LMH7322_14 Datasheet, PDF (25/33 Pages) Texas Instruments – Dual 700 ps High Speed Comparator with RSPECL Outputs

LMH7322
www.ti.com
SNOSAU8H – MARCH 2007 – REVISED MAY 2011
Transmission Lines & Termination Technologies
The LMH7322 uses complementary RSPECL outputs and emitter followers, which means high output current
capability and low sensitivity to parasitic capacitance. The use of Reduced Swing Positive Emitter Coupled Logic
reduces the supply voltage to 2.7V, being the lowest possible value, and raises the maximum frequency
response. Data rates are growing, which requires increasing speed. Data is not only connected to other IC’s on a
single PCB board but, in many cases, there are interconnections from board to board or from equipment to
equipment. Distances can be short or long but it is always necessary to have a reliable connection, which
consumes low power and is able to handle high data rates. The complementary outputs of the LMH7322 make it
possible to use symmetrical transmission lines The advantage over single ended signal transmission is that the
LMH7322 has higher immunity to common mode noise. Common mode signals are signals that are equally
apparent on both lines and because the receiver only looks at the difference between both lines, this noise is
canceled.
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 36.) 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 1 01 01 0
Figure 36. Bit Rates
Need for Terminated Transmission Lines
During the 1980’s and 90’s, TI fabricated the 100K ECL logic family. The rise and fall time specifications were
0.75 ns, which are 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 a rise and fall time specification
of 2 ns. This was much slower and easier to use. The RSPECL output signals of the LMH7322 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 (Figure 37).
D
2h
d
d
Coax Cable
Parallel Wire
Figure 37. Cable Types
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