Quick Tech Tips: Oscilloscope Probing

Probe Tips

Recently someone contacted me for troubleshooting their boards for noise problems. In one of the pics they send, they were probing the circuit with one of those long standard alligator clip-styled probes for ground leads. Let’s discuss that.

Oscilloscope Probes

A probe’s primary job is to try to reliably transfer a signal from your circuit to the input circuitry of your oscilloscope for measurement. All the probes out there will have an input capacitance because of the compensation circuitry and the coaxial cable. You can represent this capacitor in a simplistic electrical circuit of the probe. Now the ground lead of your probe can be modeled as an inductor. When you are probing across a point in your circuit(Referenced to ground) with the long ground lead(for maybe ease of connection), what happens is that you are introducing a larger inductor in that return path. Or a fancy way of saying that is “Your loop inductance increases”. That inductance along with the probe capacitance can and will resonate when a signal with a high freq component is being measured by the probe. You will see ringing and overshoot at the edges of the signal. If you are let’s say probing a data signal(square waveform), the fundamental frequency or the data switching rate can be small, still, the ringing will be there because most circuits have a very fast-rising edge. That’s where all the high-frequency stuff lies and you will see your probe misbehaving.

So what’s the solution? Reduce the ground lead wire length to a bare minimum reducing the inductance of the leads. For that you have probes with tiny needle pins with the positive and earth leads very close to each other. This will ensure that resonance is at a much higher freq and your signal will be cleaner. You don’t have that tip in your oscilloscope kit, you can wind a small wire at the GND part as shown in the figure.

PS: Oscilloscope probes and measuring signals in itself is a fascinating topic with many nuances. If there is any interest in that, do let me know in the comments section. Might have to do a long series of posts to cover that.

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