Datasheet

Differential
Input Signal
Differential
Output Signal
PW
t
PDH
Output Q
10% or 20%
80% or 90%
80% or 90%
10% or 20%
't
PDd
= | t
PDHd
- t
PDLd
|
t
PDd
= (t
PDHd
+ t
PDLd
)/ 2
t
fd
t
rd
t
r
t
f
0
V
O
V
O
0
t
PDHd
t
PDLd
t
PDL
't
PDLH
't
PDHL
V
overdrive
Output
Q
t
PDL
t
PDH
t
PD
= (t
PDLH
+ t
PDHL
)/ 2
't
PDQ
= | t
PDH
- t
PDL
|
't
PD
= 't
PDLH
+ 't
PDHL
)/ 2
t
PDHL
= (t
PDL
+ t
PDH
)/ 2
t
PDLH
= (t
PDH
+ t
PDL
)/ 2
't
PDQ
= | t
PDL
- t
PDH
|
't
PDLH
= | t
PDH
- t
PDL
|
't
PDHL
= | t
PDL
- t
PDH
|
LMH7220
SNOSAL3E –SEPTEMBER 2006–REVISED MAY 2013 www.ti.com
Figure 21. Propagation Delay Definition
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 switching levels. The
higher the frequency, the more important the timing properties of the comparator become, because the response
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 parameters are defined which describe the behavior of the comparator. For a good understanding of the
timing parameters 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 definition there are two parameters, t
PDH
and t
PDL
(Figure 22). Both parameters don’t
necessarily have the same value. It is possible that differences will occur due to a different response of the
internal circuitry. As a result of this effect another parameter is defined: Δt
PD
. This parameter is defined as the
absolute value of the difference between t
PDH
and t
PDL
.
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