Hello,
So when I first learned about probing, I learned that you should use the 10x setting of the probe whenever possible so that you don't load down your circuit very much.
Recently, I came upon a video by Mr. Ganssle.
https://youtu.be/aJsJibDNg9M
He shows that a 10pF capacitor goes from over 300ohms of capacitive reactance to just 159ohms of capacitive reactance.
What happened to the 9Mohm resistor in the probe?
Like, it should still be a 10 to 1 input impedance, right?
Formula: sqrt((1/(PI×2×100 000 000×0.000 000 000 01))^2+(9 000 000^2))
The capacitance is effectively in
parallel with the 10M

input resistance so the capacitive reactance will dominate and the input impedance will be ever so slightly less than 159

capacitive reactance. The formula you have used would apply for the
series connection of a capacitor and resistor, which is not what a probe looks like.
He goes on to discuss using just a piece of coax and 1K resistor.
Wouldn't that need some sort of capacitive compensation?
And if this really is *the answer* to high speed probing, why are we using 10x probes at all? Why not just a resistor hanging off of a piece of coax with maybe an attached pogo-pin and alligator clip on the ground terminal?
Such probes are really only useful for higher frequency signals e.g. >100Mhz and the lower impedance circuits typically found with high speed signals. There are commercial probes available, known as Z
0 probes, that have a series 450

resistor and have a x10 attenuation when coupled to a scope with a 50

input. Since the cable is terminated at the scope with the cable characteristic impedance it still looks like 50

at the probe end of the cable and doesn't have the excess cable capacitance that is seen with cable terminated by the 1M

input of a scope as for a 10M

probe. Without the excess capacitance there is no need for compensation.
However such probe present an excessive loading, especially DC loading, on lower frequency and higher impedance circuits so are not generally suitable for most circuit probing needs.