Analysis and application of probe bias capability and dynamic range

When using active probes for testing, the most common mistake is using the probe beyond its dynamic range. We often receive calls from customers saying that when they use active and passive probes to measure the same signal, the voltage of the active probe is lower than that of the passive probe. The reason for this is that in most cases it is caused by a misunderstanding of the dynamic range and the withstand voltage range.

Taking the Keysight N2795A active probe as an example, its withstand voltage range is plus or minus 20V, input dynamic range is plus or minus 8V, and offset range is plus or minus 8V:

>>> Input dynamic range is defined as

The input dynamic range refers to the voltage range above and below the center line of the oscilloscope screen that the probe can test. For example, a probe with a ±2.5V dynamic input range can only measure voltages within 2.5V above and below the center line of the oscilloscope screen. If the input signal fluctuates beyond this range, it will be reflected in the measured waveform as clipping, and the measured amplitude will be smaller.

By definition, this means that when using the N2795A probe, you can measure waveforms within 8V above and below the center line of the oscilloscope screen:

When we directly measure the 0-16V sine wave, since the waveform exceeds the range of 8V above the center line of the screen, it will cause waveform distortion and make the test result low.

This is where the probe/oscilloscope bias capability comes into play:

>>> The definition of bias capability is:

Bias capability refers to the ability to adjust the 0V voltage reference line to the voltage difference with the center line of the oscilloscope screen. By setting a suitable bias according to the DC component of the signal, the dynamic signal with a DC component can be adjusted to the vicinity of the center line of the oscilloscope screen to meet the requirements of the dynamic input range of the probe. 

For example, if the distorted test waveform above is set to an 8V bias so that the waveform continues to be displayed in the center of the screen, the 0-16V sine wave voltage can be tested correctly:

Therefore, the maximum input voltage of the probe is not necessarily the measurable voltage range. The measurable voltage range should be its dynamic range plus the offset range. Of course, the offset setting should be used reasonably at this time so that the test waveform is always displayed within its dynamic range.