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Imagine trying to measure a tiny 50Β mV50\text{ mV} signal riding on top of a lethal 400Β V400\text{ V} electric vehicle battery rail. If you connect a standard op-amp directly to that high rail, it will instantly vaporize in a puff of smoke.
To safely read current at high voltages, we use precision resistive dividers to step down the deadly rail voltage while capturing the microscopic shunt drop.
How do we split a voltage measurement into the dangerous background voltage and the useful signal we actually want?

Common-Mode vs. Differential Voltage

The voltage across any sensor has two components: differential voltage (VdiffV_{\text{diff}}), which is the true voltage drop across the component, and common-mode voltage (VcmV_{\text{cm}}), which is the average potential of both terminals relative to ground.
Mathematically, for terminals at potentials V1V_1 and V2V_2, the differential voltage is Vdiff=V1βˆ’V2V_{\text{diff}} = V_1 - V_2, while the common-mode voltage is Vcm=V1+V22V_{\text{cm}} = \frac{V_1 + V_2}{2}.
Can a simple four-resistor network strip away that huge 400Β V400\text{ V} pedestal without losing the tiny millivolt drop?

The High-Voltage Difference Stage

A standard difference amplifier uses four resistors (R1,R2,R3,R4R_1, R_2, R_3, R_4) arranged around an operational amplifier to subtract two input voltages.
For high-voltage sensing, we choose R1>R2R_1 > R_2 and R3>R4R_3 > R_4 so the network acts as a voltage attenuator, scaling the inputs down into the op-amp's safe supply rails (such as 0Β V0\text{ V} to 5Β V5\text{ V}).