User manual
Advantech SOM-Express Design Guide
Chapter 5 Carrier Board Design Guidelines 71
5.8.2.2 ESD diode
ESD diodes are required for each TV DAC channel output. The diodes should
connect between the 3.3 V power plane (from the regulator) and ground. These
diodes should have a low C rating (~ 5 pF max) and a small leakage current (~ 10 uA
at 120℃).The diodes should be placed to keep the inductance of the 3.3 V power rail
connection as low as possible. The diode placement should be similar for all three
channel outputs and should not be shared with any other signals, especially video or
clock signals. In addition, one decoupled capacitor, C1= 0.1 uF, should be placed in
close proximity and across the ESD diodes to reduce noise on the 3.3 V rail.
5.8.2.3 TV DAC Video Filter
A video filter is required for each TV DAC channel output signal. This video filter is to
be placed in close proximity to the connector. The separation between each of the 3
video filters for the TV DAC channels should be a minimum of 50 mils or greater if
possible in order to minimize crosstalk. This is especially important for the TVDAC_B
and TVDAC_C channels (S-video signals). Figure 5-36 shows the TV DAC Video
Filter.
Input
FB
C1 C2
TV DAC Filter
Figure 5-36 TV DAC Video Filter
The video filter is designed for a cutoff frequency of at least 30 MHz and a gain of -3
dB. Table 5-21 shows the TV DAC Video Filter component descriptions.
Table 5-21 TV DAC Video filter component descriptions
Component Value Tolerance Voltage/Current Type
C1 6 pF
± 20%
16 V Ceramic
C2 6 pF
± 20%
16 V Ceramic
FB
150 #@
100 MHz
± 25%
100 mA
5.8.3 Layout Guidelines
5.8.3.1 TV DAC routing
The minimum spacing between each TV DAC signal is 40 mils, but 50 mils is
preferred. A maximum amount of spacing should be used between each TV DAC
signal as well as to all other toggling signals. This helps prevent crosstalk between
the TV DAC signals and other toggling signals. The routing for each TV DAC signal
should also be matched and balanced as much as possible. All TV DAC signals
should be routed on the same layer, have a similar number of bends, the same
number of vias, etc. All routing should be done with ground referencing as well.
Figure 5-37 shows the TV DAC routing Topology.