Question

Why do we need either a differential amplifier or differential probes to measures voltages across two floating point using an Oscilloscope?

Answer 1

In precise measurement, it is essential that the measuring instrument shouldn't affect the quantity being measured.

In resistance measurement, the basic relationship R = V/I is used. In measurement of resistance, ammeter is connected in series with resistance and voltmeter across it to read these quantities. In normal measurement, two wires from resistor connect to voltage source in the meter and same wires also measure the current. Since resistance of these lead wires is usually negligible compared to that being measured, this is sufficiently accurate.

However, when the resistance is very low (which is usually the case when for resistivity measurement), the lead wire resistance is comparable to the one being measured, these wires themselves will drop considerable voltage . So the voltage being measured across unknown resistance cannot be measured properly.

To overcome this problem, two sets of wires are used. One set serves to supply current to the resistance, and second pair to measure voltage across it. This second pair (voltage probes) do not draw appreciable current, as voltmeter resistance is high. Current probe drop is not in the picture.

The reading thus obtained is quite accurate. Hence for measurement of small resistance in milli ohm and micro ohm range, always 4 terminal measurement is used.