LMH7322_14 Datasheet, PDF (21/33 Pages) Texas Instruments – Dual 700 ps High Speed Comparator with RSPECL Outputs

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LMH7322
SNOSAU8H – MARCH 2007 – REVISED MAY 2011
Vin cm
+
Vin cm
0
time
-
Common Mode Dispersion
+
Dispersion
0
time
-
Figure 30. 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 improvement hysteresis can give.
Using No Hysteresis
Figure 31 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 prevent 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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