Identifying Parasitic Inductance and AC Step Responses

Side-effects of parasitic impedances can cause signal glitches Click to subscribe: http://bit.ly/Scopes_Sub Scope tips eBook ► http://bit.ly/2MG_eBook ◄

Parasitic elements - parasitic inductance and parasitic capacitance - can cause some confusing glitches in your signals.

What is a parasitic element? A parasitic element is a naturally occurring capacitance, inductance, or resistance that appears in non-ideal circuit elements.

In this episode, we look at the simple capacitive circuit from the last 2-Minute Guru video: https://youtu.be/m_P1rvhEeiI

The circuit we are looking at is simply a capacitor connected to a square wave from the oscilloscope's function generator (WaveGen).

A commenter noticed that there was a weird signal spike at the signal edge transitions. After doing some debugging, we were able to identify that the spike was a result of parasitic inductance in a jumper wire.

To fully understand what's happening, we first have to understand how a capacitor responds to an step function and how an inductor responds to a step function. This understanding is essential to basic circuit theory and basic electronics. ( 1:14 )

A capacitor responds to a step function as a short circuit. At the instant the step function encounters capacitance, it can be modeled as a short circuit. As the capacitor begins to charge up, it transitions from a short circuit to an open circuit. After an effectively long time (infinitely long), the capacitor will completely charge and act as an open circuit. The rate at which a capacitor charges and discharges depends on the size of the capacitor (in Farads). In a simple circuit like the one in this video, the square wave acts as a step impulse, and the capacitor will charge or discharge to match the source voltage.

An inductor responds to a step function as an open circuit. At the instant the step function encounters inductance, it can be modeled as an open circuit. As the inductor begins to charge up, it transitions from an open circuit to a short circuit. After an effectively long time, the inductor will completely charge and act as a short circuit. The rate at which an inductor charges and discharges depends on the size of the inductor (in Henries). In a simple inductive circuit, the square wave acts as the step impulse, and the inductor will charge or discharge to become a short circuit.

( 3:29 ) We then model the effective circuit, which is more than just a capacitor. The breadboard and the jumper wires have significant parasitic inductance. The inductance of the jumper connecting the waveform generator's square wave and the capacitor is the main source of the glitch.

( 4:05 ) When we evaluate how the circuit will respond to a rising edge from the waveform generator

( 5:02 ) Then, we plot what the ide ... https://www.youtube.com/watch?v=heufatGyL1s