In some cases, we often encountered a differential input voltage required for measurement. Using only one operational amplifier, you can build an adapter to give your input for floating-ground reference voltage. In the circuit diagram is one example and you should try. Circuit diagram is very simple in using IC-OP Amp CA3140. Here is a schematic picture:
Use 1% tolerance metal film resistor for R1 and R2. Adjust VR1 to give zero output voltage when the input is shorted probe. You can use the supply voltage + UB-UB and between 3 to 20V, this is a symmetrical supply.
