Datasheet

4. Circuit Guidelines

4.1 More Information

Refer to Application Note QTAN0002, Secrets of a Successful QTouch Design and the Touch Sensors

Design Guide (both downloadable from the Microchip website), for more information on construction and

design methods.

4.2 Sample Capacitor

Cs is the charge sensing sample capacitor. The required Cs value depends on the thickness of the panel

and its dielectric constant. Thicker panels require larger values of Cs. Typical values are 2.2 nF to 50 nF

depending on the sensitivity required; larger values of Cs demand higher stability and better dielectric to

ensure reliable sensing.

The Cs capacitor should be a stable type, such as X7R ceramic or PPS film. For more consistent sensing

from unit to unit, 5% tolerance capacitors are recommended. X7R ceramic types can be obtained in 5%

tolerance at little or no extra cost. In applications where high sensitivity (long burst length) is required the

use of PPS capacitors is recommended.

For battery powered operation a higher value sample capacitor may be required.

4.3 Rs Resistor

Series resistor Rs is in line with the electrode connection and should be used to limit ESD currents and to

suppress radio frequency interference (RFI). It should be approximately 4.7 kΩ to 33 kΩ.

Although this resistor may be omitted, the device may become susceptible to external noise or RFI. See

Application Note QTAN0002, Secrets of a Successful QTouch Design, for details of how to select these

resistors.

4.4 Power Supply and PCB Layout

See Section 5.2 for the power supply range.

If the power supply is shared with another electronic system, care should be taken to ensure that the

supply is free of digital spikes, sags, and surges which can adversely affect the QT1012. The QT1012 will

track slow changes in Vdd, but it can be badly affected by rapid voltage fluctuations. It is highly

recommended that a separate voltage regulator be used just for the QT1012 to isolate it from power

supply shifts caused by other components.

If desired, the supply can be regulated using a Low Dropout (LDO) regulator, although such regulators

often have poor transient line and load stability. See Application Note QTAN0002, Secrets of a Successful

QTouch Design, for further information on power supply considerations.

Parts placement: The chip should be placed to minimize the SNSK trace length to reduce low frequency

pickup, and to reduce stray Cx which degrades gain. The Cs and Rs resistors (see Figure 1-1) should be

placed as close to the body of the chip as possible so that the trace between Rs and the SNSK pin is very

short, thereby reducing the antenna-like ability of this trace to pick up high frequency signals and feed

them directly into the chip. A ground plane can be used under the chip and the associated discrete

components, but the trace from the Rs resistor and the electrode should not run near ground, to reduce

AT42QT1012

Datasheet

DS40001948A-page 22