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PDF AN11157 Data sheet ( Hoja de datos )

Número de pieza AN11157
Descripción Capacitive touch sensing
Fabricantes NXP Semiconductors 
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AN11157
Capacitive touch sensing with high EMC performance
Rev. 1 — 3 February 2012
Application note
Document information
Info Content
Keywords
EMC performance, Touch sensing, Design, Guidelines, PCF8885,
PCA8885
Abstract
NXP’s touch sensor family provides one of the highest immunity sensor
circuits on the market. Nevertheless some considerations in the design
are necessary to get a high EMC performance in the application. A
description and discussion about the EMC immunity and how it can be
improved are made in this document.

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AN11157 pdf
NXP Semiconductors
AN11157
Capacitive touch sensing with high EMC performance
fclk = fosc n , where n = 1, 4, 16, 64, and 16 is the default value.
The sensor sampling frequency (fs) is derived from the internal clock frequency with
fs = fclk 8
The eight sensors are sampled sequentially, which results in a default sensor sampling
rate of fs = 0.55 kHz.
Running autonomously, this method provides efficient immunity against RF noise down to
kHz ranges. There is no need for advanced computing of the absolute capacitance level,
noise level in the system, and so on.
Not even a fly nor a turtle will activate the sensor!
4. EMC design considerations for board layout
Thanks to the properties explained in the previous section, the sensor plates and routing
may be done with high immunity in mind and without major concerns for reducing
sensitivity.
For a two-layer FR-4 based PCB taken as a reference, the following design rules are
recommended:
Components and routing should be on the bottom layer.
On the top layer, the sensor plates should be surrounded by GND. The separation to
GND is recommended to be equal to the overlay thickness. However a minimum of
2.2 mm will work for a wide range of thicknesses. The GND is needed for high
immunity. There will be stray capacitance between a touching finger and GND which
is not good for sensitivity. So, a simple way of providing high immunity without
decreasing the sensitivity is to make hatched GND around sensor plates.
On the bottom layer, the GND under sensor plates is recommended to be hatched
with 20 % metallization. The gap between traces and GND should be kept as small as
possible; 0.1 mm to 0.15 mm but the total capacitance budget on each channel must
not be exceeded. The width of traces may be kept small since the resistance and
inductance is not critical. The traces should be kept as short as possible for immunity
reasons.
A trace on a typical FR4 board with GND both on the rear side and on the primary side is
illustrated in Figure 3. The graph shows that the parasitic capacitance contribution from
the trace will decrease with increased separation to GND on the sides. In order to obtain
lower capacitance, the GND metallization on the rear side of the board might be hatched
or removed with lower RF immunity as a consequence for the latter case.
AN11157
Application note
All information provided in this document is subject to legal disclaimers.
Rev. 1 — 3 February 2012
© NXP B.V. 2012. All rights reserved.
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AN11157 arduino
NXP Semiconductors
AN11157
Capacitive touch sensing with high EMC performance
7. Legal information
7.1 Definitions
Draft — The document is a draft version only. The content is still under
internal review and subject to formal approval, which may result in
modifications or additions. NXP Semiconductors does not give any
representations or warranties as to the accuracy or completeness of
information included herein and shall have no liability for the consequences of
use of such information.
7.2 Disclaimers
Limited warranty and liability — Information in this document is believed to
be accurate and reliable. However, NXP Semiconductors does not give any
representations or warranties, expressed or implied, as to the accuracy or
completeness of such information and shall have no liability for the
consequences of use of such information. NXP Semiconductors takes no
responsibility for the content in this document if provided by an information
source outside of NXP Semiconductors.
In no event shall NXP Semiconductors be liable for any indirect, incidental,
punitive, special or consequential damages (including - without limitation - lost
profits, lost savings, business interruption, costs related to the removal or
replacement of any products or rework charges) whether or not such
damages are based on tort (including negligence), warranty, breach of
contract or any other legal theory.
