The first rule of thumb to be observed in choosing a test probe involves the probe’s bandwidth. “The bandwidth of the probe needs to be three to five times the signal bandwidth,” explains Jae-Yong Chang, Agilent Technologies’ product manager for probes. “Customers often are confused by signal bandwidth, which is driven by the fastest rise time of your circuitry.”
When it comes to using high-end active test probes, it’s important to remember that these devices are high-quality measurement instruments in their own right and are one component of a highly tuned system.
“They require some additional care compared to traditional passive probes,” says Richard Van Epps, senior design engineer at Tektronix. “The cables we use on these probes are costly and they can be damaged if improperly used or bent or twisted to an extreme. The high-bandwidth buffer amplifiers used in the probes tend to be ESD-sensitive (electrostatic discharge), just as a sampling scope’s input module.”
To get the maximum lifespan out of high-end probes, they should be used with wrist straps and in static-controlled environments. Users should be mindful of the probe’s maximum voltage rating and be careful not to exceed it.
Most designers and test engineers are more accustomed to using passive probes, which are not only very robust in general, but also inexpensive. If you lose or damage one, who cares? On the other hand, active probes can be quite pricey. They’re not exactly delicate flowers, but neither would you cavalierly toss one into the bottom of a tool box, so be gentle with them.
Even though this relates to a broader testing issue, it still bears mentioning that test strategy is a very important part of maximizing probe use. Just as carpenters should “measure twice and cut once,” test engineers should plan first and connect second. With test setups that can get quite complicated, it will pay off (see the figure). Otherwise, you may find yourself hours into the construction of a complex maze of probe tips, only to find that your sequence of connections made a signal of interest inaccessible.
When working with solder-in probe tips, take some time to consider how you’ll secure the tip. Solder joints aren’t necessarily a mechanically robust connection. Your toolbox should include things like a hot-glue gun, double-stick tape, and epoxy for use in affixing tips to the board so the weight of the cable won’t pull the solder joint apart.