Protected: Oscilloscope Probe Loading Calculator

Connecting an oscilloscope probe can change the signal you’re trying to measure. How much it changes depends on the frequency, the circuit’s source resistance and the probe’s input resistance and capacitance.

Use the calculator below to estimate how much signal amplitude you lose at the test point when you connect a probe.

1. Enter the signal frequency. For a DC measurement, enter zero.
2. Enter the circuit source resistance seen looking back into the circuit from the test point with the probe disconnected. The default of 50 Ω is an example.
3. Choose a probe preset or enter the input resistance and capacitance from your probe datasheet. Use the values for the attenuation setting and oscilloscope input specified by the manufacturer.

The result shows the estimated amplitude reduction compared with the signal at the same point before you connected the probe. Open Calculation details to see the probe impedance, capacitive reactance and estimated phase lag.

At 10 MHz, the example 10× probe gives an estimated amplitude reduction of 0.05% with a 50 Ω source. Change only the source resistance to 1 kΩ and the estimated reduction rises to 15.3%. The same probe can have a much bigger effect on a higher-resistance test point.

The calculator estimates loading at one frequency using a resistive source and a parallel resistance–capacitance model for the probe. Probe bandwidth, ground-lead inductance and reactive source impedance can also affect the measurement.