# Glitch vs runt pulse due to hysteresis

This is a conceptual question. What I understand is that runt pulse is like a glitch but its amplitude is less than the main pulse's amplitude. If Im not wrong on this definition, below illustartes the difference:

Since I observe all of these kind of outputs time to time; can a very low hysteresis be the source of the problem by cosnidering the shapes of the pulse outputs? I can understand a Schmitt trigger with very low hysteresis can cause a glitch but I dont get if it can cause a runt pulse or other type of noisy outputs above. Is that possible? Or runt pulse's source should be searched some where else?

• Can you share a circuit that is generating these pulses?
– Seth
Nov 14, 2016 at 18:42
• In my book a runt is usually one that should be there with proper amplitude. Since both terms don't have a rock solid industry standard definition, I am not sure if it makes sense to bother about the semantics in context of this question, but rather give detail about what you do there. Without knowing what you do, we can only guess how that unknown thing misbehaves Nov 14, 2016 at 18:46
• @PlasmaHH Maybe I couldn't express it enough. My question is actually very simple. Lets say you have a Schmit trigger and at the output along with the expected pulses you also see runt pulses(or call it glitches which have "lesser" amplitudes). I wonder if a Schmitt trigger would output such lower amplitude pulses due to very low hysteresis? Nov 14, 2016 at 18:51
• Bus turn-around can cause temporary bus contention, which will produce runts. Depending on quality of design of bus drivers, this is either a "don't care" condition, or should not happen at all. Nov 15, 2016 at 0:40
• As I see, more info about the issue is here, electronics.stackexchange.com/questions/266347/… The case needs detailed analysis of your schematics and sensors, and environment. Nov 15, 2016 at 0:48

can a very low hysteresis be the source of the problem by considering the shapes of the pulse outputs?

A runt pulse is due to less than sufficient full swing.

It can be caused by;

• insufficient gain or input level of an isolated input pulse ( for whatever reason)
• or insufficient time due to;

• slew rate dV/dt (or)
• current limit (or)
• pulse width , dt
• because dV = Ic/C * dt for any current limit charging an input capacitance

It is often caused by race or metastable conditions when the threshold of one device is Vss/2 while feeding a Schmitt trigger gate with a threshold of Vss/3 (typ @25'C) then the runt pulse becomes a missing pulse. Another case is a phase detector with a small skew value almost in sync.

This is not due to excessive Hysteresis or insufficient hysteresis, rather due to mismatched design requirements for such purposes as PWM control or phase detection or a race condition and thus a design challenge to recognize and avoid.

• Thanks for the reply. I have been struggling with an issue for a long time. This question was a part of that to eliminate some possibilities. Actually the system and the issues I'm experiencing are better explained in my previous question without any luck: electronics.stackexchange.com/questions/266347/… Nov 20, 2016 at 3:51

I think you have the definitions slightly wrong.

A glitch is a very short pulse.
If it's at the start or end of a pulse the cause is normally a combination of a noisy input transition (either due to noise or poor termination) and a lack of hysteresis.
It it's in the middle of a pulse then the cause is normally either a lot of noise on the input, the transition threshold being poorly set or poorly designed asynchronous logic.

A glitch is typically driven to one rail or the other however due to the length of the pulse and the edge rate of the signal it may not get all the way there before starting to return to the correct state.

A runt on the other hand is a signal which has reached a stable state at an invalid level, it can last for a full clock period or more. Runt pulses are normally caused by a conflict, two different sources trying to drive the same wire to different states. They can also be caused by insufficient drive capability on an output.

• Thanks for your answer. However in my case there is no digital circuit(besides some inner circuitry of the daq board). The pulses in fact are created by an "analog" Schmitt trigger. These pulses goes to a data acquisition board. And I sometimes experience the types of unwanted outputs as shown in the figures in my question. So there is no digital circuit, or state ect., how is that possible I do have runt pulses? Can we say the runt pulse cannot come from analog circuit? Nov 15, 2016 at 10:38
• A runt pulse is any pulse that is not reaching the voltage that it should get to. In the digital world that is easy to define since everything should be one of two voltages, anything else is invalid. There is no reason why you can't have them from an analog circuit but since in an analog circuit any voltage is valid telling the difference between a runt and a valid level is harder. If this is supposed to be a digital output from an analog circuit and it's reaching a stable invalid level then to me that implies you need more gain on the output stage. Nov 15, 2016 at 10:45
• what do you think abut these weird outputs?: i.stack.imgur.com/CXn1L.png i.stack.imgur.com/mDtga.png it seems to me i cannot be the low hysteresis here. and happens verrandomly Nov 15, 2016 at 10:52
• I think that they are weird. As others have already said, it's impossible to help find the cause without knowing the circuit. Nov 15, 2016 at 11:23
• @Andrew, I don't believe this issue is about definitions. The issue is, why a heavily-clipped signal (nearly digital) gets acquired with a fairly slow analog data acquisition system. The OP needs to understand what kind of "rotating instrument signal" he is dealing with, and what properties of this signal are meant to be collected/analysed. Nov 17, 2016 at 6:18