LMH7322SQ National Semiconductor Corporation, LMH7322SQ Datasheet - Page 19

• Manufacturer Part Number: LMH7322SQ
• Description: The ADC14V155 is a high-performance CMOS analog-to-digital converter with LVDS outputs. It is capable of converting analog input signals into 14-Bit digital words at rates up to 155 Mega Samples Per Second (MSPS). Data leaves the chip in a DDR (Dual
• Manufacturer: National Semiconductor Corporation

In this case the current I
through the termination resistor. This means that the voltage
drops at the input and the rise and fall times are dramatically
different from the specified numbers for this part.
Another parasitic capacity that can affect the output signal is
the capacity directly between both outputs, called C
Figure 18). The LMH7322 has two complementary outputs so
there is the possibility to transport the output signal by a sym-
metrical transmission line. In this case both output tracks form
a coupled line with their own parasitics and both receiver in-
puts connected to the transmission line. Actually the line
termination looks like 100Ω and the input capacities, which
are in series, are parallel to the 100Ω termination. The best
way to measure the input signal is to use a differential probe
directly across both inputs. Such a probe is very suitable for
measuring these fast signals because it has good high fre-
quency characteristics and low parasitic capacitance.
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 Re-
duced 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 sig-
nals 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.
FIGURE 18. Parasitic Capacities
P
has the same value as the current
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PAR
(see
19
Maximum Bit Rates
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 spec-
ified at 20% and 80% of the signal amplitude (60% difference).
The maximum toggle rate is defined at an amplitude of 50%
of the nominal output signal. In order to know what the max-
imum toggle rate is, it is required to recalculate the rise and
fall times to 50% swing instead of 60%.
Using a rise and fall time of 160 ps the calculated maximum
bit rate is as follows:
f = 1/ (2*((50/60) * 160e-12))
f = 3.75 GHz
which means a maximum bit rate of 7.5 Gb. During one fre-
quency period, 2 bits of NRZ (Non Return to Zero) format can
be transmitted (see Figure 19).
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 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 intro-
duced. 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
20).
FIGURE 20. Cable Types
FIGURE 19. Bit Rates
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