why we need hysterisis characteristics for input/RX

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chang830

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Hi,
we know there is always hysterisis characetristics for input/RX. Someone told me there are two resions for the necessarity for this hysterisis.
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Primary reason for hysterisis is to avoid erros due to issues such as signal bounce/chatter as you mentioned.

It is also used to clean up low speed or slow slewing signal curves so as to avoid nasty oscillations when the input signal is near the trigger voltage/threshold.
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The first reson is easy to understand for me. But I have some puzzles on the second, would anyone pls. help me on it?

thanks
 

They are really all the same reason: to reduce the effect of noise.

When a signal changes state rapidly, it is only near the threshold for a very short period of time and thus not so sensitive to small noise levels.

When a signal slowly slews past the threshold, it remains near the threshold for a much longer time. During this time any small noise would trigger it back and forth. So it is much more susceptible to noise.

The worst case is with a dc signal that is exactly at the threshold. You can easily understand what even a very small noise signal would do in this case.
 

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    chang830

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agree, the reason is mainly noise.

think about his situation:
input slew rate is very slow, input voltage just reach the threshold, output switches, large current occurs at gnd, device internal gnd goes up due to the ground bond wire resistance, then input voltage is lower than threshold now! then output switches back! ....................

OSCILLATION!!!!!!!!
 

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    chang830

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To make output stable instead of the input noise signal !
 

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