LMH7220_0805 Datasheet, PDF (12/20 Pages) National Semiconductor (TI) – High Speed Comparator with LVDS Output

Symbol
t_PDHd resp t_PDLd
Text
Δt_PDLH resp Δt_PDHL Q to Q time skew
Δt_PD
Δt_PDd
t_OD-disp
t_SR-disp
t_CM-disp
t_r / t_rd
t_f / t_fd
Average Q to Q time skew
Average diff. time skew
Input overdrive dispersion
Input slew rate dispersion
Input Common Mode dispersion
Output rise time (20% - 80%)
Output fall time (20% - 80%)
Description
Delay time between the moment the input signal crosses the switching
level L to H and the zero crossing of the rising edge of the differential
output signal (t_PDHd), or delay time between the moment the input
signal crosses the switching level H to L and the zero crossing of the
falling edge of the differential output signal (t_PDLd)
Time skew between 50% levels of rising edge of Q output and falling
edge of Q output (Δt_PDLH ), or time skew between 50% levels of falling
edge of Q output and rising edge of Q output (Δt_PDHL)
Average of t_PDLH and t_PDHL for L to H and H to L transients
Average of t_PDHd and t_PDLd for L to H and H to L transients
Change in t_PD for different overdrive voltages at the input pins
Change in t_PD for different slew rates at the input pins
Change in t_PD for different common mode voltages at the input pins
Time needed for the (single ended or differential) output voltage to
change from 20% of its nominal value to 80%
Time needed for the (single ended or differential) output voltage to
change from 80% of its nominal value to 20%
FIGURE 3. Propagation Delay Definition
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DELAY AND DISPERSION
Comparators are widely used to connect the analog world to
the digital one. The accuracy of a comparator is dictated by
its DC properties such as offset voltage and hysteresis and
by its timing aspects such as rise and fall times and delay. For
low frequency applications most comparators are much faster
than the analog input signals they handle. The timing aspects
are less important here than the accuracy of the input switch-
ing levels. The higher the frequency, the more important the
timing properties of the comparator become, because the re-
sponse of the comparator can give e.g. a noticeable change
in time frame or duty cycle. A designer has to know these
effects in order to deal with them. In order to predict what the
output signal will do compared to the input signal, several pa-
rameters 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 defined as the time it
takes for the comparator to change the output level halfway
in its transition from L to H or H to L, in reaction to the moment
the input signal crosses the switching level. Due to this defi-
nition there are two parameters, t_PDH and t_PDL (Figure 4). Both
parameters don’t necessarily have the same value. It is pos-
sible that differences will occur due to a different response of
the internal circuitry. As a result of this effect another param-
eter is defined: Δt_PD. This parameter is defined as the abso-
lute value of the difference between t_PDH and t_PDL.
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