LMH7324_14 Datasheet, PDF (19/30 Pages) Texas Instruments – Quad 700 ps High Speed Comparator with RSPECL Outputs

LMH7324
www.ti.com
SNOSAZ2F – SEPTEMBER 2007 – REVISED JULY 2010
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 26 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.
mV
V_REF
time
1
time
0
1
time
0
t=0
t=1
Figure 26. 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 27.
mV
V_REF
A
B
1
0
t=0
t=1
Figure 27. Hysteresis
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