Design Considerations
AN64846 - Getting Started with CapSense
®
Doc. No. 001-64846 Rev. *X
43
3.3.1.1 General EMI/EMC Guidelines
3.3.1.1.1 Ground Plane
In general, a proper ground plane on the PCB reduces both RF emissions and interference. However, solid grounds
near CapSense sensors or traces connecting these sensors to the PSoC pins increase the parasitic capacitance of the
sensors. It is thus recommended that you use hatched ground planes surrounding the sensor and on the bottom layer
of the PCB, below the sensors, as explained in the
section. A solid ground may be used below the device
and other circuitry on the PCB, which is far from CapSense sensors and traces. A solid ground flood is not
recommended within 10 mm of CapSense sensors or traces. Multiple-layer boards should be the preferred choice. If
you are using a board with four layers or more, you can provide a complete layer for ground that will further help to
reduce emissions as it reduces the ground bounce significantly.
3.3.1.1.2 Series Resistor
Every CapSense controller pin has some parasitic capacitance (C
P
) associated with it. Adding an external resistor
forms a low-pass RC filter that can dampen the RF noise amplitude coupled with the pin. This resistance also forms a
low-pass filter with the parasitic capacitance of the trace connected to the pin (for example, sensor trace and sensor
pad as shown in
) that can significantly reduce RF emissions. Thus, series resistors help in eliminating higher-
order harmonics and attenuating the RF interference and emission.
Figure 3-5. RC Filter
CapSense
Controller
CapSense
Sensor
External Series
Resistor
Pins
Capacitance
Series resistors should be placed close to the PSoC pins so that the radiated noise picked by the traces gets filtered at
the input of the PSoC device. Thus, it is recommended that you place series resistors within 10 mm of the PSoC pins.
3.3.1.1.2.1
CapSense Input Lines
The sensor must be fully charged and discharged during each switching cycle for proper operation of CapSense. The
charge and discharge paths of the sensor capacitor include series resistances that slow down the charging / discharging
process.
shows an equivalent circuit and resulting waveforms. Adding a resistance changes the time
constant of the switched-capacitor circuit that converts C
P
into an equivalent resistor. If the series resistance value is
set high, the slower time constant of the switching circuit suppresses the emission but limits the amount of charge that
it can transfer. Thus the sensors may not get charged and discharged completely. This lowers the signal level, which
in turn lowers the SNR. Smaller values are better, but are less effective at blocking RF emissions and interference.
The recommended series resistance for CapSense input lines on general copper PCBs is 560
Ω. An ITO panel already
provides a high resistance; one may not have too much flexibility in the value selection (range 100 Ω–1 kΩ). Series
resistors are generally used in the range of 560 Ω–4.7 kΩ for EMC purpose. The actual maximum value of the series
resistor that can be used varies from device to device. This depends on multiple factors such as the resistance of the
GPIO used as sensor, the switching frequency used to scan sensors, and the SNR required.
