Multimeters For High-End Testing

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Multimeters—the simplest electrical testing tools—have evolved as high-end electrical and electronic testing equipment to serve complex testing needs. Today, these incorporate a higher level of integration, connectivity and advanced features

SHWETA DHADIWAL BAID


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Figure courtesy: www.eevblog.com

FEBRUARY 2011: Multimeters are the most essential and simplest test and measurement (T&M) equipment. These can aptly be called the screwdrivers of the T&M world. Originally designed for technicians to measure mere voltage, current and resistance, they saw a revolution of sorts in the last decade.

“Rapid technological developments have resulted in the need for accurate data and quick results, and instrumentation plays a very decisive role in everyday science,” shares Milind Thakkar, marketing manager, Industrial Group India, Fluke South East Asia. So, he adds, “From industrial electronic installation, maintenance and service to precision measurement and q

Multimeters, or multiple testers, are the basic testing tools that find diverse use across industries. To meet the needs of every segment, the design of multimeters has evolved from bench-top to portable handheld and lately modular form-factor. Also, their sampling rate, resolution and precision have improved to provide better measurements and greater usability at an affordable price.

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From semiconductor testing and chip designing to aerospace and automotive testing, or precision measurement and data logging in industrial testing, multimeters today fit all the requirements. T&M manufacturers are offering innovatively designed, reliable and high-precision equipment to cater to these segments.

Integration of new capabilities
To serve the complex testing requirements of today, integrating more parts, devices or features that will complement the use of particular equipment has become a unique selling point for many T&M manufacturers.

Anant K. Waghchoure, market development executive, Rishabh Instruments, says, “Adding new features to the instrument in order to give more value to the user is essential. This has been very useful as the user seeks to extract more from the given product.”

“Major features and functions of a digital multimeter include diode test, low-battery indication, continuity buzzer, data hold, auto power-off, bar-graph display, RS-232 interface jack, USB interface, function protection, shockproof protection, frequency/duty cycle (0.1-99.9 per cent) and AC frequency response (40-400Hz),” shares Rajesh Joshi, manager, education solutions group, Dynalog India.

Many more measurement capabilities could be added by using accessories and integrating more equipment. Manjunath C.S., business unit head, TTL Technologies, says, “Your digital multimeter can become a thermometer, ampere-meter or even pressure gauge. For example, current clamps can extend the current ranges of digital multimeters and are available for both AC and AC-DC environments.”

Companies have also started selling hybrid or two-in-one kind of equipment. Sadaf Siddique, senior technical marketing engineer from Agilent Technologies, says, “There are equipment that combine multimeters and power supply to address specific testing needs.”

Thakkar adds, “You also have equipment that integrate an insulation tester with a digital multimeter. High-end engineering and safety features make these lightweight tools ideal for field use. They meet the basic multimeter needs of electricians and technicians while giving them an insulation tester that generates up to 1 kV.”

Harmonic ratio measurement (for maintenance of facilities, generators and transformers), switch count (for detection of glitches in switch and relay systems) and built-in square wave generation (for designing and troubleshooting motor drivers) are some of the enhanced measurement capabilities offered by companies today.

Bob Green, senior market development manager, Keithley, says, “More intelligence has been incorporated in some digital multimeters (DMMs) so that these can act as partial controllers. Code can be downloaded into the instrument so that the instrument can control other instruments and make decisions. It is designed to reduce bus traffic and speed-up test processing. Some DMMs include switching capability for automated testing. More intelligence has been placed on switch cards to monitor relay life with switch closure counts and to automatically identify the card type and connect it to an analogue backplane.”

Data logging capabilities
Multimeters today can store the readings or perform automated data logging. It’s an essential feature when repetitive test is required for qualification or troubleshooting.

