AT42QT1040-MMH Atmel, AT42QT1040-MMH Datasheet - Page 11

• Manufacturer Part Number: AT42QT1040-MMH
• Description: IC TOUCH SENSOR 4KEY 20-VQFN
• Manufacturer: Atmel
• Series: QTouch™r
• Type: Resistiver

Specifications of AT42QT1040-MMH

• Touch Panel Interface: 4, 2-Wire
• Number Of Inputs/keys: 4 Key
• Data Interface: I²C, Serial
• Data Rate/sampling Rate (sps, Bps): 200k
• Voltage Reference: Internal
• Voltage - Supply: 1.8 V ~ 5.5 V
• Current - Supply: 10mA
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 20-VQFN Exposed Pad, 20-HVQFN, 20-SQFN, 20-DHVQFN
• Output Type: Logic
• Input Type: Logic
• Supply Voltage: 1.8 V to 5.5 V
• Dimensions: 3 mm L x 3 mm W x 0.8 mm H
• Output Voltage: 0.7 V to 0.8 V
• Temperature Range: - 40 C to + 85 C
• Termination Style: SMD/SMT
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Interface: -
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant
• Other names: AT42QT1040-MMH; AT42QT1040-MMHTR

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