QT60248C-ASG Atmel, QT60248C-ASG Datasheet - Page 38

• Manufacturer Part Number: QT60248C-ASG
• Description: IC TOUCH SENSOR 24KEY 32TQFP
• Manufacturer: Atmel
• Series: QMatrix™, QProx™r
• Type: Capacitiver

Specifications of QT60248C-ASG

• Number Of Inputs/keys: 24 Key
• Resolution (bits): 9, 11 b
• Data Interface: Serial, SPI™
• Voltage - Supply: 3 V ~ 5 V
• Current - Supply: 25mA
• Operating Temperature: -40°C ~ 105°C
• Mounting Type: Surface Mount
• Package / Case: 32-TQFP, 32-VQFP
• Output Type: *
• Interface: *
• Input Type: *
• For Use With: 427-1087 - BOARD EVAL QT60248-AS QMATRIX
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

10620D–AT42–04/09
Mutual-capacitance Zero-dimensional Sensors
4.2.3
4-6
Ground Loading
One big advantage of a mutual-capacitance sensor is that because the coupling capacitance from X to Y
is being measured, any parasitic effects on X or Y are not so acute as they are with a self-capacitance
electrode.
Placing a ground plane in close proximity to the non-touch side of such a sensor should still be avoided
because it will desensitize the key. However, the overall effect is far less dramatic than for a self-
capacitance sensor.
Bringing ground floods or traces close to the X element of the sensor has little or no effect on sensitivity.
This is beneficial in most cases as it helps to shield X and Y traces on the layers below from interference
from strong electromagnetic fields nearby. The ground flood can be made very close to X. Another
benefit is that the ground flood helps to improve the overall free-space return path, which is particularly
important for small battery powered devices.
Figure 4-6.
Bringing ground floods or traces close to Y adds parasitic capacitance (Cp) and can cause loss of
sensitivity. Keep this in mind when flooding around keys; the interconnecting Y traces on layers below
the flood will suffer some additional Cp loading. Try to keep the Y interconnections to the layer farthest
from the flood (see
build-up on Y could be substantial due to the reduced separation between layers.
Figure 4-7.
Ground Flood Around X
Cross-section of a Multi-layer PCB
Ground
Figure
Y Trace Located Farthest
From Ground Flood
4-7). Be careful when adding ground floods on thin PCBs or FPCBs, as the Cp
X
Y
Via
Front Panel
Touch Sensors Design Guide