HF Differential Probe

This project is an isolated high frequency differential probe ideal for monitoring signals such as RS-485, CAN, MIL-STD-1553 and so on. It is DC coupled, and can also be used for differential DC voltage measurements such as Vbe on a transistor. The floating battery supply eliminates the possibility of ground loops.

Right-click on the images to view them at a larger size.

Version 3 PCB, shared at OSH Park

The version 3 PCB was sized to allow mounting the battery directly beneath the board

Version 3 block diagram

Version 3 Schematic. Download PDF

The version 2 prototype used individual boards to separate the power supply from the differential amplifier. A dual boost power supply was created to convert the voltage from a LiPo cell into +/-15V. This works very well, with no noticable noise being coupled into the signal under test. Both of these PCBs have been shared at OSH Park.

Version 2 PCB with +/-15V converter and LiPo battery

Measured frequency response of version 2 PCB - note the peak at 80MHz in the version 1 PCB has been eliminated

Version 2 Top

Version 2 Bottom

Version 1 PCB under test, powered by two 9V batteries.

Measured frequency response of first prototype - note the peak at 80MHz was eliminated in the second version

Version 1 LT spice schematic

Version 1 Altium schematic

Visual comparison of a 1Vpp 10MHz sine wave, yellow = input, blue = output

Last update: 13 Sep 2017
Steven J. Merrifield