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While digital power analyzers from different manufacturers operate on similar fundamental principles, critical differences in accuracy specifications can often be overlooked. Figure 1 illustrates key factors affecting measurement accuracy. This article provides an in-depth examination of these factors, with particular focus on easily-missed specifications that are frequently buried in fine print.
Common-Mode Rejection Ratio (CMRR)
CMRR quantifies an instrument's ability to reject signals common to both positive and negative inputs (common-mode signals), which can significantly impact measurement accuracy. In theory, an ideal power analyzer would feature infinite CMRR, though material and design limitations make this unachievable in practice. The uncompensated common-mode component consequently superimposes onto the measured load, affecting overall accuracy - with higher CMRR values yielding smaller impacts.
Consider the inverter-powered three-phase motor application shown in Figure 4. In such configurations, PWM switching generates undesirable leakage currents when capacitances at the inverter output undergo repeated charging. These currents flow through motor cables and insulation, creating noise that ultimately superimposes on the power analyzer's measurement inputs and compromises accuracy.
Comparative Analysis Example
For an application with 100V/50Hz sinusoidal input signals referenced to ground, we evaluate how common-mode voltage affects measurements differently between Manufacturer X and Y's power analyzers, given their divergent CMRR specifications:
