When making oscilloscope measurements, one occasionally encounters signals not referenced to ground, as shown. Point B, the reference for point A, is not ground. Voltage V(subB), between B and ground, may be line voltage. Thus, you cannot connect the oscilloscope probe ground lead to point B without causing serious problems.
We can, however, measure V(subA) and V(subB) separately, with respect to ground, and subtract the value at B from that at A. To measure V(subA-B), instruments must be able to reject V(subB). For V(subB) large compared to V(subA-B), the equipment's rejection of this common voltage must be very high. This Common Mode Rejection Ratio (CMRR) is the magnitude of the output signal to the differential input signal, and a function of frequency and amplitude.
Cutting the safety ground on the oscilloscope "floats" its chassis to V(subB), rejecting this common voltage, but is dangerous to the operator. CMRR is also poor at higher frequencies, and circuit loading can be a problem. Alternatively, measuring V(subA) on one oscilloscope channel and V(subB) on another, then using the subtraction math function, yields limited results--for typical 1% channel accuracies, CMRR is seldom better than 100:1. For a 100V common mode V(subB), CMRR should be at least 10,000:1 to view an uncorrupted 100-mV V(subA-B) signal.
Differential amplifiers designed for such measurements provide CMRR to 100,000:1, measuring safely with minimal degradation. They have a wide common mode range (maximum V(subB) amplitude) to permit viewing small differences in large signals.
To speak with a Preamble Instruments engineer, call 800-376-7007 or fax 503-646-1604.