LMH7324_14 Datasheet, PDF (21/30 Pages) Texas Instruments – Quad 700 ps High Speed Comparator with RSPECL Outputs

LMH7324
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SNOSAZ2F – SEPTEMBER 2007 – REVISED JULY 2010
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Both outputs are activated when current is flowing through a resistor that is externally connected to V_T. The
termination voltage should be set 2V below the V_CCO. This makes it possible to terminate each of the outputs
directly with 50Ω, and if needed to connect through a transmission line with the same impedance. (See
Figure 30.) Due to the low ohmic nature of the output emitter followers and the 50Ω load resistor, a capacitive
load of several pF does not dramatically affect the speed and shape of the signal. When transmitting the signal
from one output to any input the termination resistor should match the transmission line. The capacitive load (C_P)
will distort the received signal. When measuring this input with a probe, a certain amount of capacitance from the
probe is parallel to the termination resistor. The total capacitance can be as large as 10 pF. In this case there is
a pole at:
f = 1/(2*π*C*R)
f = 1e9/ π
f = 318 MHz
For this frequency the current I_P has the same value as the current 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_PAR. (See Figure 30.) The LMH7324 has two complementary outputs so there is the possibility that the output
signal will be transported by a symmetrical transmission line. In this case both output tracks form a coupled line
with their own parasitics and both receiver inputs are 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 frequency characteristics and low
parasitic capacitance.
I_P
C_P
V_CCI
V_CCO
R_T
IN+
+
Q
-
Q
IN-
C_PAR
V_EE
R_T
C_P
I_P
V_T
Figure 30. Parasitic Capacities
TRANSMISSION LINES & TERMINATION TECHNOLOGIES
The LMH7324 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
gives advantages concerning speed and supply. 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 LMH7324 make it possible to use symmetrical transmission lines. The advantage
over single ended signal transmission is that the LMH7324 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.
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