What are common mode and differential mode signals and why is a high CMRR important for optically isolated probes?
Using the testing of GaN half-bridge circuits as a practical example, we show how common mode noise arises and how optically isolated probes with high CMRR, such as those based on SigOFIT technology, can effectively suppress unwanted noise. By reducing common mode noise, these probes significantly improve the signal-to-noise ratio and measurement accuracy in high-speed and high-voltage applications.
Common mode and differential mode signals
Optically isolated probes offer several advantages over conventional differential probes, with exceptional common mode rejection being one of the most important. To understand why this is important, it is first necessary to clarify how electrical signals are classified.
In electronics, signals are generally categorised into two basic forms: Common mode signals and differential mode signals. These concepts are important for analysing signal integrity, electromagnetic interference and measurement accuracy - especially in high-speed power electronics.
What is a common mode signal?
A common mode signal is a voltage component that occurs with the same amplitude and phase on two signal conductors in relation to a common reference point, usually ground. In other words, it represents the portion of a signal that is the same on both lines in relation to the reference point.
Common mode signals usually contain no useful information and are often the result of noise coupling, switching or external electromagnetic interference.
What is a signal in differential mode?
A signal in differential mode is defined as the voltage difference between two signal lines. This difference represents the actual information that is transmitted in the circuit.
In most electronic systems, the differential mode component is the desired signal, while the common mode component is considered a disturbance that should ideally be suppressed by the measuring system.
Understanding the common-mode rejection ratio (CMRR)
The common-mode rejection ratio (CMRR) is an important specification for voltage probes. It describes how effectively a touch probe amplifies differential signals and suppresses common-mode signals.
In an ideal measurement system, only the differential signal would be amplified and any common-mode voltage at the inputs would have no effect on the output. In practice, however, a portion of the common mode signal is always present. A higher CMRR value means greater suppression of this unwanted component and generally corresponds to better measurement performance.
CMRR is expressed in decibels (dB) and is calculated as the ratio between the differential gain and the common mode gain.
For example, if a probe amplifies the differential signal with a gain of 1000 (30 dB) and the common mode signal with a gain of 1 (0 dB), the CMRR is 30 dB. As the CMRR increases, the influence of common mode interference on the measured waveform decreases, resulting in a higher signal-to-noise ratio and more accurate results.
Analysing the measurement of GaN half-bridge circuits
Consider the measurement of the gate-source voltage (Vgs) of the upper switch in a GaN half-bridge circuit. In this scenario, the Vgs signal of the upper device represents the differential signal, while the drain-source voltage (Vds) of the lower device acts as a common-mode voltage source.
GaN half-bridge circuits are particularly susceptible to common-mode interference due to their extremely fast switching speeds and high dv/dt characteristics. Large transient voltages are generated during the switching processes. These rapid voltage changes generate high-frequency common mode components that can couple into the gate-source loop.
In addition, strong electromagnetic fields generated during switching operations can induce common-mode voltages in nearby conductors. The radiated electromagnetic energy can couple into the measuring loop at the exact moment of the switching process and further increase the common mode noise.
Common mode interference caused by measuring devices
In high-speed switching environments, long test leads can behave like antennas, picking up electromagnetic energy from the environment and converting it into common mode voltage. In addition, the input capacitance and resistance of a probe can form unintended voltage divider networks with the circuit under test, amplifying common mode interference.
Conventional differential probes are particularly susceptible to these effects. Optically isolated probes with MMCX or MCX connectors, on the other hand, have extremely short signal paths and a very low input capacitance, which significantly reduces susceptibility to interference.
The effects of common mode interference can be considerable. For example, in a GaN circuit with a common-mode voltage of 500 V and a dv/dt of 250 V/ns:
With a CMRR of 60 dB, approximately 500 mV of common mode interference occurs at the output.
With a CMRR of 80 dB, the interference is reduced to around 50 mV.
At a CMRR of 100 dB, only about 5 mV remain, which is normally negligible.
This clearly shows that a higher CMRR value leads directly to improved measurement accuracy.
Advantages of optically isolated SigOFIT probes
Optically isolated probes based on SigOFIT achieve CMRR values of up to 180 dB, while still offering over 100 dB CMRR at a bandwidth of 1 GHz. Thanks to this performance, they can almost completely eliminate oscillations caused by high-frequency common mode noise and display clean waveforms without unwanted artefacts.
These features make these probes ideal for testing third-generation semiconductors, including GaN and SiC power devices. In addition, impedance mismatch in high-speed signal paths can lead to reflections, which in turn contribute to common mode interference. By using a 50 Ω impedance design, SigOFIT probes minimise reflections and maintain signal integrity.
Conclusion
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