Sensing high differential voltages using operational amplifiers
You know the inputs are in the (+80V, -30V) range.  So, you could apply
a battery-powered voltmeter and read the difference directly.

Yes, of course it can be also be done with low-voltage op amps.   The high voltage can be turned into
a modest current, and with a few (-30, -35, -40V) power rails to power
an op amp, one can generate a ground-referenced (negative) voltage from each
input.   Do it twice, and you can apply a normal difference
amplifier on the translated/scaled voltages.







simulate this circuit &ndash; Schematic created using CircuitLab

The scheme (1) reduces the voltages
to something less, (2) applies an offset to near-ground levels.
$$A_{scaled} = -(A + 30) *( R8*R4)/(R1*R7) $$

so (80V, -30V) inputs are changed to (-0.550V, 0V) outputs

(There's some linearity errors near the (-30V) value, if that
end of the range is important, moving the power supplies down
by ten volts will clean it up).

Actually, though, voltage dividers aren't so bad; the proliferation
of power supplies is rather a nuisance, doing it this way.