Differential amplifier with common voltage far above the rail?
This is a suggestion only.  Do a precision voltage-to-current conversion using 25V as your "ground", then do the rest of the work ground referenced.  None of the component values I show should be trusted.  R1, R2, R4, and R5 set the gain.  You need to choose a part for OP1 that works decently with it's common mode voltage at or close to the positive rail -- even with rail-rail amps this is not a hugely common thing.

A bit of theory:  M1 doesn't flow current in the gate, so the voltage that you maintain across R4 sets the current.  R5 "catches" this current and turns it into a voltage (you want to buffer it immediately with a voltage follower, BTW).  R3, C1, and R1 &amp; R2 in parallel compensate the circuit for the FET's gate capacitance (use a wimpy fet with low gate capacitance).  The 79L05 provides a regulated rail-5V supply for the high-side circuit.







simulate this circuit &ndash; Schematic created using CircuitLab