qt220 Quantum Research Group, qt220 Datasheet - Page 4

• Manufacturer Part Number: qt220
• Description: 2 Key Qtouch? Sensor Ic
• Manufacturer: Quantum Research Group

Multiple touch electrodes connected to either SNSnK can be
used, for example to create control surfaces on both sides of
an object.
It is important to limit the amount of stray capacitance on the
SNS terminals, for example by minimizing trace lengths and
widths to allow for higher gain without requiring higher values
of Cs. Under heavy delta-Cx loading of one key, cross
coupling to another key’s trace can cause the other key to
trigger. Therefore, electrode traces from adjacent keys
should not be run close to each other over long runs in order
to minimize cross-coupling if large values of delta-Cx are
expected, for example when an electrode is directly touched.
This is not a problem when the electrodes are working
through a plastic panel with normal touch sensitivity.
1.4 Sensitivity
1.4.1 Introduction
Sensitivity can be altered to suit various applications and
situations on a channel-by-channel basis. The easiest and
most direct way to impact sensitivity is to alter the value of
each Cs; more Cs yields higher sensitivity. Each channel has
its own Cs value and can therefore be independently
adjusted.
LQ
SPEED
Threshold
OPT
Output
S1
OUT1
OUT2
VDD
1M
R1
Figure 1.2 Fast, Spread-Spectrum Circuit
Figure 2.1 Drift Compensation
10nF
CS1
2.2K
RS1
22K
RSNS1
Signal
Reference
22K
RSNS2
10nF
2.2K
CS2
RS2
Hysteresis
10 second
timeout shown
360K
R5
C1
47nF
VDD
1M
R3
62K R4
OPT2
S3
220K
R6
OPT1
4
two Rs resistors have no effect on sensitivity and should not
be altered. Load capacitances can also be added to overly
sensitive channels to ground, to reduce their gains. These
should be in the order of a few picofarads.
2 QT220 Specifics
2.1 Signal Processing
2.1.1 Introduction
These devices process all signals using 16 -bit math, using a
number of algorithms pioneered by Quantum. These
algorithms are specifically designed to provide for high
survivability in the face of adverse environmental changes.
2.1.2 Drift Compensation
Signal drift can occur because of changes in Cx , Cs, and
Vdd over time. If a low grade Cs capacitor is chosen, the
signal can drift greatly with temperature. If keys are subject
to extremes of temperature or humidity, the signal can also
drift. It is crucial that drift be compensated, else false
detections, nondetections, and sensitivity shifts will follow.
Drift compensation (Figure 2.1) is a method that makes the
VDD
1M
S2
R2
reference level track the raw signal at a slow rate,
only while no detection is in effect. The rate of
reference adjustment must be performed slowly
else legitimate detections can also be ignored. The
IC drift compensates each channel independently
using a slew-rate limited change to the reference
level; the threshold and hysteresis values are
slaved to this reference.
Once an object is sensed, the drift compensation
mechanism ceases since the signal is legitimately
high, and therefore should not cause the reference
level to change.
1.4.2 Alternative Ways to Increase
Sensitivity
Sensitivity can also be increased by using bigger
electrode areas, reducing panel thickness, or
using a panel material with a higher dielectric
constant.
1.4.3 Decreasing Sensitivity
In some cases the circuit may be too sensitive.
Gain can be lowered further by a number of
strategies: a) making the electrode smaller, b)
making the electrode into a sparse mesh using a
high space-to-conductor ratio, or c) by
decreasing the Cs capacitors.
1.4.4 Key Balance
A number of factors can cause sensitivity
imbalances. Notably, SNS wiring to electrodes
can have differing stray amounts of capacitance
to ground. Increasing load capacitance will
cause a decrease in gain. Key size differences,
and proximity to other metal surfaces can also
impact gain.
The two keys may thus require ‘balancing’ to
achieve similar sensitivity levels. This can be
best accomplished by trimming the values of the
two Cs capacitors to achieve equilibrium. The
QT220R R1.03/1006