load capacitance (Cx) will cause a decrease in gain. Key
size differences, and proximity to other metal surfaces can
also impact gain.
The keys may thus require ‘balancing’ to achieve similar
sensitivity levels. The NTHR parameter in the Setups
functions is one easy way to trim and balance key sensitivity
(Section 4.1).
Balancing can also be achieved by adjusting the Cs
capacitor values to achieve equilibrium. The Rs resistors
have no effect on sensitivity and should not be altered. Load
capacitance to ground (to boost Cx) can also be added to
overly sensitive channels to reduce gain; these should be on
the order of a few picofarads.
2.6 Power Supply
The power supply can range from 3.3 to 5.0 volts. If this
fluctuates slowly with temperature, the device will track and
compensate for these changes automatically with only minor
changes in sensitivity. If the supply voltage drifts or shifts
quickly, the drift compensation mechanism will not be able to
keep up, causing sensitivity anomalies or false detections.
The power supply should be locally regulated using a
3-terminal device. If the supply is shared with another
electronic system, care should be taken to ensure that the
supply is free of digital spikes, sags, and surges which can
cause adverse effects.
For proper operation a 0.1µF or greater bypass capacitor
must be used between Vdd and Vss; the bypass cap acitor
should be routed with very short tracks to the device’s Vss
and Vdd pins.
2.7 PCB Layout and Construction
Ground Planes: The PCB should if possible include a
copper pour under and around the IC, but not under the SNS
lines after the Rsns resistors. Ground planes increase
loading capacitance (Cx) on the SNS lines and can
dramatically degrade sensitivity.
Part Placement: The resistors and capacitors associated
with each key should be placed physically as close to the
body of the QT1100A as possible, with the shortest possible
trace lengths, to minimize the influence of external fields
(see Section 2.9.2). The QT1100A should be placed as close
to the key electrodes as possible to reduce wiring lengths, to
minimize stray capacitances on and between SNS traces
and to reduce interference problems.
PCB Cleanliness: All capacitive sensors should be treated
as highly sensitive analog circuits which can be influenced
by stray conductive leakage paths. QT devices have a basic
resolution in the femtofarad range; in this range, there is no
such thing as ‘no-clean flux’. Flux absorbs moisture and
becomes conductive between solder joints, causing signal
drift, false detections, and transient instabilities. Conformal
coatings will trap in existing amounts of moisture which will
then become highly temperature sensitive.
The designer should specify ultrasonic cleaning as part of
the manufacturing process, and in cases where a high level
of humidity is anticipated, the use of conformal coatings after
cleaning to keep out moisture.
LQ
12
Copyright © 2003-2005 QRG Ltd
QT1100A-ISG R3.02/1105
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