In Voltmeter impedance I presented a 2-voltmeter way of measuring AC voltmeter impedance, and in Measuring voltmeter input impedance I presented a way of measuring the AC voltmeter impedance with just the voltmeter itself, a function generator, and a resistor that is around the DC resistance of the meter (or a little smaller).
I got measurements of
|Radio Shack||10.87MΩ || 18.54pF|
|DT-830B||0.42MΩ || 31.59pF|
|DT-9205A||13MΩ || 22pF|
I decided to apply the same techniques to the BitScope BS-10 USB oscilloscope, getting the following result:
The BitScope has the standard 1MΩ input impedance with a fairly small 10pF parallel capacitance (probably largely from the 20cm leads I was using). The measurements are a bit noisy, because I was using the provided peak-to-peak voltage measurement, which varies quite a bit from trace to trace. At high frequencies the waveform is not much like a sine wave, so the results are bit dubious that far out—I did not include frequencies greater than 100kHz in the fit.
The voltage measurements look pretty good, though getting consistent measurements from BitScope’s peak-to-peak measurements with the cursor is a bit difficult:
I should probably average hundreds of waveforms to get a more precise and accurate measurement, but setting that up would be tedious. I did gather 576 traces of the 200kHz waveform and averaged them together to get a 2.2334V peak-to-peak waveform that looks much more like a sine-wave than I would have expected from the individual traces:
The BitScope is capable of seeing the glitches in the waveform at lower frequencies, like 2kHz, but only barely. The nonlinearities are much better viewed with the PteroDAQ running at lower frequencies.