I'm working on a Geiger-counter, and trying to get an accurate measurement of the high-voltage Tube Bias.
The issue I'm having is that the impedance of the high-voltage source is really, really high, I'd guess somewhere in the range of 100-500 MΩ.
Right now, I have 4 10MΩ resistors in series with my scope probe, which is set at 10X (I've measured the scope input impedance to be 10MΩ). This should produce a effective 50X probe, with a 50MΩ input impedance. Then, since the Input impedance of the prove is still enough to significantly draw-down the high-voltage rail, I'm using a sampling scope to capture the voltage as I touch the scope probe to the high-voltage rail.
Unfortunately, I'm still getting voltages that are far higher then they should be (it should be ~500V. I'm measuring ~1.2 KV). However, as this is a spark-fun product, and the circuit that I have so far studied has been extensively studded with truly WTF inducing design decisions.
I am having trouble determining if the incorrect voltage measurement is due to the circuit not behaving as designed, or measurement error on my part.
Are there any other simple ways I can measure a high voltage source with an extremely high source impedance to cross-check the results?