3: Integration Guidelines
Micro125 Embedded Device Server Integration Guide
15
Figure 3-1 Multi-Layer Board Strategy
Virtual Ground
The device server provides a virtual ground at a (plated) mounting hole. It is a ground
imitation. It uses the tap of two capacitors (ceramic 0.1
μF) conducted symmetrically to
ground and VCC. In the absence of a solid ground (earth), this virtual ground can be
used for shielding or balancing metal parts of the case.
Serial Signals
Device server TTL-level serial input and output signals are protected by 220-Ohm
resistors. These resistors provide a simple output shortage protection for infinite duration
(by limiting the current). They also reduce conducted interferences at higher frequencies
to the base board.
Additional Emission Improvements
Depending on the voltage regulator used and base board design, the power supply cord
may sometimes emit conducted interferences. If the voltage level there is low, common
mode chokes are the appropriate barrier to avoid these frequencies being emitted via the
power cord as an antenna.
Common mode chokes help pass the conducted emission requirements of the EN55022
for frequencies below 30 MHz. Metal cases or partial metal shielding inside the unit can
also help to reduce emission levels so that even more stringent standards can be
passed.
