Active Probe for an oscilloscope
Now and then I need to check out sensitive circuit which cannot tolerate impedance of standard scope probe even if set to 10:1 attenuation. Such probe loads the circuit under test with a few pF of probe's input capacitance. Crystal stabilized clocks are one type of such circuits - connecting extra 5-6 pF to the crystal can easily stop oscillation. HF filters are one other class of such circuits - they will work but will immideately get out of tune as soon as loaded with such scope probe. Anyway, there is good reason active probes, single ended or differential, exist. You can find quite a few commercial probes as well as private projects of inclied individuals designing their own probes.
Full credit for the design presented here goes to the brilliant engineer I admire - Jim Williams, who published it in one of his app notes while working at Linear Technology. There are two flavors of the gadget: one with lower bandwidth using IC and another using discrete compoents, which thus is more complicated to put together, but should have better performance - that's the one I chose to replicate. It may not make much sense to construct a probe far exceeding bandwidth of the scope itself, however as this is investment of time and effort, I consider possibility of upgrading my lab scope(s), and wouldn't want probes to become bandwidth limiting bottleneck.
The only difference between published design and this implementation is substitute of a few transistors that are either no longer available or very hard to get. Another circuit I'm planning to add is -5V generaotr
All I did was designed a printed circuit board for the probe. I have to tell the reader that this is not as pressing project as some others, so it is on the back burner. The probe was actually never built and I will do it at my first opportunity when I'll have to make a PCB for another project - this will be a bread-away section. As soon as it's done, I'll publish results and physical construction. For now here is all the material I will use to proceed.
Schematic of the probe in CIRCAD format. For those of you who have never heard of CIRCAD, here isPDF version of it.
Likewise, here is proposed PCB layout of the probe in CIRCAD format. Gerbers are here, PDF version of layout is here, and below is screen shot image of proposed PCB.
You can email meto share your thoughts or ask questions.