LMH7220_14 Datasheet, PDF (17/34 Pages) Texas Instruments – LMH7220 High Speed Comparator with LVDS Output

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LMH7220
SNOSAL3E – SEPTEMBER 2006 – REVISED MAY 2013
PW
80%
V_IN 50%
20%
t_PDH
80%
50%
20%
t_PDL
Output Q
80%
50%
20%
t_r
t_f
Figure 22. Pulse Parameter
80%
50%
20%
If Δt_PD isn’t 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 produces a symmetrical square wave at the output with
a duty cycle of 50%. In case of different t_PDH and t_PDL the duty cycle of the output signal will not remain at 50%,
but will be lower or higher. In addition to the propagation delay parameters for single ended outputs discussed
before, there are other parameters in 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 23). 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 ΔtP_DLH. similar definitions for the falling slope of the input signal can be seen in Figure 21.
V_REF
time
t_PDH
V_O
time
't_PDLH
t_PDL
V_O
time
Propagation Delay
Figure 23. Propagation Delay
Both output circuits should be symmetrical. At the moment one output is switching ‘on’ the other is switching ‘off’
with ideally no skew between them. The design of the LMH7220 is optimized to minimize this timing difference.
Propagation delay t_PD is defined as the average delay of both outputs at both slopes: (t_PDLH + t_PDHL) / 2.
DISPERSION
There are several circumstances that will produce a variation of the propagation delay time. This effect is called
dispersion.
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