A common misconception about isolated fiber optic probes: Differential voltage vs. common-mode voltage
Why are isolated fiber optic probes so frequently used in these applications?
A key feature of these systems is extremely high voltage rise rates (dv/dt). Until now, engineers have relied on high-voltage differential probes for such measurements. However, conventional differential probes often have significant problems with common-mode rejection ratio (CMRR), isolation voltage, signal-to-noise ratio (SNR), usable bandwidth, bandwidth flatness, and immunity to electromagnetic interference.
The development of power semiconductors towards higher blocking voltages, lower on-resistances, and reduced switching losses leads to higher operating voltages, faster switching times, and larger current amplitudes. These conditions push conventional high-voltage differential probes to their practical limits.
In contrast, optically isolated probes with their optoelectronic isolation architecture overcome these limitations and are better suited for modern high-speed and high-voltage measurements.
Nevertheless, misunderstandings still exist among engineers regarding the use of high-voltage differential probes or optically isolated probes – especially concerning the difference between differential voltage and common-mode voltage.
Differential voltage vs. common-mode voltage
A common misconception is that isolated fiber optic probes can withstand extremely high common-mode voltages and therefore can also directly measure differential voltages of the same order of magnitude. This assumption is incorrect.
Common-mode voltage
The common-mode voltage is defined relative to the probe's reference point, usually ground. When both the positive and negative inputs of a probe are at a specific potential relative to ground, this is the common-mode voltage. To ensure measurement safety, the probe's common-mode voltage—also called insulation voltage—must be higher than the highest voltage present in the circuit under test. A higher permissible common-mode voltage provides greater protection for personnel and equipment.
Differential voltage
The differential voltage is the actual signal voltage between the two input terminals of the probe - the quantity to be measured and analyzed.
In conventional high-voltage differential probes, the maximum measurable common-mode voltage is limited by the probe's internal circuitry and typically ranges from a few hundred volts to a few kilovolts. In many cases, a compromise exists between common-mode voltage and measurement resolution. For example, engineers may require a high common-mode voltage for safety reasons while simultaneously needing to accurately measure small differential signals. Since the attenuation ratio of most differential probes is fixed, a reduction in the differential signal amplitude directly degrades the signal-to-noise ratio.
Isolated fiber optic probes function differently. Thanks to their optical isolation, they can withstand common-mode voltages of several tens of kilovolts, while interchangeable attenuation modules enable the measurement of differential voltages from millivolts to several kilovolts. This flexibility allows the probe to maintain a high signal-to-noise ratio over a wide range of signal amplitudes.
Conclusion
Common-mode voltage is not the same as differential voltage ; isolated fiber optic probes offer significantly higher common-mode voltage capability.
Thanks to interchangeable damping options, optically isolated probes can adapt to different differential voltage levels while maintaining measurement accuracy and superior signal-to-noise ratio – a crucial advantage over conventional high-voltage differential probes.
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