SN74LVC1G125: Vol is negative during H to L signal transition

Hi Jason,

It's very common to have some undershoot at the output of a logic gate. It's most common with very light loads, but can occur under other circumstances as well.

What's the load that you have connected to the device?

Hello Emrys,

I don't know the specific system load, but could we add dummy load to solve the issue? If ok, what dummy load value is suggested?

in addition, may I learn what the reason you mentioned the undershoot is common at light load on the output of logic gate?

~Jason

Hi Jason,

I need to have at least some idea of the load for your system to help -- do you not know what you are connecting your device to? Are there wires involved? Coaxial cable? PCBs? Connectors? Probes? What's the receiving device? Any information would be helpful.

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The reason for ringing from a light load lies in second order effects of linear systems. Every logic output is a second order system, with some inherent resistance, inductance, and capacitance. Here's an example showing how different loading can affect ringing in second order systems:

If you look at the rising edge of the above image, you can see that the slower ramp rates result in smoother outputs without any ringing (overdamped and critically damped cases).

With a typical load (50pF), the output of a logic gate will usually appear very smooth. When you reduce that capacitance, it will speed up the transition time at the output due to the reduced load, and thus will start to increase ringing. With very light loads, for example 5pF, we expect to see some overshoot (and undershoot).

It's also possible to introduce ringing through poor measurement techniques -- ie there may be no ringing at all in the system, until you hook up a scope to see what it's doing.

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Please note that this discussion is specific to capacitive loads connected directly to the output of the device -- adding any sort of transmission line complicates things and adds other factors -- thus the reason I asked for details regarding your loading.