Notwithstanding any damages that customer might incur for any reason
whatsoever, NXP Semiconductors’ aggregate and cumulative liability towards
customer for the products described herein shall be limited in accordance
with the Terms and conditions of commercial sale of NXP Semiconductors.
Right to make changes — NXP Semiconductors reserves the right to make
changes to information published in this document, including without
limitation specifications and product descriptions, at any time and without
notice. This document supersedes and replaces all information supplied prior
to the publication hereof.
Suitability for use — NXP Semiconductors products are not designed,
authorized or warranted to be suitable for use in life support, life-critical or
safety-critical systems or equipment, nor in applications where failure or
malfunction of an NXP Semiconductors product can reasonably be expected
to result in personal injury, death or severe property or environmental
damage. NXP Semiconductors and its suppliers accept no liability for
inclusion and/or use of NXP Semiconductors products in such equipment or
applications and therefore such inclusion and/or use is at the customer’s own
risk.
Applications — Applications that are described herein for any of these
products are for illustrative purposes only. NXP Semiconductors makes no
representation or warranty that such applications will be suitable for the
specified use without further testing or modification.
Customers are responsible for the design and operation of their applications
and products using NXP Semiconductors products, and NXP Semiconductors
accepts no liability for any assistance with applications or customer product
design. It is customer’s sole responsibility to determine whether the NXP
Semiconductors product is suitable and fit for the customer’s applications and
products planned, as well as for the planned application and use of
customer’s third party customer(s). Customers should provide appropriate
design and operating safeguards to minimize the risks associated with their
applications and products.
NXP Semiconductors does not accept any liability related to any default,
damage, costs or problem which is based on any weakness or default in the
customer’s applications or products, or the application or use by customer’s
third party customer(s). Customer is responsible for doing all necessary
testing for the customer’s applications and products using NXP
Semiconductors products in order to avoid a default of the applications and
the products or of the application or use by customer’s third party
customer(s). NXP does not accept any liability in this respect.
Export control — This document as well as the item(s) described herein
may be subject to export control regulations. Export might require a prior
authorization from competent authorities.
Evaluation products — This product is provided on an “as is” and “with all
faults” basis for evaluation purposes only. NXP Semiconductors, its affiliates
and their suppliers expressly disclaim all warranties, whether express, implied
or statutory, including but not limited to the implied warranties of
non-infringement, merchantability and fitness for a particular purpose. The
entire risk as to the quality, or arising out of the use or performance, of this
product remains with customer.
In no event shall NXP Semiconductors, its affiliates or their suppliers be liable
to customer for any special, indirect, consequential, punitive or incidental
damages (including without limitation damages for loss of business, business
interruption, loss of use, loss of data or information, and the like) arising out
the use of or inability to use the product, whether or not based on tort
(including negligence), strict liability, breach of contract, breach of warranty or
any other theory, even if advised of the possibility of such damages.
Notwithstanding any damages that customer might incur for any reason
whatsoever (including without limitation, all damages referenced above and
all direct or general damages), the entire liability of NXP Semiconductors, its
affiliates and their suppliers and customer’s exclusive remedy for all of the
foregoing shall be limited to actual damages incurred by customer based on
reasonable reliance up to the greater of the amount actually paid by customer
for the product or five dollars (US$5.00). The foregoing limitations, exclusions
and disclaimers shall apply to the maximum extent permitted by applicable
law, even if any remedy fails of its essential purpose.
Translations — A non-English (translated) version of a document is for
reference only. The English version shall prevail in case of any discrepancy
between the translated and English versions.
7.3 Licenses
ICs with capacitive sensing functionality
This NXP Semiconductors IC is made under license to European Patent
No. 0723339, owned by EDISEN - SENSOR SYSTEME GmbH & CO KG
and counterparts. Any license fee is included in the purchase price.
7.4 Trademarks
Notice: All referenced brands, product names, service names and trademarks
are the property of their respective owners.
AN11157
Application note
All information provided in this document is subject to legal disclaimers.
Rev. 1 — 3 February 2012
© NXP B.V. 2012. All rights reserved.
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