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A cheap, high quality probe

June 25, 2012

Jack Ganssle-June 25, 2012

In a pair of columns recently, Probing pointers and Probing Pointers: Take 2, I discussed some of the issues that arise in using scope probes as frequencies increase.

Alas, too few engineers do a real analysis.

Turns out, a decent (think over $300) 10 pF 10X probe at 100 MHz looks like a 160 ohm load! That’s 14 mA at 2.3V, which is more than many gates (not to mention CPUs) can drive. In other words, putting a probe on a perfectly good circuit may cause the system to stop functioning.

But it gets worse as the frequency goes up. In the following graph probe impedance in ohms is shown on the vertical axis and frequency along the horizontal. Impedances are shown for three different probes. Note that 10 pF is pretty common for decent probes; better ones, like Tektronix’s very nice 5 pF TPP1000 approach a thousand dollars.

Agilent recently introduced the Infinium 90000, a 33 GHz scope. (I want one of those!) But how do you probe a signal that fast? A 5 pF probe will look like a one ohm load at that speed. That’s over 2 amps at 2.3 volts.

The answer, of course, is to buy special probes, which run about $30k a pop. A set of four will buy a house in the Midwest.

For those of us working at more modest frequencies, say 50 to a few hundred MHz, one can steal an idea from High-Speed Digital Design by Howard Johnson and Martin Graham. I briefly mentioned this in the second of the two referenced articles, but quite a few people asked for more details.


Click on image to enlarge.

The probe is simplicity itself. Note in the figure above that a typical quarter watt resistor has about 0.5 pF of capacitance. Get a meter of RG-58/U coax. On one end install a decent BNC connector (or, just buy cable with a preinstalled BNC) and solder the resistor (with short leads) to the inner conductor at the far end. Then as show in the figure below solder the other end of the resistor to the node being probed, and solder the braid (very short) to a ground near the node. The result looks like this:


Click on image to enlarge.

(Note the SOT-23 package I’m probing is so small you can’t really see it in this picture, or in real life if you have the slightest myopia). 

It’s very important to set your scope’s input impedance to 50 ohms, since most scopes default to 1 M? or so. If your scope doesn’t have a selectable impedance buy a 50 ? attenuator (Agilent's N5442A or Test Products International’s 120082, which is $56 from Digi-key).

Now you have a 0.5 pF probe which divides the input by a factor of 21 (i.e., this is a 21X probe). And its performance, shown in the red line in the figure below is pretty stunning:

The downside is that this probe isn’t as easy to use as one from a vendor. After all, you will have to solder it in place every time it’s moved. An alternative is a commercial low-capacitance probe, which will set you back about five grand each.

Probably the most important take-away here is to understand that everything is part of your circuit. Even humidity can affect sensitive analog designs. And your test equipment is part of the circuit. No scope, logic analyzer or any other device is perfect; they will all interact in lesser or larger ways with your board. Always do an engineering analysis to understand how things will behave.

Jack G. Ganssle is a lecturer and consultant on embedded development issues. He conducts seminars on embedded systems and helps companies with their embedded challenges. Contact him at jack@ganssle.com. His website is www.ganssle.com.

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