DS90UB901Q Datasheet, PDF (40/45 Pages) Texas Instruments – 10 - 43MHz 14 Bit Color FPD-Link III Serializer and Deserializer with Bidirectional Control Channel

TRANSMISSION MEDIA
The Ser/Des chipset is intended to be used over a wide variety
of balanced cables depending on distance and signal quality
requirements. The Ser/Des employ internal termination pro-
viding a clean signaling environment. The interconnect for
FPD-Link III interface should present a differential impedance
of 100 Ohms. Use of cables and connectors that have
matched differential impedance will minimize impedance dis-
continuities. Shielded or un-shielded cables may be used
depending upon the noise environment and application re-
quirements. The chipset's optimum cable drive performance
is achieved at 43 MHz at 10 meters length. The maximum
signaling rate increases as the cable length decreases.
Therefore, the chipset supports 50 MHz at shorter distances.
Other cable parameters that may limit the cable's perfor-
mance boundaries are: cable attenuation, near-end crosstalk
and pair-to-pair skew. The maximum length of cable that can
be used is dependant on the quality of the cable (gauge,
impedance), connector, board (discontinuities, power plane),
the electrical environment (e.g. power stability, ground noise,
input clock jitter, PCLK frequency, etc.) and the application
environment.
The resulting signal quality at the receiving end of the trans-
mission media may be assessed by monitoring the differential
eye opening of the CMLOUT P/N output. A differential probe
should be used to measure across the termination resistor at
the CMLOUT P/N pins.
For obtaining optimal performance, we recommend:
• Use Shielded Twisted Pair (STP) cable
• 100Ω differential impedance and 24 AWG (or lower AWG)
cable
• Low skew, impedance matched
• Ground and/or terminate unused conductors
Figure 39 shows the Typical Performance Characteristics
demonstrating various lengths and data rates using Rosen-
berger HSD and Leoni DACAR 538 Cable.
*Note: Equalization is enabled for cable lengths greater than 7 meters
30113557
FIGURE 39. Rosenberger HSD & Leoni DACAR 538 Cable Performance
PCB LAYOUT AND POWER SYSTEM CONSIDERATIONS
Circuit board layout and stack-up for the Ser/Des devices
should be designed to provide low-noise power feed to the
device. Good layout practice will also separate high frequency
or high-level inputs and outputs to minimize unwanted stray
noise pickup, feedback and interference. Power system per-
formance may be greatly improved by using thin dielectrics (2
to 4 mils) for power / ground sandwiches. This arrangement
provides plane capacitance for the PCB power system with
low-inductance parasitics, which has proven especially effec-
tive at high frequencies, and makes the value and placement
of external bypass capacitors less critical. External bypass
capacitors should include both RF ceramic and tantalum elec-
trolytic types. RF capacitors may use values in the range of
0.01 uF to 0.1 uF. Tantalum capacitors may be in the 2.2 uF
to 10 uF range. Voltage rating of the tantalum capacitors
should be at least 5X the power supply voltage being used.
Surface mount capacitors are recommended due to their
smaller parasitics. When using multiple capacitors per supply
pin, locate the smaller value closer to the pin. A large bulk
capacitor is recommend at the point of power entry. This is
typically in the 50uF to 100uF range and will smooth low fre-
quency switching noise. It is recommended to connect power
and ground pins directly to the power and ground planes with
bypass capacitors connected to the plane with via on both
ends of the capacitor. Connecting power or ground pins to an
external bypass capacitor will increase the inductance of the
path.
A small body size X7R chip capacitor, such as 0603, is rec-
ommended for external bypass. Its small body size reduces
the parasitic inductance of the capacitor. The user must pay
attention to the resonance frequency of these external bypass
capacitors, usually in the range of 20-30 MHz. To provide ef-
fective bypassing, multiple capacitors are often used to
achieve low impedance between the supply rails over the fre-
quency of interest. At high frequency, it is also a common
practice to use two vias from power and ground pins to the
planes, reducing the impedance at high frequency.
Some devices provide separate power for different portions
of the circuit. This is done to isolate switching noise effects
between different sections of the circuit. Separate planes on
the PCB are typically not required. Pin Description tables typ-
ically provide guidance on which circuit blocks are connected
to which power pin pairs. In some cases, an external filter
many be used to provide clean power to sensitive circuits
such as PLLs.
Use at least a four layer board with a power and ground plane.
Locate LVCMOS signals away from the differential lines to
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