Stitching Capacitors; Stitching Capacitors Could Create Low Impedance Path For Return Currents - Intel Quark SoC X1000 Design Manual

17.4.4.1

Stitching Capacitors

Stitching capacitors are used to create return current path for signal traces with
different references. Using stitching capacitors for split-crossing is one example. The
separated reference planes have different voltages. They are, therefore, not able to be
merged. As a result, there is no low impedance path for the return currents. The
current loop becomes large and unpredictable. Connecting one or two capacitors
between the two planes could provide low impedance return paths. Therefore, the new
current loop is controlled.
Figure 63.

Stitching Capacitors Could Create Low Impedance Path for Return Currents

Figure 64
simulated results show the different emissions of a microstrip line with a perfect
referencing, a microstrip line with split-crossing, a split-crossing microstrip line with
one stitching capacitor, and a split-crossing microstrip line with two stitching
capacitors.It is observed that split-crossing results in much higher emission. Placing
one stitching capacitor reduces emissions. Placing two stitching capacitors
symmetrically to the trace greatly reduces EMI.
Stitching capacitors should be placed as close as possible to the troubled signal traces.
Figure 65
2-mm away from the trace. To maximize stitching capacitor effect:
• Two capacitors placed symmetrically to the split-crossing trace show much better
mitigation than one capacitor only.
• Capacitors should be placed as close as possible to the split-crossing trace.
®
Intel Quark™ SoC X1000
PDG
116
No cap case No cap case
shows an example of stitching capacitor impact on EMI radiation. The
shows different emissions when the stitching capacitors are placed 1-mm and
®
Intel Quark™ SoC X1000—Electromagnetic Interference
cap cap
with cap case with cap case
June 2014
Order Number: 330258-002US