LMH7324_14 Datasheet, PDF (16/30 Pages) Texas Instruments – Quad 700 ps High Speed Comparator with RSPECL Outputs

LMH7324
SNOSAZ2F – SEPTEMBER 2007 – REVISED JULY 2010
www.ti.com
response of the comparator can make a noticeable change in critical parameters such as time frame or duty
cycle. 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 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 described in the definition section. Two delay parameters can be
distinguished, t_PDH and t_PDL as shown in Figure 21. Both parameters do not necessarily have 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_PD. This parameter is defined as the absolute value of the difference
between t_PDH and t_PDL.
PW
80%
V_IN 50%
20%
t_PDH
80%
50%
20%
t_PDL
Output Q
80%
50%
20%
80%
50%
20%
t_r
t_f
Figure 21. Propagation Delay
If Δt_PD is not zero, duty cycle distortion will occur. For example 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_PDH and t_PDL are different, the duty cycle of the 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 parameters
describe the delay from input to each of the outputs and the difference between both delay times. (See
Figure 22.) 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_PDH for the Q output and t_PDL for the Q output, while the
difference between both signals is defined as Δt_PDLH. Similar definitions for the falling slope of the input signal
can be seen in Figure 15.
time
t_PDH
time
't_PDLH
time
t_PDL
V_REF
V_O
V_O
Figure 22. t_PD with Complementary Outputs
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