LMH7322_0706 NSC [National Semiconductor], LMH7322_0706 Datasheet - Page 18

• Manufacturer Part Number: LMH7322_0706
• Description: Dual 700 ps High Speed Comparator with RSPECL Outputs
• Manufacturer: NSC [National Semiconductor]

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The overdrive dispersion is caused by the switching currents
in the input stage which is dependent on the level of the dif-
ferential input signal.
Slew Rate Dispersion
The slew rate is another parameter that affects propagation
delay. The higher the input slew rate, the faster the input stage
switches (See Figure 13).
A combination of overdrive and slew rate dispersion occurs
when applying signals with different amplitudes 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.
FIGURE 12. Overdrive Dispersion
FIGURE 13. Slew Rate Dispersion
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Common Mode Dispersion
Dispersion will also occur when changing the common mode
level of the input signal (Figure 14). When V
through the CMVR (Common Mode Voltage Range), It results
in a variation of the propagation delay time. This variation is
called 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 phenom-
ena in situations where no hysteresis is applied, and discuss
the possible improvement hysteresis can give.
Using No Hysteresis
Figure 15 shows what happens when the input signal rises
from just under the threshold V
From the moment the input reaches the lowest dotted line
around V
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.
REF
FIGURE 14. Common Mode Dispersion
at t=0, the output toggles on noise etc. Toggling
REF
to a level just above it.
REF
is swept
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