C-2
Circuit Board Layout Issues
C.1 Signal Separation
Avoid crosstalk problems by providing a good separation between LVD
and TTL/CMOS signals. Crosstalk is proportional to dv/dt. TTL/CMOS
signals have larger voltage swings than LVD and can effect them if lines
are running in close proximity. The best means of separation is to provide
a ground trace between the two types of signals. Another means of
keeping the two kinds of signals apart is to place them on separate
layers. If LVD and TTL/CMOS signals need to be on the same layer, they
should be separated by as much distance as possible.
C.2 Routing Signal Lines
Routing of differential lines is an important factor in maintaining signal
integrity. Differentially paired traces must be kept equidistant. Each line
should be kept as parallel as possible to its counterpart. To avoid skew
issues, the two lines should be exactly the same in length. Abiding by
these rules ensures that the rejection of common mode noise, inherent
to differentially paired signals, remains intact. Another consideration in
laying out these traces is to avoid sharp orthogonal turns. This type of
turn needs to be angled to avoid sharp changes in impedance.
C.3 Impedance Matching
Trace impedance should match the impedance of the media as close as
possible to avoid signal reections. A typical differential impedance for
the cable is about 120
. The impedance of a trace on the PCB is
controlled by its height and width, as well as the thickness of the
dielectric. The impedance of a trace pair is controlled by the distance
between the two traces.
C.4 Termination and Stub Length
The impedance of the terminator should match that of the cable.
Terminators need to be placed at the far ends of the cable and as close
to the receiver inputs as possible. Stub lengths of any device placed