What are the Advantages and Disadvantages of Optically Isolated Measurement Technology?

Leveraging its exceptional electrical isolation and interference immunity capabilities, optically isolated measurement technology demonstrates significant advantages in complex high-voltage and high-frequency environments; however, it also faces limitations such as high cost and large physical size.

I. Core Technical Advantages

1. Ultra-High Common Mode Rejection Ratio (CMRR)

Optically isolated probes can achieve a CMRR of -160 dB in the low-frequency range, and still maintain a level of over -100 dB at a high frequency of 1 GHz-far exceeding that of traditional differential probes (which typically reach only -26 dB at 100 MHz).

They are capable of precisely capturing millivolt-level differential signals within environments subject to ±6 kV common mode voltages, making them particularly well-suited for measuring switching transients in third-generation semiconductors (such as SiC and GaN).

2. High Isolation Voltage for Enhanced Safety

Based on the principle of electrical isolation via optical fiber, this technology can achieve isolation voltages exceeding 60 kV, thereby completely severing the electrical connection between the circuit under test and the measurement equipment.

This effectively prevents high-voltage breakdown of the oscilloscope and safeguards operating personnel, making it ideal for high-risk testing scenarios involving new energy vehicle motor controllers, photovoltaic inverters, and similar systems.

3. High Signal Integrity and Minimal Loading Effects

With an input impedance reaching as high as 10 MΩ || 1 pF, the loading effect on the circuit under test remains below 0.1%, thereby avoiding the signal distortion typically caused by the lead inductance found in traditional probes.

Rise times can be as low as 700 ps, ​​enabling the accurate reproduction of voltage slew rates (dV/dt) of up to 100 V/ns in GaN devices, and providing reliable waveforms for the calculation of switching losses.

4. Strong Immunity to Electromagnetic Interference (EMI)

The optical fiber transmission path is free from electromagnetic coupling; consequently, it remains unaffected by strong electromagnetic environments (such as those found near communication base stations or CNC machine tools), thereby ensuring signal purity.

This makes the technology particularly suitable for measurements involving floating-ground circuits and multiple potential points, eliminating the need for a common ground connection.

II. Primary Application Limitations

1. Limited Bandwidth and Response Speed

Constrained by the modulation rates of laser diodes and the response times of photodiodes, the bandwidth of mainstream models is typically limited to within 1 GHz-a range lower than that of top-tier passive probes (which can reach 6 GHz). In ultra-high-frequency signal measurements (e.g., 5G RF power amplifiers), frequency response attenuation may occur, leading to signal distortion.

2. High Power Consumption and Large Physical Size

The front-end module requires an independent power supply (either battery or laser-based). Due to the thermal management design, its physical volume is 3 to 5 times larger than that of traditional probes, with a weight exceeding 500g.

Operation is restricted when testing densely populated PCBs; prolonged handheld use causes fatigue, necessitating the use of a support stand.

3. High Cost and Complex Maintenance

Comprising a laser driver, a high-precision optoelectronic conversion module, and a temperature control system, the manufacturing cost is 3 to 5 times that of comparable differential probes.

Entry-level models are priced upwards of 10,000 RMB, while high-end models reach hundreds of thousands of RMB. Furthermore, periodic calibration of gain and linearity is required, resulting in high long-term operating costs.

4. Strict Usage and Maintenance Requirements

The bending radius of the optical fiber must exceed 30mm (some models require ≥10cm); excessive bending will result in optical signal attenuation.

Prior to measurement, the voltage range must be estimated and an appropriate attenuator selected; recalibration is required if the ambient temperature fluctuates by more than 5°C.

III. Summary of Applicable Scenarios

Trend Outlook: The bandwidth of the new generation of optically isolated probes has now surpassed 2GHz. By integrating AI-driven signal processing and edge computing, these probes are evolving toward higher levels of integration and intelligence.