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.
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.
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