User guide
Getting Started with CapSense Document No. 001-64846 Rev. *A 33
3.3.1.2.1 CapSense Input Lines
The recommended series resistance for CapSense input lines is 560 ohms. Adding resistance changes the time
constant of the switched capacitor circuit that converts C
P
3.3.1.2.2 Digital Communication Lines
into an equivalent resistor. If the value is set larger than
560 ohms, the slower time constant of the switching circuit limits the amount of charge that can transfer, and this
lowers the signal level, which in turn lowers SNR. Smaller values are better, but are less effective at blocking RF.
Communication lines, such as I
2
3.3.1.3 Trace Length
C, and SPI, also benefit from series resistance. 330 ohms is recommended for
communication lines. Communication lines have long traces that act as antennae like the CapSense traces. If more
than 330 ohms is placed in series on these lines, the voltage levels fall out of spec with the worst case combination of
supply voltages between systems and the input impedance of the receiver.
Long traces can pick up more noise than short traces. Long traces also add to C
P
3.3.1.4 Current Loop Area
. Minimize trace length whenever
possible.
Another important layout consideration is to minimize the return path for current. General system emission
suppression techniques include adding a decoupling capacitor network, and reducing current loops. Current loops
create issues for both emission and immunity. A proper ground plane scheme can make a big reduction in path
length. Hatched ground instead of solid fill should be provided near the sensors or traces to reduce the impact of
parasitic capacitance. A solid ground flood is not recommended within 1 cm of CapSense sensors or traces due to an
increase the parasitic capacitance. Figure 3-5 shows an example of an improper grounding scheme. The layout
would greatly improve by reducing the loop area.
Figure 3-5. Improper Ground Scheme and Ground Loop
CapSense
Controller
On Board Driver
circiuts
External Circiuts
1) Decoupling loop inductance and
switching currents combine to create
an AC voltage on the local ground
2) Each output (and input) drives the AC
voltage out onto the PCB. Each signal
will have a loop area associated with it.
3) Cables magnify the problem as loop
areas are proportional to cable length