Application of Multi-channel Isolation Oscilloscope in Floating Measurement

1. Introduction

Technicians and engineers in many industries often need to perform floating measurements on two or more signals at work, that is, to measure multi-terminal signals without reference ground potential between points. This measurement is usually also called differential measurement, and the measured signal is also called differential signal.

To achieve the purpose of differential measurement, existing technologies include isolation transformers, differential probes, and isolation channels. The advantages and disadvantages of these technologies can be seen in Table 1.

From the table above, we can see that the traditional method generally uses the technology to invalidate the grounding system; the back-end isolation method generally uses the technology of isolation transformer to artificially disconnect the signal common line from the ground potential; but this technology has certain dangers, because it not only increases the voltage on the oscilloscope, increasing the possibility of electric shock to the operator, but also accumulates stress on the insulator of the oscilloscope power transformer, which will not cause failure immediately, but may cause dangers such as electric shock or fire in the future. (See Figure 1)

Figure 1: Disconnecting the ground wire to perform floating measurements is dangerous to people and equipment.

Differential or isolated probes provide a safe and reliable way to make floating measurements with a grounded oscilloscope. Neither probe contact needs to be grounded, and the probe system as a whole is isolated from the chassis ground of the oscilloscope. However, they impose new requirements on the cost and complexity of the measurement equipment; independent power supplies may be required, and their gain and deviation characteristics must be factored into each measurement; oscilloscopes equipped with differential probes focus on performance and safety (bandwidth, isolation), sacrificing the advantages of form factor construction, such as portability and cost.
In the past five years, in response to the defects of back-end isolation transformers and front-end differential probe technology, a new floating measurement technology, mid-end multi-channel isolation technology (IsolatedChannel technology), has emerged, making it possible to make floating measurements safely, economically and conveniently.

2. Working principle of multi-channel isolated oscilloscopes

A multi-channel isolated oscilloscope is a field-usable laboratory oscilloscope with 2 or 4 isolated channels. Its four isolated channel input architecture provides true and complete safe electrical isolation between channels for the "positive" input and negative reference wires.

This multi-channel isolation technology is different from traditional isolation technology. It changes the circuit structure in the middle of the measurement system, that is, the internal circuit structure of the oscilloscope. (See Figure 2)

Figure 2 Principle comparison between traditional oscilloscope and multi-channel isolated oscilloscope

The essence of a multi-channel isolated oscilloscope is a highly integrated and multifunctional laboratory digital storage oscilloscope. Its basic structure is the same as that of a traditional oscilloscope, but the isolation is based on the isolation of the instrument itself, which is mainly reflected in two aspects: (1) the channels are completely electrically isolated, and (2) the input ground wire is isolated from the mains ground wire by high impedance. At the same time, on the basis of the multi-functions of the traditional oscilloscope, it adds battery power supply and power analysis functions, and integrates the external connection interface RS232, printer parallel port and CF card drive.

3. Application of multi-channel isolated oscilloscopes

The development of multi-channel isolation technology has basically matured, with the TPS2000 series oscilloscope recently launched by Tektronix as the most representative. Its unique multi-channel isolation technology has attracted the attention of a large number of scientific researchers and engineering technicians since its launch. In a short period of time, it has been applied in fields that require floating measurements, such as medical equipment maintenance, industrial power supply design, and motor power analysis. [page]

3.1 Application in medical equipment maintenance

Medical equipment is closely related to human life. In modern treatment, its role is becoming more and more important, more and more advanced, and its size is getting larger and larger, and the cost is getting higher and higher. How to repair equipment in a timely, safe and reliable manner has become an important issue that major medical institutions need to solve. The TPS2000 series oscilloscope is compact and portable, and can be powered by batteries, which greatly improves the speed of equipment maintenance. Because most medical equipment directly acts on the human body, it requires absolute safe grounding to prevent electric shock; medical equipment is mostly large electronic equipment, which puts forward requirements for the anti-interference of electronic instruments used for maintenance. The TPS2000 channels are isolated, and its superiority in the maintenance of medical equipment is very obvious.

3.2 Application in industrial switching power supply design

In the current situation of increasingly tight energy, how to reduce energy consumption and improve product competitiveness has become the top priority in the design of industrial power supplies. Therefore, it is important to detect power and power-related parameters at any time during design. TPS2000 has a corresponding power analysis kit, which includes a high-voltage probe, a current probe, a power analysis module and a power analysis software. It can analyze switching losses (including turn-on, turn-off and conduction losses), measure harmonic distortion up to 50 times, and provide dv/dt and di/dt cursors, which can greatly shorten the design cycle.

Figure 4 Harmonic analysis using TPS2000

It can also perform power waveform analysis, including:

– Phase analysis, active power, reactive power, power factor, crest factor, phase relationship, etc.

Figure 5 Power analysis using TPS2000

3.3 Application in field measurement

In addition, multi-channel isolated oscilloscopes are widely used in field measurement. Some engineers often need to measure some signals in the field, but the power supply in the field is inconvenient or there is no power supply at all, and the measurement conditions are poor. This series of oscilloscopes provides a battery power supply solution of up to eight hours and a backlight function, which solves the problem of engineers' measurement; it also greatly improves the work efficiency of the measurement in the maintenance and repair of satellite launch equipment.

4. Conclusion

It can be seen that the multi-channel isolated oscilloscope provides engineers and scientific researchers with fast, accurate and economical multi-channel floating measurement solutions with its superior performance, and puts personal and equipment safety first. In the future, the application of multi-channel isolated oscilloscopes will increase to meet the more complex measurements of technicians.

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