I some cases, a differential input is needed for voltage measurement. Using the only one operational amplifier, you can build an adapter to provide a floating input for your ground-referenced  voltmeter.

differential input adapter circuit schematic

Use 1% tolerance metal film resistor for R1 and R2.  Adjust VR1 to give zero voltage output when the input probe is shorted.  You can use voltage supply +Ub and -Ub between 3 to 20V, as long as it is a symmetric supply.