LMH7322_14 Datasheet, PDF (24/33 Pages) Texas Instruments – Dual 700 ps High Speed Comparator with RSPECL Outputs

LMH7322
SNOSAU8H – MARCH 2007 – REVISED MAY 2011
www.ti.com
Output Q
V_OH
V_OD
V_O
Output Q
V_OL
Figure 34. Output Swing
Loading the Output
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 35). 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)
(1)
f = 1e9/ π
(2)
f = 318 MHz
(3)
In this case 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 capacitance that can affect the output signal is the capacitance directly between both outputs,
called C_PAR (see Figure 35). The LMH7322 has two complementary outputs so there is the possibility to transport
the output signal by a symmetrical transmission line. In this case both output tracks form a coupled line with their
own parasitics and both receiver inputs connected to the transmission line. Actually the line termination looks like
100Ω and the input capacitances, 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.
VCCO
VCCI
I_P
C_P
R_T
IN+
+
Q
-
Q
IN-
V_T
C_PAR
VEE
R_T
C_P
I_P
V_T
Figure 35. Parasitic Capacitance
24
Submit Documentation Feedback
Product Folder Links: LMH7322
Copyright © 2007–2011, Texas Instruments Incorporated