LMH7220_0805 Datasheet, PDF (14/20 Pages) National Semiconductor (TI) – High Speed Comparator with LVDS Output

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FIGURE 7. Slew Rate Dispersion
A combination of overdrive- and slew rate dispersion occurs
when applying signals with different amplitude 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.
Common Mode Dispersion
Dispersion will also occur when changing the common mode
level of the input signal (Figure 8). When V_REF is swept
through the CMVR (Common Mode Voltage Range), this re-
sults in a variation of the propagation delay time. This varia-
tion is called Common Mode Dispersion.
All of the dispersion effects discussed before influence the
propagation delay. In practice the dispersion is often caused
by a combination of more than one varied parameter. It is
good to realize this if there is the need to predict how much
dispersion a circuit will show.
HYSTERESIS & OSCILLATIONS
In contrast to an op amp, the output of a comparator has only
two defined states ‘0’ or ‘1’. Due to finite comparator gain
however, there will be a small band of input differential voltage
where the output is in an undefined state. An input signal with
fast slopes will pass this band very quickly without problems.
During slow slopes however, passing the band of uncertainty
can be relatively long. This enables the comparator outputs
to switch back and forth several times between ‘0’ and ‘1’ on
a single slope. The comparator will switch on its input noise,
ground bounce (possible oscillations), ringing etc. Noise in
the input signal will also contribute to these undesired switch-
ing effects.
In the next sections an explanation follows about these phe-
nomena in situations where no hysteresis is applied, and the
possible improvement hysteresis can give.
Using No Hysteresis
In Figure 9 can be seen what happens when the input signal
rises from just under the threshold V_REF to a level just above
it. From the moment the input reaches the lowest dotted line
around V_REF at t=0, the output toggles on noise etc. Toggling
ends when the input signal leaves the undefined area at t=1.
In this example the output was fast enough to toggle three
times. Due to this behavior digital circuitry connected to the
output will count a wrong number of pulses. One way to pre-
vent this is to choose a very slow comparator with an output
that is not able to switch more than once between ‘0’ and ‘1’
during the time the input state is undefined.
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FIGURE 8. Common Mode Dispersion
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FIGURE 9. Oscillations & Noise
In most circumstances this is not an option because the slew
rate of the input signal will vary.
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