Checking the Effects of External Noise on Atmel® Capacitive-touch ICs 1. Introduction Although Atmel capacitive-touch integrated circuits (QT ICs) include many noise suppression features, to optimize product design and improve performance it is useful to measure the effects of noise on raw touch-detection signals and signal delta values. 2. Sources of Noise Electrical noise can emanate from one or more sources such as: • Noisy power supply • Insufficient or improperly located decoupling capacitors • Decoupling capacitors that are too large or too small • Interfering signals near the electrodes or electrode traces Effects of External Noise on QT ICs Application Note QTAN0017 • Nearby devices such as liquid crystal displays (LCDs) and oscillators • Radiating devices such as television signals and induction heating (IH) cookers 3. Jitter In an optimum design, the signal levels display little variation when there is no change in touch. Jitter of no more than ±2 points is typical. In a noisy design, the signal can vary significantly from plus or minus ten points or even hundreds of points. Extreme jitter can cause false, or missed, detections. 4. Reading Signal Data Many QT ICs permit the signal levels for each sensor to be read through a serial connection such as serial peripheral interface (SPI), universal asynchronous receiver transmitter (UART), Inter Integrated Circuit (I2C-compatible). Refer to the datasheet of the QT IC used for details on retrieving signal level data. 5. Retrieving and Displaying Information If the product has a display that can show multiple numeric values, the use of a diagnostic mode that can present signal level data is beneficial. Otherwise, to retrieve data, it may be necessary to use an isolated connection such as an optically-coupled RS-232 port or USB connection. A wireless connection such as Bluetooth® is also an option. 10708A–AT42–10/08 6. Connections to External Devices If the product has different connections to external devices such as battery chargers, headsets, or download cables, make sure that the product is tested in every possible combination of connection. Some types of noise change significantly – depending on the combination of connections and whether or not they are grounded. 7. Quick Tests Figures 7-1 and 7-2 illustrate methods of quickly testing the product to determine whether shielding, synchronization, or other noise suppression measures are necessary. Figure 7-1. Quick Test A Conductive Sheet Test Device (LCD, RF) Figure 7-2. Demo Unit (Touch Side Down) Quick Test B Jumper Leads (Connected to Conductive Sheets) Conductive Sheet Conductive Sheet Test Device (LCD, RF) Demo Unit (Touch Side Down) 1. Place the demonstration unit and the test device (for example, LCD or RF) on the conductive sheet, sensor side to sheet (see Figure 7-1). Alternatively, use two conductive sheets connected by a jumper wire (see Figure 7-2). Ensure that the conductive sheet (typically copper) faces the touch sensor. For example, in a touch LCD design, the conductive sheet should face the view side of the LCD. 2. Connect the Ground of the demonstration unit to the Ground of the test device. 3. Connect the appropriate software for the demonstration unit to the PC. 4. Ensure the devices are mechanically stable. 5. Recalibrate the demonstration unit. 6. Using the demonstration unit software check for noise affecting the touch signals. 2 Effects of External Noise on QT ICs 10708A–AT42–10/08 Effects of External Noise on QT ICs 8. Associated Publications The following documents published by Atmel’s Touch Technology Division are also of interest: • QTAN0015 – Power Supply Considerations for Atmel Capacitive-touch ICs • QTAN0016 – Diagnostic Modes for Products with Atmel Capacitive-touch ICs 3 10708A–AT42–10/08 Headquarters International Atmel Corporation 2325 Orchard Parkway San Jose, CA 95131 USA Tel: 1(408) 441-0311 Fax: 1(408) 487-2600 Atmel Asia Unit 01-05 & 16, 19/F BEA Tower, Millennium City 5 418 Kwun Tong Road Kwun Tong Kowloon Hong Kong Tel: (852) 2245-6100 Fax: (852) 2722-1369 Atmel Europe Le Krebs 8, Rue Jean-Pierre Timbaud BP 309 78054 Saint-Quentin-enYvelines Cedex France Tel: (33) 1-30-60-70-00 Fax: (33) 1-30-60-71-11 Atmel Japan 9F, Tonetsu Shinkawa Bldg. 1-24-8 Shinkawa Chuo-ku, Tokyo 104-0033 Japan Tel: (81) 3-3523-3551 Fax: (81) 3-3523-7581 Technical Support [email protected] Sales Contact [email protected] Touch Technology Division 1 Mitchell Point Ensign Way Hamble Southampton Hampshire SO31 4RF United Kingdom Tel: (44) 23-8056-5600 Fax: (44) 23-8045-3939 Product Contact Web Site www.atmel.com Literature Requests www.atmel.com/literature Disclaimer: The information in this document is provided in connection with Atmel products. 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