LMH7324_14 Datasheet, PDF (22/30 Pages) Texas Instruments – Quad 700 ps High Speed Comparator with RSPECL Outputs

LMH7324
SNOSAZ2F – SEPTEMBER 2007 – REVISED JULY 2010
www.ti.com
Maximum Bit Rates
The maximum toggle rate is defined at an amplitude of 50% of the nominal output signal. This toggle rate is a
number for the maximum transfer rate of the part and can be given in Hz or in Bps. When transmitting signals in
a NRZ (Non Return to Zero) format the bitrate is double this frequency number, because during one period two
bits can be transmitted. (See Figure 31.) The rise and fall times are very important specifications in high speed
circuits. In fact these times determine the maximum toggle rate of the part. Rise and fall times are normally
specified at 20% and 80% of the signal amplitude (60% difference). Assuming that the edges at 50% amplitude
are coming up and down like a sawtooth it is possible to calculate the maximum toggle rate but this number is
too optimistic. In practice the edges are not linear while the pulse shape is more or less a sinewave.
period period
1
2
V_OUT
80%
20%
Decision Level
1
bit
0
Ideal Pulse Out
0 101010
Figure 31. Bit Rates
Need for Terminated Transmission Lines
During the 1980’s and 90’s, National fabricated the 100K ECL logic family. The rise and fall time specifications
were 0.75 ns, which were considered very fast. If sufficient care has not been given in designing the transmission
lines and choosing the correct terminations, then errors in digital circuits are introduced. To be helpful to
designers that use ECL with “old” PCB-techniques, the 10K ECL family was introduced with rise and fall time
specifications of 2 ns. This is much slower and easier to use. The RSPECL output signals of the LMH7324 have
transition times that extend 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. (See Figure 32.)
D
2h
d
d
Coax Cable
Parallel Wire
Figure 32. Cable Types
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 microstrip line. 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 33.)
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