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ABCs of Probes Primer
Precision Measurements Start at the Probe Tip
Probes are vital to oscilloscope measurements. To understand how vital, disconnect the probes from an oscilloscope and try to make a measurement. It can’t be done. There has to be some kind of electrical connection, a probe of some sort between the signal to be measured and the oscilloscope’s input channel.
In addition to being vital to oscilloscope measurements, probes are also critical to measurement quality. Connecting a probe to a circuit can affect the operation of the circuit, and an oscilloscope can only display and measure the signal that the probe delivers to the oscilloscope input.
Thus, it is imperative that the probe have minimum impact on the probed circuit and that it maintain adequate signal fidelity for the desired measurements.
If the probe doesn’t maintain signal fidelity, if it changes the signal in any way or changes the way a circuit operates, the oscilloscope sees a distorted version of the actual signal. The result can be wrong or misleading measurements. In essence, the probe is the first link in the oscilloscope measurement chain. And the strength of this measurement chain relies as much on the probe as the oscilloscope.
Weaken that first link with an inadequate probe or poor probing methods, and the entire chain is weakened.
What Is a Probe?
As a first step, let’s establish what an oscilloscope probe is. Basically, a probe makes a physical and electrical connection between a test point or signal source and an oscilloscope. Depending on your measurement needs, this connection can be made with something as simple as a length of wire or with something as sophisticated as an active differential probe. At this point, it’s enough to say that an oscilloscope probe is some sort of device or network that connects the signal source to the input of the oscilloscope.
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