LMH7322_0706 NSC [National Semiconductor], LMH7322_0706 Datasheet - Page 17

• Manufacturer Part Number: LMH7322_0706
• Description: Dual 700 ps High Speed Comparator with RSPECL Outputs
• Manufacturer: NSC [National Semiconductor]

cle. A designer has to know these effects and has to deal with
them. In order to predict what the output signal will do, several
parameters are defined which describe the behavior of the
comparator. For a good understanding of the timing parame-
ters discussed in the following section, a brief explanation is
given and several timing diagrams are shown for clarification.
PROPAGATION DELAY
The propagation delay parameter is described in the definition
section. Due to this definition there are two parameters, t
and t
the same value. It is possible that differences will occur due
to a different response of the internal circuitry. As a derivative
of this effect another parameter is defined: Δt
eter is defined as the absolute value of the difference between
t
If Δt
when applying a symmetrical waveform (e.g. a sinewave) at
the input, it is expected that the comparator will produce a
symmetrical square wave at the output with a duty cycle of
50%. When t
output signal will not remain at 50%, but will be increased or
decreased. In addition to the propagation delay parameters
for single ended outputs discussed before, there are other
parameters in the case of complementary outputs. These pa-
rameters describe the delay from input to each of the outputs
and the difference between both delay times (See Figure
11.) When the differential input signal crosses the reference
level from L to H, both outputs will switch to their new state
with some delay. This is defined as t
t
nals is defined as Δt
slope of the input signal can be seen in Figure 3.
PDH
PDL
PD
for the Q output, while the difference between both sig-
PDL
and t
is not zero, duty cycle distortion will occur. For example
(Figure 10). Both parameters do not necessarily have
PDL
.
PDH
FIGURE 10. Propagation Delay
and t
PDLH
PDL
. Similar definitions for the falling
are different, the duty cycle of the
PDH
for the Q output and
PD
. This param-
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PDH
17
Both output circuits should be symmetrical. At the moment
one output is switching ‘on’ the other is switching ‘off’ with
ideally no skew between both outputs. The design of the
LMH7322 is optimized so that this timing difference is mini-
mized. The propagation delay, t
delay of both outputs at both slopes: (t
Both overdrive and starting point should be equally divided
around the V
DISPERSION
There are several circumstances that will produce a variation
of the propagation delay time. This effect is called dispersion.
Amplitude Overdrive Dispersion
One of the parameters that causes dispersion is the amplitude
variation of the input signal. Figure 12 shows the dispersion
due to a variation of the input overdrive voltage. The overdrive
is defined as the ‘go to’ differential voltage applied to the in-
puts. Figure 12 shows the impact it has on the propagation
delay time if the overdrive is varied from 10 mV to 100 mV.
This parameter is measured with a constant slew rate of the
input signal.
FIGURE 11. t
REF
(absolute values).
PD
with Complementary Outputs
PD
, is defined as the average
PDLH
+ t
PDHL
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)/2.
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