LMH7324_0710 Datasheet, PDF (14/20 Pages) National Semiconductor (TI) – Quad 700 ps High Speed Comparator with RSPECL Outputs

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 pa-
rameters describe the delay from input to each of the outputs
and the difference between both delay times. (See Figure
10.) 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 sig-
nals is defined as Δt_PDLH. Similar definitions for the falling
slope of the input signal can be seen in Figure 3.
FIGURE 11. Overdrive Dispersion
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The overdrive dispersion is caused by the switching currents
in the input stage which are dependent on the level of the
differential 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. (See Figure 12.)
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FIGURE 10. t_PD with Complementary Outputs
Both output circuits should be symmetrical. At the moment
one output is switching ‘on’ the other is switching ‘off’ with
ideally no skew between both outputs. The design of the
LMH7324 is optimized so that this timing difference is mini-
mized. The propagation delay, t_PD, is defined as the average
delay of both outputs at both slopes: (t_PDLH + t_PDHL)/2. Both
overdrive and starting point should be equally divided around
the V_REF (absolute values).
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 11 shows the dispersion
due to a variation of the input overdrive voltage. The overdrive
is defined as the ‘go to’ differential voltage applied to the in-
puts. Figure 11 shows the impact it has on the propagation
delay time if the overdrive is varied from 10 mV to 100 mV.
This parameter is measured with a constant slew rate of the
input signal.
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FIGURE 12. Slew Rate Dispersion
A combination of overdrive and slew rate dispersion occurs
when applying signals with different amplitudes at constant
frequency. A small amplitude will produce a small voltage
change per time unit (dV/dt) but also a small maximum switch-
ing current (overdrive) in the input transistors. High ampli-
tudes produce a high dV/dt and a bigger overdrive.
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