ATQT600 Atmel, ATQT600 Datasheet - Page 11
![KIT EVAL TOUCH FOR QT600](/photos/9/18/91802/qt600_sml.jpg)
ATQT600
Manufacturer Part Number
ATQT600
Description
KIT EVAL TOUCH FOR QT600
Manufacturer
Atmel
Series
QTouch™r
Specifications of ATQT600
Sensor Type
Touch Screen
Interface
USB
Embedded
Yes, Other
Utilized Ic / Part
ATtiny88, ATmega324PA, ATxmega128A1
Processor To Be Evaluated
ATtiny88, ATmega324, ATxmega128
Data Bus Width
8 bit, 16 bit
Interface Type
USB
Maximum Operating Temperature
+ 85 C
Minimum Operating Temperature
- 40 C
Operating Supply Voltage
1.6 V to 3.6 V
Silicon Manufacturer
Atmel
Kit Application Type
Sensor
Application Sub Type
Touch Sensor
Kit Contents
USB Bridge, MCU Cards, Touchpad Cards
Svhc
No SVHC (15-Dec-2010)
Mcu Supported Families
ATtiny88,
Rohs Compliant
Yes
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
Voltage - Supply
-
Sensitivity
-
Sensing Range
-
Lead Free Status / Rohs Status
Lead free / RoHS Compliant
Available stocks
Company
Part Number
Manufacturer
Quantity
Price
Company:
Part Number:
ATQT600
Manufacturer:
Atmel
Quantity:
135
2.1
Touch Sensors Design Guide
Charge Transfer
Atmel’s capacitive sensors work on a principle called charge transfer. This uses a switched capacitor
technique to assess relative changes in a sensor’s capacitance as it is touched.
Charge transfer works by applying a voltage pulse to series connection of the unknown capacitance Cx
and a charge integrator capacitor Cs. By repeating the pulse multiple times, a high resolution
measurement system is realized that can detect changes in capacitance of just a few femtofarads
In order to obtain stable and repeatable results, it is important that the voltage pulse is allowed to settle
properly and hence transfer all the charge into Cx and Cs (see
Because Cx and Cs are normally connected with some amount of series resistance, the RC time
constants so formed will tend to slow down this settling process.
It is therefore important that when designing a capacitive touch system that the amount of series
resistance is kept in mind in combination with the size of Cx (the sensor’s capacitance).
Series resistance is normally deliberately introduced to improve electromagnetic interference (EMI) and
ESD behavior (see the device datasheets for recommended series resistors). For some designs,
significant extra resistance can be introduced because of the resistivity of the tracks connecting the
sensor, or the resitivity of the electrode material itself. This is normally only true for designs using
material like Indium Tin Oxide (ITO), Orgacon
situations where adding deliberate extra series resistance can be beneficial, most notably in designs with
difficult ESD conditions or where emissions must be controlled to very low levels. A 10 k series resistor
can be a good choice. In either case, RC time constants can degrade the charge-transfer process so
some precautions must be taken.
For all designs, it is good practice to measure the settling time of the charge-transfer pulses to make
sure they are fast enough to work reliably. This can be done by observing the pulses using an
oscilloscope and coupling the scope probe to the sensor electrode capacitively
top of the overlying panel, or using a small piece of copper tape instead (see
The charge pulses should have substantially flat tops, that is the voltage has settled before the pulse
ends (see
1. One femtofarad is 1/1000 of a picofarad.
2. Orgacon™ is a trademark of Agfa-Gevaert Group. Orgacon is a printable conductive ink using a PEDOT polymer
3. Probing the sensor directly will add the capacitance of the probe and so give an unrealistic wave shape.
base. Care should be taken when assessing this material’s suitability for sensor construction due to the high
starting resistivity and the fact that this resistivity is known to increase over life depending on its exposure to UV,
heat and moisture (and any other oxidising substance). Consult Agfa before making any design decisions.
Figure 2-2 on page
2-2).
™ (2)
or Carbon with high /sq values. However, there are
Figure 2-2 on page
General Advice
Figure 2-1 on page
(3)
using a small coin on
2-2).
Section 2
10620D–AT42–04/09
(1)
2-2).
.
2-1