LMH7220_0805 Datasheet, PDF (13/20 Pages) National Semiconductor (TI) – High Speed Comparator with LVDS Output

FIGURE 4. Pulse Parameter
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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 sym-
metrical 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 be-
tween both delay times (see Figure 5). When the differential
input signal crosses the reference level from L to H, both out-
puts 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 3.
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.
Amplitude Overdrive Dispersion
One of the parameters that causes dispersion is the amplitude
variation of the input signal. Figure 6 shows the dispersion
due to a variation of the input overdrive voltage. The overdrive
is defined as the ‘goto’ differential voltage applied to the in-
puts. Figure 6 shows the impact it has on the propagation
delay time if overdrive is varied from 10 millivolts to 100 mil-
livolts. This parameter is measured with a constant slew rate
of the input signal.
FIGURE 5. Propagation Delay
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FIGURE 6. Overdrive Dispersion
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The overdrive dispersion is caused by the fact that switching
currents in the input stage depend on the level of the differ-
ential input signal.
Slew Rate Dispersion
The slew rate is another parameter that affects propagation
delay. The higher the input slew rate, the faster the input stage
switches (Figure 7).
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