User guide

Getting Started with CapSense Document No. 001-64846 Rev. *A 35
3.3.1.5 RF Source Location
When systems like computer monitors or digital photo frames are designed with CapSense devices, care must be
taken care to prevent noise from LCD inverters and switched-mode power supplies (SMPS) from upsetting the
CapSense system. A simple technique to minimize this kind of interaction is to partition the system with noise sources
from CapSense inputs, as demonstrated in Figure 3-8. Due to the practical limitations of product size, the noise
source and the CapSense circuitry may only be separated by a few inches. This small separation can provide the
extra margin required for good sensor performance compared to the case with close proximity between noise source
and CapSense.
Figure 3-8. Separating Noise Sources
Computer monitor
SMPS/LCD Inverter
CapSense interface
Computer monitor
SMPS/LCD Inverter
CapSense interface
NOT Recommended Recommended
3.3.2 Radiated Emissions
Figure 3-9 shows the impact of rise/fall time of a square wave on the radiated emissions. It is clear that slowing the
transitions introduces the cutoff point and damps the radiated energy level. The internal clock signals of the
CapSense controller are slew-controlled to reduce the radiated emission.
Figure 3-9. Impact of Slew Rate on Emissions
The CapSense sensing methods use a switched capacitor front end to interact with the sensors. Selecting a low
frequency for the switched capacitor clock will help to reduce the radiated noise from the CapSense sensor.