“Parameters like short-term stability, drift and fluctuations are critical to understand a circuit’s behaviour in either controlled or uncontrolled environments,” explains Manjunath. He adds, “In some cases, to troubleshoot a device, the engineer may find himself looking for elusive problems like intermittent dropouts or glitches. When tasked to measure these parameters, he may have to gather multiple instruments and write custom software routines to capture and analyse thousands of measurement data points.”

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“A common test would be voltage measurements at room, hot and cold temperatures. In such cases, it’s more convenient and efficient to automate recording of measurements while you perform other tasks at hand. It’s even better if you can record as many data points as required to further analyse the situation without worrying about the storage capacity of the instrument,” says Siddique.

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Dynaxact DX-6101 handheld DMM

Justifying with an example, Vineet Aggarwal, product manager, National Instruments, says, “In the semiconductor industry, for chip testing there are a number of different parameters to be measured. Without automation, it would be very difficult to perform complex tests. Having the software that can run through test sequences, integrating the channels and performing necessary switching operations are key aspects of PC-based ATE systems.” In addition, some modular instruments have built-in synchronisation for stimulus-response testing.

Reading the digits of multimeter
Rakesh Joshi, manager, Dynalog India, shares, “By convention, a half digit can display either a zero or a one, while a three-quarters digit can display a numeral higher than a one but not nine. Commonly, a three-quarters digit refers to a maximum value of 3 or 5. The fractional digit is always the most significant digit in the displayed value. A 3¾-digit meter can display a quantity from 0 to 3999 or 5999, depending on the manufacturer. While a digital display can easily be extended in precision, the extra digits are of no value if not accompanied by care in the design and calibration of analogue portions of the multimeter. Meaningful high-resolution measurements require a good understanding of the instrument specifications, good control of the measurement conditions, and traceability of the calibration of the instrument.

“Many vendors have invested in providing an efficient software for data logging. These software are trend-plot-style graphical representation of a logged session which has been recorded by the meter. Today, not only can the user immediately view recorded results from a logging session which could have been up to 200 hours long, allowing for an instant analysis of any unusual event without the need of a PC, multiple recording sessions can also be completed and viewed right on the meter without the need for a download first. In addition to being able to view the trend information right on the screen, a cursor is available to highlight a specific event (including event detail). You can also zoom in for a closer look at the data,” explains Thakkar.

Advancement in ADCs
Analogue-to-digital converters (ADCs) are at the heart of multimeters. So when talking of multimeters, it is important to mention the improvements in ADCs in terms of sampling rate, accuracy, form-factor as well as price.

Aggarwal says, “While DMMs and oscilloscopes are distinctly different equipment, the advancement in ADCs and other analogue components has created a lot of flexible designs.”

“While selecting an instrument, typically, the trade-off that you make is faster sampling rate and less precision, or slower sampling rate and more precision. But with advanced ADC technology, you can get both faster sampling and high precision.”

Technologies like FlexADC used in high-end digitisers and multimeters give you flexible precision and dynamic range for dynamic measurements. You can switch the sample rate as per your measurement need.

Aggarwal comments, “Earlier, in the DMM architecture, a combination of an ADC and a multiplexer was used to reduce the cost, ADC being the most expensive component of the system. But now because of the overall ADC technology, the prices have come down. We are looking at devices which have more than one ADC.” He explains, “In a switching system, if you look at multiplexing across channels, there is some type of charge build-up from channel to channel. You have to allow it to settle so that it does not affect your measurement accuracy. But with a dedicated ADC for every channel, we are creating systems with higher accuracy.”

Modular and PC-based form-factors
“A multimeter with flexibility of being modular gives an upper hand as it can be clubbed with other equipment to enhance its usability in bigger applications,” says Waghchoure. Especially for diagnostics, design and high-end applications, multimeters are always clubbed with other equipment.
Today, multimeters are available in a variety of form-factors like bench-top, handheld, USB modular, PXI modular and PC-based to fit in your application and testing requirements.

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Siddique says, “For field usage handheld models that are easy to carry, portable and very user-friendly are recommended, while for automated environments there are USB- or PXI-based modular multimeters.”

