LMH7324_0710 Datasheet, PDF (15/20 Pages) National Semiconductor (TI) – Quad 700 ps High Speed Comparator with RSPECL Outputs

Common Mode Dispersion
Dispersion will also occur when changing the common mode
level of the input signal. (See Figure 13.) When V_REF is swept
through the CMVR (Common Mode Voltage Range), it results
in a variation of the propagation delay time. This variation is
called Common Mode Dispersion.
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FIGURE 13. Common Mode Dispersion
All of the dispersion effects described previously influence the
propagation delay. In practice the dispersion is often caused
by a combination of more than one varied parameter.
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 take a relatively long time. This enables the comparators
output 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
switching actions.
The next sections explain these phenomena in situations
where no hysteresis is applied, and discuss the possible im-
provement hysteresis can give.
Using No Hysteresis
Figure 14 shows 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 14. Oscillations on Output Signal
In most circumstances this is not an option because the slew
rate of the input signal will vary.
Using Hysteresis
A good way to avoid oscillations and noise during slow slopes
is the use of hysteresis. With hysteresis the switching level is
forced to a new level at the moment the input signal crosses
this level. This can be seen in Figure 15.
FIGURE 15. Hysteresis
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In this picture there are two dotted lines A and B, both indi-
cating the resulting level at which the comparator output will
switch over. Assume that for this situation the input signal is
connected to the negative input and the switching level
(V_REF) to the positive input. The LMH7324 has a built-in hys-
teresis voltage that is fixed at approximately 20 mV_PP. The
input level of Figure 15 starts much lower than the reference
level and this means that the state of the input stage is well
defined with the inverting input much lower than the non-in-
verting input. As a result the output will be in the high state.
Internally the switching level is at A, with the input signal slop-
ing up, this situation remains until V_IN crosses level A at t = 1.
Now the output toggles, and the internal switching level is
lowered to level B. So before the output has the possibility to
toggle again, the difference between the inputs is made suf-
ficient to have a stable situation again. When the input signal
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