Electronic Design

Probe Priority

Your scope’s measuring results will only be as good as the probe you use (see the figure). As data rates and frequencies of operation rise, figuring out how to connect the scope input to the circuit under test without compromising the signal has become an ever-daunting challenge.

Probes and cables introduce capacitance and inductance that distort and attenuate the signal. That’s why scope manufacturers spend lots of time creating the best probe for their scopes, including the best way to attach the probe to the test point. By all means, use only the probes designed for your scope to get the best results.

Several different types of probes are available today. The older passive attenuator probes still suit low-frequency (less than 100 MHz) scopes, and some of the better ones will get you to about 500 MHz. Beyond that, active probes are more common. These feature amplification, isolation, and impedance matching between the connection point and the scope input.

Differential probes are available for working with most of the high-speed serial buses. Current probes and high-voltage probes are available for working with power supplies. Shunt capacitances are now down in the hundreds of femtofarads (fF) range.

Probes are surprisingly expensive, but they’re the secret to getting the most accurate measurements.

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