time

Dispersion

Common Mode Dispersion

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LMH7322

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SNOSAU8H – MARCH 2007 – REVISED MAY 2011

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

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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