This is a DIY 100MHz version of a new differential probe that was mostly designed by my friend Bud Bennett and tested and verified by me. It can be used with oscilloscopes to look at signals that are not earth ground referenced.

Using the same PCB, you can also build a 10MHz probe and a 1MHz probe. The latter two have really low noise contributions.

There are three BOM's available for the three different versions.

Specifications 100MHz:

• Input impedance: 20MegOhm//1.25pF - differential, 10MegOhm//2.5pF for each terminal to GND.
• Differential Gain = 1/10V/V. Any lower than this and most hobby DSO's cannot resolve a 1V input signal with any clarity.
• Maximum AC Common Mode Voltage (with 50V differential input) = 350VAC
• CMRR > 90dB @ DC, ~60dB @ 1MHz.
• 100MHz: Differential Voltage Range > +/-25V for 240VAC common-mode, +/-25V for 0V common-mode.
• 10MHz: Differential Voltage Range > +/-80V for 240VAC common-mode, +/-80V for 0V common-mode.
• 3dB bandwidth >= 100MHZ (depends on signal amplitude)
• DC offset < 1mV (trimmed)
• Noise: 30mVpp or lower at output.
• Power supply: 5.25V +/- 0.25V. This can be a USB-C PD supply plus a regulator, see the power options.
• Cost: ~$50 not including shipping and handling costs. Not including 3D printed enclosure.

Specifications 10MHz:

• Input impedance: 10MegOhm//2.5pF - differential, 5MegOhm//5pF each input to GND.
• Differential Gain = 1/10V/V.
• Maximum AC Common Mode Voltage (with 140Vpp differential input) = 350VAC
• Maximum input voltage = +/-600VDC or 424Vrms.
• CMRR > 85dB @ DC, ~50dB @ 1MHz.
• Differential Voltage Range > +/-70V for 240VAC common-mode, +/-70V for 0V common-mode.
• 3dB bandwidth >= 10MHZ ( for Vin < 60Vpp) Slew rate limited.
• DC offset < 1mV (trimmed)
• Noise: 1.5Vrms, 2.2mVpp, input referred.
• Power input 15V +/-0.5V. Can be USB-C PD, see power possibilities.
• Cost: ~$30 not including shipping and handling costs. Not including 3D printed enclosure.

Specifications 1MHz:

• Input impedance: 10MegOhm//2.5pF - differential, 5MegOhm//5pF each terminal to GND.
• Differential Gain = 1/10V/V.
• Maximum AC Common Mode Voltage (with 140V differential input) = 270VAC
• Maximum input voltage = +/-600VDC or 424Vrms.
• CMRR > 85dB @ DC, ~50dB @ 100KHz.
• Differential Voltage Range > +/-70V for 240VAC common-mode, +/-70V for 0V common-mode.
• 10MHz: Differential Voltage Range > +/-80V for 240VAC common-mode, +/-80V for 0V common-mode.
• Bandwidth >= 1MHZ (-3dB) 
• DC offset < 1mV (trimmed)
• Noise: 0.5mVrms (3mVpp), input referred.
• Power input 15V +/-0.5V. Can be USB-C PD, see power possibilities.
• Cost: ~$30 not including shipping and handling costs. Not including 3D printed enclosure.

The complete project including design, building and calibration is described in detail on my Blog here:

https://www.paulvdiyblogs.net/2025/08/building-new-100mhz-differential-probe.html

There are a number of additional boards that can be configured to power the probe from either battery of USB-C PD supplies. Two of the boards are listed here as another project. The 10MHz and 1MHz probes share the same PCB as the 100MHz probe, but with different parts. They are also listed.

https://www.pcbway.com/project/shareproject/Diff_Probe_Power_Supply_VREG_SOIC_8_ce2b5e93.html

https://www.pcbway.com/project/shareproject/Diff_Probe_Power_Supply_Discrete_c5b5e2fb.html

https://www.pcbway.com/project/shareproject/Diff_probe_switching_mode_power_supply_e369360a.html

There is a 3D printable enclosure available.

All the files and details to build the probes can also be found on my Github project here:

https://github.com/paulvee/New-Differential-Probe-1-10-100MHz

Here is Bud's project with lots of details about the design:

https://hackaday.io/project/203296-a-slim-10x-100mhz-10mhz-1mhz-differential-probe/

Success and have fun!

Paul