What is a floating signal and how to measure it with an oscilloscope

There are many challenges in troubleshooting in industrial environments. Modern production sites need to analyze low-voltage digital signals when debugging electronic systems, and also need to verify the quality of 380V high-voltage power supply or test the power efficiency of electrical drive equipment. Testing floating signals has always been a problem that electrical engineers cannot avoid.

The reason why ordinary digital oscilloscopes are not suitable for testing such systems is that most ordinary oscilloscopes are safely connected to the earth through an AC power supply. In general, such a ground connection improves the safety of the test. However, for floating ground systems, this will cause the reference ground of the system under test to be forcibly connected to the earth through the reference ground of the oscilloscope single-ended probe, thereby changing the ground loop structure of the system under test, affecting the normal working characteristics of the system under test, and even causing damage to the system under test and the test instrument.

1. What is a floating ground signal?

Ground Measurement

◾Measurement of ground signal

◾Signal reference terminal ground

◾Measurement is possible using a single-ended probe

Floating measurement

◾Measurement of differential signals

◾The signal reference end is not grounded, and there is a common mode voltage

◾Cannot measure with single-ended probe

2. Where does the floating ground signal appear?

◾Transportation: cars, ships, airplanes, railway systems

◾Power supply and power electronic systems: MOSFET or IGBT, GaN or SiC power semiconductor devices, motors

◾ Differential signals in digital circuits: MI PI, USB, Ethernet, MIL-STD1553B, ARINC429

◾Power transmission: power supply network, power line carrier communication, super charging station

Since most oscilloscope channels use a common ground design, that is, the reference grounds of different input channels are connected together. When the signals of different subsystems with isolated reference grounds are tested and debugged simultaneously, the measured signals with different reference grounds will be forced to connect the reference points together through the oscilloscope's common ground design, affecting the normal operation of each subsystem. Even if powered by a battery, the isolation of each test channel cannot be achieved.

Comparison of test methods:

Three factors to consider when choosing an oscilloscope for floating measurements

Method 1: Differential Probe

High cost, limited voltage resistance of specific differential probes, and not portable.

Method 2: Subtracting two single-ended probes

The CMRR (common mode rejection ratio) capability is poor, the noise is large, and the smaller differential mode cannot be measured.

Method 3: Floating the oscilloscope

Danger! The oscilloscope case and the exposed BNC interface are charged. Electric shock may occur when the voltage at the floating reference terminal is too high.

Method 4: Isolate the power supply

Do not measure multiple floating signals with different references at the same time. Even if the oscilloscope is floating, there is still a risk of electric shock.

Method 5: Isolate the Benchtop Oscilloscope

The floating reference voltage has limited voltage withstand capability, generally around 30 Vr ms, is slightly less portable, and has average performance (bandwidth/sampling rate/storage depth).

Method 6: Channel Isolation Handheld Oscilloscope

◾Channel isolation, single-ended probe can be used directly to measure floating signals, with low cost;

◾ Isolation between channels enables simultaneous measurement of multiple floating signals with different references;

◾Portable battery-powered, suitable for different measurement environments (laboratory + field);

◾Stronger pressure resistance.

In fact, the battery-powered and isolated channel design of handheld digital oscilloscopes have a good response to the above test challenges. However, most handheld oscilloscopes on the market cannot meet the current floating signal test needs in terms of test performance. Compared with desktop oscilloscopes with continuously improved performance, most handheld oscilloscopes have a large gap with user needs in terms of bandwidth, sampling rate, recording depth, waveform capture rate, trigger capability and display resolution, which greatly affects the application of handheld oscilloscopes in this test field.

In addition to being an excellent oscilloscope, the R&S®RTH can also be

logic analyzer

Protocol Analyzer

Data logger

Digital Multimeter

Spectrum Analyzer

Harmonic Analyzer

frequency meter

The R&S®RTH integrates many innovative functions.

To meet the test application requirements of various environments

◾Out-field power testing: high voltage and high energy (CA TI II&IV) installation and maintenance;

◾Laboratory electronic testing: Testing ungrounded power electronic equipment, such as "floating" measurements;

◾Electrical and electromechanical testing: multi-purpose (measurement, monitoring, high voltage).