High Impedance Differential Probe For Oscilloscopes & Multimeters

The High Impedance Differential Probe is an essential analog conditioning circuit designed to allow standard single-ended oscilloscopes and multimeters to measure voltage differences between two non-grounded test points safely. At the heart of the circuit are operational amplifiers—specifically the dual TL072 (or TL082/TL062) and single LF356/LF357 (or TL071/TL061/TL081) ICs—which feature JFET inputs that provide extremely high input impedance. This high input impedance ensures that the probe draws minimal current from the circuit under test, preventing loading effects that could distort the measured signals. Resistors (R1, R2) and a multi-turn trimmer potentiometer (VR1) form the balancing and attenuating network, allowing the user to precisely tune the common-mode rejection ratio (CMRR) so that unwanted noise picked up equally by both input lines is effectively cancelled out. The circuit relies on a set of diodes (D1 through D4) and Zener diodes (ZD1, ZD2) configured at the inputs as protective clamping circuitry, which safely shunts over-voltage transients away from the sensitive op-amp inputs. Switches (S1, S2) are integrated to control operational modes or power states. Power is delivered to the active components via dedicated power supply connectors (CON2), while the processed differential output signal is routed out through a standard BNC connector (CON1) to be displayed on the test equipment. All components are mounted cleanly onto a custom PCB.