Oscilloscope Probe Compensation – an essential routine
These probes have a switch on them to select between 1X and 10X. The 10X setting is often needed when the probing signal is larger than the input of the oscilloscope is capable or signal nodes that are high impedance or susceptible to capacitive loading. The 10X probe setting introduces a 9 MΩ (mega-Ohm) resistor in series with the probe tip. That resistor is part of a voltage divider, with the input resistance of the oscilloscope channel being the other resistor.
All oscilloscopes available today have an internal resistor placed in parallel with each channel input. When the probe is switched to 10X, the input signal appearing at the scope input channel is divided by ten and that provides 10X scaling of the displayed amplitude of whatever signal is being probed. That works fine at d.c., but when probing a.c. signals whose frequency is higher that a few kHz (kilohertz) something unintended can happen.
The input of each oscilloscope channel also has a shunt capacitance which is typically about 25 pF (picofarad). That capacitance along with the unavoidable capacitance of the probe cable produces an resistor-capacitor (RC) network. That RC network attenuates higher frequencies and if not compensated, can produce moderate measurement error. At the probe handle or more commonly at the probe cable BNC connector, there is a little screw head which provides access to small adjustable capacitor. That capacitor is in parallel with the 9 MΩ resistor and those two components form a second RC network. When the capacitor is adjusted correctly, the effects of the two RC networks cancel, leaving only the resistor divider ratio (10:1) for all frequencies the oscilloscope can accurately display.
You can find a 1 kHz square wave on a little terminal on the front panel of your oscilloscope and you use that to adjust the 10X probe compensation for the best-looking square wave on the screen. A properly compensated probe ensures that what you see on the oscilloscope screen is what is happening at the circuit node being probed. A poorly uncompensated ‘scope probe can distort edges, exaggerate ringing, hide overshoot, and produce incorrect amplitude at higher frequencies. It can also capacitively load the circuit node being probed and that can alter the waveform too.
The 1X probe setting is fine for smaller, low frequency signals, but the scope cable capacitance coupled with the input capacitance of the channel input can seriously load a high frequency circuit node. A properly compensated 10X probe will not.
Probe compensation is one of those measurement fundamentals that separates “good enough” measurements from accurate ones. The good news is that probe compensation takes less than a minute. Make it a habit to do it every time you power up the oscilloscope for use, and you will be assured your scope is showing you the “real picture”.
Last Updated on 2026-04-30 by Eve