SN75LVDS83B_17 Datasheet, PDF (28/43 Pages) Texas Instruments – FlatLink Transmitter

SN75LVDS83B
SLLS846C – MAY 2009 – REVISED AUGUST 2014
11 Power Supply Recommendations
Power supply PLL, IO, and LVDS pins must be uncoupled from each.
www.ti.com
12 Layout
12.1 Layout Guidelines
12.1.1 Board Stackup
There is no fundamental information about how many layers should be used and how the board stackup should
look. Again, the easiest way the get good results is to use the design from the EVMs of Texas Instruments. The
magazine Elektronik Praxis has published an article with an analysis of different board stackups. These are listed
in Table 3. Generally, the use of microstrip traces needs at least two layers, whereas one of them must be a
GND plane. Better is the use of a four-layer PCB, with a GND and a VCC plane and two signal layers. If the
circuit is complex and signals must be routed as stripline, because of propagation delay and/or characteristic
impedance, a six-layer stackup should be used.
Layer 1
Layer 2
Layer 3
Layer 4
Decoupling
EMC
Signal Integrity
Self Disturbance
Table 3. Possible Board Stackup on a Four-Layer PCB
MODEL 1
SIG
SIG
VCC
GND
Good
Bad
Bad
Satisfaction
MODEL 2
SIG
GND
VCC
SIG
Good
Bad
Bad
Satisfaction
MODEL 3
SIG
GND
SIG
VCC
Bad
Bad
Good
Satisfaction
MODEL 4
GND
SIG
VCC
SIG
Bad
Bad
Bad
High
12.1.2 Power and Ground Planes
A complete ground plane in high-speed design is essential. Additionally, a complete power plane is
recommended as well. In a complex system, several regulated voltages can be present. The best solution is for
every voltage to have its own layer and its own ground plane. But this would result in a huge number of layers
just for ground and supply voltages. What are the alternatives? Split the ground planes and the power planes? In
a mixed-signal design, e.g., using data converters, the manufacturer often recommends splitting the analog
ground and the digital ground to avoid noise coupling between the digital part and the sensitive analog part. Take
care when using split ground planes because:
• Split ground planes act as slot antennas and radiate.
• A routed trace over a gap creates large loop areas, because the return current cannot flow beside the signal,
and the signal can induce noise into the nonrelated reference plane (Figure 22).
• With a proper signal routing, crosstalk also can arise in the return current path due to discontinuities in the
ground plane. Always take care of the return current (Figure 23).
For Figure 23, do not route a signal referenced to digital ground over analog ground and vice versa. The return
current cannot take the direct way along the signal trace and so a loop area occurs. Furthermore, the signal
induces noise, due to crosstalk (dotted red line) into the analog ground plane.
28
Submit Documentation Feedback
Copyright © 2009–2014, Texas Instruments Incorporated
Product Folder Links: SN75LVDS83B