QT240-ISSG Atmel, QT240-ISSG Datasheet - Page 4

• Manufacturer Part Number: QT240-ISSG
• Description: IC SENSOR QTOUCH 4CHAN 20SSOP
• Manufacturer: Atmel
• Series: Quantumr
• Type: Signal Conditionerr

Specifications of QT240-ISSG

• Input Type: Logic
• Output Type: Logic
• Current - Supply: 1.5mA
• Mounting Type: Surface Mount
• Package / Case: 20-SSOP
• For Use With: 427-1113 - BOARD EVAL FOR QT240-IS QTOUCH
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Interface: -
• Other names: 427-1102-2; QT240-ISS-G

Multiple touch electrodes connected to any 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
Threshold
SPEED
OPT
Output
S1
VDD
1M
R1
Figure 1.2 Fast, Spread-spectrum Circuit
OUT1
OUT2
OUT3
OUT4
22K
RSNS1
10nF
CS1
RS1
2.2K
Figure 2.1 Drift Compensation
Signal
10nF
CS2
RS2
2.2K
22K
RSNS2
QT240_ISS
Reference
22K
RSNS3
10nF
CS3
2.2K
RS3
22K
RSNS4
Hysteresis
10nF
CS4
2.2K
RS4
360K
R5
C1
22nF
62K
VDD
10 second
timeout shown
1M
180K
R3
R6
OPT2
R4
S3
OPT1
4
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 QT240 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
reference level track the raw signal at a slow rate, only while
VDD
R2
1M
S2
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 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 four keys may thus require ‘balancing’ to
achieve similar sensitivity levels. This can be best
accomplished by trimming the values of the four
Cs capacitors to achieve equilibrium. The four Rs
resistors have no effect on sensitivity and should
Sensitivity
QT240R R1.11/1006