Isolation improves measurement results

Several factors have an impact on the quality of measurement results. In some applications galvanic isolation is the key to major improvements. This article presents the different topologies and discusses their advantages and limitations.

Application requirements

A typical measurement problem can easily be illustrated by the example of large machinery or a production line. Figure 1 depicts a situation where two measurement points (e.g. two motors in a paper mill) are 50 meter apart from each other. A user would most likely place the measurement system in the middle and run cables to the two measurement points.

Typically both devices under test (DUT) are connected to ground and the data acquisition system (DAQ) too is grounded by its mains connection. One would assume that the potential of each ground connection is identical.

This assumption is unfortunately not always true, mainly due to load-switching combined with inappropriate ground wiring (too small wire diameter, bad connections, etc.). The potential of the ground connections at the different measurement points can change for a short period of time.

Even though these differences may occur only for a short period of time and with only a few volts potential difference, the effect produced is still significant.

When two points with different potential are connected using low-impedance cable (e.g. measurement cable), a current starts to flow and the potential difference is equalized. Assuming a voltage difference of only 1 Volt and a measurement cable with a resistance of 0.1 Ohm, a current of 10 Amps will flow.

This phenomenon is called ground loop and has the potential to damage measurement equipment and DUT as well as to interfere with sensitive measurements.

External isolation amplifier

The simplest approach is to use either isolated sensors or isolation amplifiers (see Figure 2) between the sensor and the measurement system. The ground loops are eliminated.

The advantages are: continued use of the existing DAQ system as well as availability of relatively inexpensive, modular and general-purpose isolation amplifiers of different brands and specifications on the market.

Limitations

Basically, external isolation amplifiers have three kinds of limitations.

First, the handling of an additional box which needs to be mains powered is a limitation in terms of usability for portable applications (fault finding, preventive maintenance work, etc.).

Second, the specifications of analog bandwidth, isolation voltage and accuracy are the limiting factors in terms of overall signal quality of the measurement chain.

The third limitation is given by the fact that isolation amplifiers do not eliminate the influence of the long analog signal path. In applications as described above, the electromagnetic environment has to be considered.

The higher the inrush currents and the inductive loads to be switched, the more disturbances are produced. Long measurement lines act as antennas and receive all sorts of electromagnetic energy which is then seen in the measured signal.

Internal isolation

Improvements in terms of usability and signal quality can be achieved by incorporating isolation into the DAQ system (see Figure 3).

This solution is often implemented in portable measurement equipment used for maintenance work as well as for more advanced DAQ systems.

Stand-alone external isolation amplifiers typically provide analog bandwidths from a few kHz up to approximately 50 kHz maximum; this is given by the technology used.

Internally isolated amplifiers use different technology and can reach bandwidths of up to a few hundred kHz.

External fiber optic isolation

Basically, there are two solutions available to address the problem of long signal lines in adverse electromagnetic environments. The traditional approach is to use higher-quality cables (double shielded, triple shielded), shielded cable ducts and to carefully place cables away from sources of disturbance.

A second possibility is to use fiber optic isolation systems (see Figure 4). They offer an analog-in/analog-out architecture, similar to external isolation amplifiers (Figure 2); the analog input signal is transmitted through an isolation barrier and the output is again an analog signal.

Similarity goes as far as this; most of the rest is different. The fiber optic isolation system consists of a battery-operated front-end which is placed close to the measurement point. This requires only a very short analog signal path and the effect of electromagnetic disturbance is significantly reduced.

The front-end transmitter (Tx) comprises the input amplifier as well as the analog–to-digital converter (A/D). The measured signal is digitized and the information is transmitted to a receiver which is placed close to the DAQ system.

In the receiver (Rx) the data is processed via a digital-to-analog converter and reconstructed as analog output signal. Fiber optic isolation systems for measurement purposes achieve maximum analog bandwidths of around 20 MHz.

They are mainly used to add isolation to existing measurement systems (oscilloscopes, transient recorders, DAQ systems).

Internal fiber optic isolation

The two external-isolation solutions (Figures 2 and 4) have one thing in common: an analog output signal is created from the analog input signal which is then fed into the analog input of the DAQ system.

Several specifications are important to the accuracy of the measurement and the user has to make sure that the overall uncertainty of the measurement chain still complies with the requirements of the application. While most of the time this is not a dominant factor in field measurements (fault finding, preventive maintenance), it is one of the key elements in certification and research applications.

Here, the lowest possible uncertainty of a measurement is often required.

Figure 5 shows an approach for these applications, where the receiver (Rx) of the fiber optic isolation system is directly integrated into the DAQ system.

The digitally transmitted data is directly stored and not reconstructed as analog signal anymore and hence does not have to be digitized a second time.

This improves the overall accuracy and significantly reduces the uncertainty of the measurement chain.

What is best?

What is the best technology? The question should be rephrased as follows: “What topology fits my requirements best?” The level of performance goes hand in hand with the required effort and is reflected in the price.

The first important question to answer is, whether the DAQ system can be replaced or has to be reused. Then the purpose of the isolation has to be defined. This can cover safety for personnel, eliminating ground loops, high isolation voltage to allow measurements on potential as well as distance to bridge between measurement point and DAQ.

Finally, the requirements in terms of analog bandwidth, accuracy and measurement uncertainty need to be defined as well. The last stage of an evaluation is to adjust the funding to enable the defined requirements to be met.

With its Genesis HighSpeed product range HBM offers solutions for all the above mentioned topologies. Visit www.hbm.com/highspeed for more information.