A Common Misunderstanding About Optical-Fiber Isolated Probes: Differential Voltage vs. Common-Mode Voltage
Why are optical-fiber isolated probes so widely used in these applications?
A defining characteristic of these systems is extremely high voltage slew rates (dv/dt). Traditionally, engineers relied on high-voltage differential probes to perform such measurements. However, conventional differential probes often face major limitations in common-mode rejection ratio (CMRR), isolation voltage rating, signal-to-noise ratio (SNR), usable bandwidth, bandwidth flatness, and resistance to electromagnetic interference.
As power semiconductor devices continue to evolve toward higher blocking voltages, lower on-resistance, and reduced switching losses, the resulting circuit behavior includes higher operating voltages, faster switching transitions, and larger current amplitudes. These conditions push traditional high-voltage differential probes beyond their practical limits.
In contrast, optical isolated probes, with their optoelectronic isolation architecture, effectively overcome these constraints and are better suited for modern high-speed, high-voltage measurements.
Despite this, misunderstandings still exist among engineers when using either high-voltage differential probes or optical isolated probes—particularly regarding the difference between differential voltage and common-mode voltage.
Differential Voltage vs. Common-Mode Voltage
A common misconception is that because optical-fiber isolated probes can tolerate extremely high common-mode voltages, they can therefore directly measure differential voltages of the same magnitude. This assumption is incorrect.
Common-Mode Voltage
Common-mode voltage is defined relative to the probe’s reference, typically earth ground. When both the positive and negative inputs of a probe float together at a certain potential relative to ground, that potential is the common-mode voltage. For measurement safety, the probe’s rated common-mode voltage—also referred to as its isolation voltage—must exceed the highest voltage present in the circuit under test. A higher allowable common-mode voltage provides greater protection for both personnel and equipment.
Differential Voltage
Differential voltage is the actual signal voltage between the probe’s two input terminals—the quantity being measured and analyzed.
For traditional high-voltage differential probes, the maximum measurable common-mode voltage is limited by the probe’s internal circuit design and typically ranges from several hundred volts to a few kilovolts. In many cases, a trade-off exists between common-mode voltage capability and measurement resolution. Engineers may require a high common-mode rating for safety while also needing to measure small differential signals accurately. Since the attenuation ratio of most differential probes is fixed, reducing the differential signal amplitude directly degrades the signal-to-noise ratio.
Optical-fiber isolated probes operate differently. Their optical isolation allows them to withstand common-mode voltages reaching tens of kilovolts, while interchangeable attenuation modules enable measurement of differential voltages from millivolt levels up to several kilovolts. This flexibility allows the probe to maintain a high signal-to-noise ratio across a wide range of signal amplitudes.
Conclusion
Common-mode voltage is not the same as differential voltage; optical-fiber isolated probes provide significantly higher common-mode voltage capability.
Interchangeable attenuation options allow optical isolated probes to adapt to different differential voltage levels, preserving measurement accuracy and superior signal-to-noise performance—an inherent advantage over traditional high-voltage differential probes.
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