LMH7220_14 Datasheet, PDF (19/34 Pages) Texas Instruments – LMH7220 High Speed Comparator with LVDS Output

LMH7220
www.ti.com
SNOSAL3E – SEPTEMBER 2006 – REVISED MAY 2013
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 switching current (overdrive) in the input transistors. High amplitudes 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 26). When V_REF is
swept through the CMVR (Common Mode Voltage Range), this results in a variation of the propagation delay
time. This variation is called Common Mode Dispersion.
Vin cm
+
0 ^{Vin cm}
time
-
Common Mode Dispersion
+
Dispersion
0
time
-
Figure 26. 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 switching effects.
In the next sections an explanation follows about these phenomena in situations where no hysteresis is applied,
and the possible improvement hysteresis can give.
Using No Hysteresis
In Figure 27 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 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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