User guide

32 Document No. 001-64846 Rev. *A Getting Started with CapSense®
A more effective method is to provide special purpose ESD protection devices on the vulnerable traces. ESD
protection devices for CapSense need to be low capacitance. Table 3-2 lists devices recommended for use with
CapSense controllers.
Table 3-2. ESD Protection Devices
ESD Protection device
Input
Capacitance
Leakage
Current
Contact
Discharge
maximum limit
Air Discharge
maximum limit
Manufacturer Part Number
Littlefuse SP723 5 pF 2 nA 8 kV 15 kV
Vishay VBUS05L1-DD1 0.3 pF 0.1 µA < +/-15 kV +/-16 kV
NXP NUP1301 0.75 pF 30 nA 8 kV 15 kV
3.3 Electromagnetic Compatibility (EMC) Considerations
EMC is concerned with the generation, transmission, and reception of electromagnetic energy that can upset the
working of a system. The source (emitter) produces the emission, and a transfer or coupling path transfers the
emission energy to a receptor, where it is processed, resulting in either desired or undesired behavior. Many
electronic devices are required to comply with specific limits for emitted energy and susceptibility to external upsets.
There are several regulatory bodies worldwide that set regional regulations to help ensure that electronic devices do
not interfere with each other. These regulations help prevent your computer from interfering with your television, or
worse, a hospital X-ray machine or ventilator, or corrupting the operation of a critical medical monitor.
CMOS analog and digital circuits have very high input impedance. As a result, they are quite sensitive to external
electric fields. Suitable precautions must be taken to ensure their proper operation in the presence of radiated and
conducted noise.
3.3.1 Radiated Interference
Radiated electrical energy can influence system measurements and potentially influence the operation of the
CapSense processor core. The interference enters the CapSense chip at the PCB level, through the sensor traces,
and via other digital and analog inputs.
Layout guidelines for minimizing the effects of RF interference are discussed in the following sections.
3.3.1.1 Ground Plane
In general, providing a ground plane on the PCB helps to reduce the RF noise picked up by the CapSense controller.
3.3.1.2 Series Resistor
Every CapSense controller pin has some parasitic capacitance, C
P
Figure 3-4. RC Filter
, associated with it. Adding an external resistor
forms a low pass RC filter that can dampen RF noise amplitude.
CapSense
Controller
CapSense
Sensor
External series
resistor
Pins
Capacitance
Series resistors should be placed within 10 mm of the CapSense controller pins.