LMH7220_0805 Datasheet, PDF (17/20 Pages) National Semiconductor (TI) – High Speed Comparator with LVDS Output

Maximum Bitrates
A very important specification in high speed circuits are the
rise and fall times. In fact these determine the maximum tog-
gle rate (TR) of the part. The LVDS standard specifies them
at 0.26 ns to 1.5 ns. Rise and fall times are normally specified
at 20% and 80% of the signal amplitude (60% difference). TR
is defined as the bitrate at which the differential output voltage
drops to 50% of its nominal value.
FIGURE 15. Bitrate
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Need for Terminated Transmission Lines
During the ‘80’s and ‘90’s National fabricated the 100k ECL
logic family. The rise and fall time specification was 0.75 ns
which was very fast and will easily introduce errors in digital
circuits if insufficient care has been taken to the transmission
lines and terminations used for these signals. To be helpful to
designers that use ECL with “old” PCB-techniques, the 10k
ECL family was introduced with a rise and fall time specifica-
tion of 2 ns. This was much slower and more easy to use.
LVDS signals have transition times that exceed the fastest
ECL family. A careful PCB design is needed using RF tech-
niques for transmission and termination. Transmission lines
can be formed in several ways. The most commonly used
types are the coaxial cable and the twisted pair telephony ca-
ble (Figure 16).
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FIGURE 16. Cable Configuration
These cables have a characteristic impedance determined by
their geometric parameters. Widely used impedances for the
coaxial cable are 50Ω and 75Ω. Twisted pair cables have
impedances of about 120Ω to 150Ω.
Other types of transmission lines are the strip line and the
micro strip. These last types are used on PCB boards. They
have the characteristic impedance dictated by the physical
dimensions of a track placed over a metal ground plane (See
Figure 17).
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FIGURE 17. PCB Transmission Lines
Differential Microstrip Line
The transmission line which is ideally suited for LVDS signals
is the differential micro strip line. This is a double micro strip
line with a narrow space in between. This means both lines
have a strong coupling and this determines mainly the char-
acteristic impedance. The fact that they are routed above a
copper plane doesn't affect differential impedance, only CM-
capacitance is added. Each of the structures above has its
own geometric parameters so for each structure there is an-
other formula to calculate the right impedance. For calcula-
tions of these transmission lines visit the National website or
feel free to order the RAPIDESIGNER. For some formula’s
given in the ‘LVDS owners manual’ see chapter 3 (see the
‘Introduction’ section for the URL). At the end of the trans-
mission line there must be a termination having the same
impedance as of the transmission line itself. It doesn’t matter
what impedance the line has, if the load has the same value
no reflections will occur. When designing a PCB board with
transmission lines on it, space becomes an important item
especially on high density boards. With a single micro strip
line, line width is fixed for given impedance and a board ma-
terial. Other line widths will result in different impedances.
Advantage of Differential Microstrip
Impedances of transmission lines are always dictated by their
geometric parameters. This is also true for differential micro
strip lines. Using this type of transmission lines, track distance
determines mainly the resulting impedance. So, if the PCB
manufacturer can produce reliable boards with narrow track
spacing the track width for a given impedance is also small.
The wider the spacing, the wider tracks are needed for a cer-
tain impedance. For example two tracks of 0.2 mm width and
0.1 mm spacing have the same impedance as two tracks of
0.8 mm width and 0.4 mm spacing. With high-end PCB pro-
cesses, it is possible to design very narrow differential mi-
crostrip transmission lines. It is desirable to use these
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