LMH7220_14 Datasheet, PDF (24/34 Pages) Texas Instruments – LMH7220 High Speed Comparator with LVDS Output

LMH7220
SNOSAL3E – SEPTEMBER 2006 – REVISED MAY 2013
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Maximum Bitrates
A very important specification in high speed circuits are the rise and fall times. In fact these determine the
maximum toggle 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.
V_OUT
period period
1
2
80%
20%
Decision Level
1
bit
0
Ideal Pulse Out
0 101010
Figure 33. Bitrate
Need for Terminated Transmission Lines
During the ‘80’s and ‘90’s TI 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 specification 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 techniques 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 cable
(Figure 34).
D
2h
d
d
Coax Cable
Parallel Wire
Figure 34. 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 35).
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