Cypress CapSense AN64846, Getting Started

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Design Considerations
AN64846 - Getting Started with CapSense
®
Doc. No. 001-64846 Rev. *X 45
3.3.1.1.3 Trace Length
Long traces can pick up more noise than short traces. Long traces also add to C
P
. Minimize trace length whenever
possible.
3.3.1.1.4 Current Loop Area
Another important layout consideration is to minimize the return path for current. A ground plane can lower the overall
ground impedance, thus reducing the high-frequency ground bounce. Ensure good GND return paths for each sensor
line. This is important as the current flows in loops. Unless there is a proper return path for high-frequency signals, the
return current will flow through a longer return path forming a larger loop; this can cause signal issues due to mutual
inductance. Thus, it leads to increased emissions and interference.
When a device package contains high-frequency current loops, energy can also be coupled out of the device through
a magnetic field. It is possible for the magnetic flux to form a current loop in the device to link to the circuit loops outside
the device. This mutual inductance can produce an unwanted voltage in the external loop. Likewise, an external
magnetic flux can induce an unwanted voltage across an interior circuit loop. Magnetic field coupling can be minimized
by keeping the power and signal loop areas as small as possible. Stitch all the grounds with as many vias as possible.
This will reduce the overall ground impedance. High-frequency traces, such as those used for clock and oscillator
circuits, should be contained by two ground lines. This will ensure that there is no coupling, which results in crosstalk.
Use separate ground plane and power planes wherever possible.
Figure 3-8 shows an example of an improper grounding scheme. The layout greatly improves by reducing the loop
area.
Figure 3-8. Improper Ground Scheme and Ground Loop
CapSense
Controller
On-Board Driver
Circuits
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
In Figure 3-9 , two sensors are surrounded by a ground plane that is connected to a CapSense controller ground, while
a third sensor is surrounded by ground. The third sensor is connected to the other ground plane through the long traces
of other circuitry, which creates a large current loop. With this layout, the third sensor may be more susceptible to
radiated noise and have increased emissions. These two sections of ground are in the same location on the schematic,
so they can potentially be one connected area with a better layout.