Schemativ differential probe.

Differential probe: active ground-free probe.

The differential probe is a special form of the active probe . The differential probe measures the difference between two voltage potentials, without affecting the oscilloscope ground with the circuit under investigation. This avoids the grounding problem .

Layout of a differential probe

The figure above shows the schematic diagram of an active differential probe. The circuit diagram is to be understood symbolically, in the detail, the circuit can vary considerably. Central element is an operational amplifier which provides a differential voltage to the BNC pin of the oscilloscope (right half in the picture). The oscilloscope ground is marked at the position where the differential probe is connected. A single operational amplifier would lead to a high input impedance at a high amplification, the circuit would be unnecessarily loaded. This can be avoided by adding two more operational amplifier. The input impedance of the probe remains low despite good amplification performance. The negative input to each operational amplifier provides the input for differential tapping.

Illustration of additional contacts on the BNC connector.

BNC connector on the oscilloscope with additional contacts.

The operational amplifiers must be supplied with a supply voltage, so they are actively fed. This can be done via batteries in the differential probe head or by additional connections next to the BNC connector. Each operational amplifier has only one specific valid boltage range in which it can work. In case of doubt, the input voltage must not exceed the battery voltage. Therefore that active probes measure a smaller voltage range than the passive probes.

In practice, differential probes are even more complex in order to avoid stability problems. Daniel Kramnik recommends in his Blog two source followers (drain circuits with transistors) instead of two operational amplifiers. This achieves a low input impedance and avoids feedback from the operational amplifier. On his page he gives tips for building your own amplifier. He has example circuit diagrams and an eagle files for download.

In the Signal-Path-Blog on Youtube there is an interesting video in which a 25GHz high frequency probe is disassembled. Special semiconductor technologies are used to produce such probes. In addition, such a probe has a processor and digital electronics for storing calibration curves.