Oscilloscope and probe fundamentals

# Understanding probe compensation

All oscilloscopes have a certain amount of inherent capacitance in parallel with their input resistance. Typically, this capacitance is in the low 10s of picofarads. When measuring DC, this is not a problem. Once measuring AC it becomes critical, because as the frequency increases, the input capacitance starts acting as a low pass filter.

## What is probe compensation?

Probe compensation is the process whereby the probe capacitance is adjusted to compensate for the effects of the inherent input capacitance of the scope. Properly compensating the probes is necessary to ensure the best possible accuracy or linearity in measurement results.

## Why is it necessary to compensate a probe?

A poorly compensated probe causes two main types of measurement inaccuracies. The first is incorrect amplitudes. Comparing a measurement made with a properly compensated probe to measurements with an undercompensated or an overcompensated probe shows significant amplitude variation even at low frequencies.

The second consequence of incorrect probe compensation is distorted waveforms, more specifically, changes in the rise and fall times of pulsed signals. It’s important to remember that these inaccuracies increase with increasing frequency.

## When should a probe be compensated?

Probes should be compensated before first use or before making any important measurements. Since different scopes have different input capacitances, a probe needs to be compensated whenever it is moved to a different scope. Moving between ports on the same scope is usually okay. Yet compensating a probe becomes more important the higher the frequency signal is.

## How are the probes compensated?

Almost all scopes have a built-in square wave generator, usually running at a frequency of 1000 Hz. It usually is labeled “probe compensation”, and/or is indicated by a square wave and ground symbol.
The probe tip is connected to the square wave source whilst the probe ground leads to the ground.

The probe tip is connected to the square wave source whilst the probe ground leads to the ground.

Then a nonconductive screwdriver or tool is used to adjust the capacitance until the square wave compensation signal appears as rectangular as possible.

A probe is properly compensated when the tops of the compensation signal are essentially horizontal.

Then a nonconductive screwdriver or tool is used to adjust the capacitance until the square wave compensation signal appears as rectangular as possible.
A probe is properly compensated when the tops of the compensation signal are essentially horizontal. Overcompensated probes create overshoot on the leading edge of the signal, and undercompensated probes cause undershoot on the leading edge.
In the case of either over- or under-compensated probes, the compensation capacitor is adjusted until the waveform has nice, square edges. This usually takes only a very small fraction of a turn.
Note that square or rectangular waves are used for probe compensation because they have both high frequency and low frequency components.

Probe compensation examples