As discussed in
our post ‘propagation
delay’, the difference in time from the input reaching 50% of the final
value of the transition to that of the output is termed as propagation delay.
It seems a bit absurd to have negative value of propagation delay as it
provides a misinterpretation of the effect happening before the cause. Common
sense says that the output should only change after input. However, under
certain special cases, it is possible to have negative delay. In most of such cases, we have one or
more of the following conditions:
i)
A high drive strength transistor
ii)
Slow transition at the input
iii)
Small load at the output
Under all of the above mentioned conditions, the output is expected to transition faster than the input signal,
and can result in negative propagation delay. An example negative delay scenario is shown in the
figure below. The output signal starts to change only after the input signal;
however, the faster transition of the output signal causes it to attain 50%
level before input signal, thus, resulting in negative propagation delay. In other words, negative delay is a relative concept.
 |
| Figure 1: Input and output transitions showing negative input delay |
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