Electronic – Oscilloscope voltage readings factor of 10 too high. Is this due to the impedance


For a university physics experiment, I built a circuit to test voltage (and conductance) steps across a nanowire break junction. The break junction was in a circuit with an I-V converter, and the I-V output was fed to an oscilloscope. The voltage steps measured matched the theory, however they were all a factor of 10 too large. Is this due to the impedance of the oscilloscope? I can't retrieve the exact model now but it was Tektronix and the standard oscilloscope used across the university, so I assume it is 1MOhm. Is there anything else involving the impedance (or any other factor) that could be causing the factor of 10?

I double checked the I-V contributions of all components in the circuit, such as the feedback resistor in the converter, and couldn't find anything so can only conclude it is the oscilloscope. But electronics is not my forte so I am not sure how an oscilloscope could cause x10 to the voltage.

Best Answer

Some of the probes used with oscilloscopes incorporate a very high impedance input circuit right at the probe tip. This circuit attenuates the signal by a factor of 10. Some oscilloscopes can automatically recognize whether such a probe, usually called a "ten-to-one" probe, is in use but other oscilloscopes require the user to manually change a setting.

My guess is that your oscilloscope assumed that such a probe was being used when in fact it was not, so the recorded voltages are too large by a factor of 10.