Measuring oscilloscope input capcity with probe calibration squarewave

[Marek85]

Hello,

I was just wondering weather I can measure the input capacity of my scope with the built in square-wave generator dedicated for probe calibration.

The scope inputs are marked with: 15 pF // 1 MOhm.

I assumed that I can apply the RC formula so:

So the RC = 15 us

Nevertheless when I measure the time when the square wave reaches about 0,63 V (63 % for 1 V) I'm getting 19 ns instead of 15 us.

Does it my that I cannot apply the RC formula for the scope inputs? Or maybe I am doing something else wrong?

Best Regards
Marek

[PSR B1257]

Does it my that I cannot apply the RC formula for the scope inputs?

Yes. You can not apply this formular in general to a RC-parallel circuit if you force a voltage to it. The voltage in a parallel circuit is everywhere the same. Unless you have a ass load of capacitance, you will not notice it this way.

You will only see the time constant of this circuit, if you disconnect the voltage source, instead of switch the voltage to zero.

[KerryW]

Just apply the square wave through a 1 meg resistor of your own, then measure the rise (and/or fall) time.  It will only go to 0.5V instead of to 1V.

Edit: Don't forget to use 500K as your R in the formula