Thakkar adds, “For design applications or high degree of accuracy, a bench-top model can be chosen with closed eyes.”

Depending on the application, sometimes you may require a multimeter in a form factor that can fit in racks, especially in automated test environments. Siddique says, “Customers may not look for big displays or panel at that time as they don’t want to manually control and look at the instrument itself. Instead, they integrate various instruments and operate it from a single node like a computer. All the modular instrumentation today offers the equivalent of bench-top model specifications.

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Tektronix benchtop DMM4050

Green says, “Manufacturers have added USB and Ethernet interfaces to DMMs. Some have added the LXI protocol and incorporated Web browsers into the DMM to include data logging software and control of the DMM through a PC browser on the DMM’s webpage.”

Another form-factor gaining importance today is PC-based multimeters. Aggarwal says, “PC-based technologies don’t just control the instruments, but the PC itself becomes the instrument.”

In PC-based multimeters, there is a piece of hardware that can handle a lot of calculations and additional signal processing is done by the software. These are extremely easy-to-use, flexible systems bringing the advantage of bigger displays with more parameters and graphs, and data logging for further analysis.

Display technologies
Display is a key part of any T&M instrument. It is the first parameter or specification that appeals to the user and makes the instrument more user-friendly.

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Agilent U1253B True RMS OLED multimeter

“Generally, a digital multimeter has a large and intuitive display that is simple to use and easy to read,” says Sai Venkat, Pacific Marketing, Tektronix.

The earlier 8-LED displays have now been replaced with LCD and organic LED (OLED) displays in many devices. Siddique shares, “DMMs with unique OLED display provide 160-degree viewing angle and 2000:1 contrast ratio. That means you can read razor-sharp readings in pitch-dark and off-angle situations.”

“DMMs show more information (graphics or statistics on measurements) on their displays. Reading rate has also increased substantially over the years,” cites Green.

Communication interfaces
Connectivity is a key specification of every equipment today. There is a need to transfer data to store and analyse as well as view on a bigger screen. This has made wired and wireless connectivity an important component of the multimeter circuitry.

USB and Ethernet connectivity allows direct transfer of readings of various parameters. In PC-based equipment, all the features of a computer, like Internet connectivity, can be effectively used to share test and measurement results.

Innovative concepts like detachable displays or remote displays have gained popularity in the industry. A wireless, removable magnetic display allows you to be 9 metres (30 feet) away from the measurement point and is perfect for those difficult measurements where viewing the display is challenging. It gives users ultimate flexibility in unusual test situations. There is no more juggling of leads and the meter while stretching into tight spots. This technology uses a 2.4GHz ISM band wireless transmitter to immediately send measurements to the detached display, improving safety and increasing productivity for today’s busy industrial and commercial electricians and electronic technicians.

Improved electronics circuitry
Refinement of the design using new technology tools has increased the performance of T&M equipment. With better circuitry and electronics, a lot of features can be integrated now. So while earlier multimeters could do only voltage and current measurements, new DMMs have far more capabilities.

Joshi cites, “Better circuitry and electronics have improved the measurement accuracy. Older analogue meters had basic accuracies of 5 to 10 per cent. On the other hand, today’s portable DMMs have accuracies as good as ±0.005 per cent, while bench-top instruments have accuracies in single-digit parts per million figures.”

PC-based multimeter (Courtesy: National Instruments
PC-based multimeter (Courtesy: National Instruments

DMM itself being an electronic device, it is necessary that its own electronics does not interfere with the measurements. “All digital meters contain a battery to power the display. So they use virtually no power from the circuit under test. This means that on their DC voltage ranges, they have a very high resistance (input impedance) of about 10 mega-ohms, and are very unlikely to affect the circuit under test,” explains Joshi.

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The design of the electronic circuitry has well adopted the use of every advanced piece of silicon available in the market. “Faster and more capable microprocessors help increase data transfer rates. Communication chips that handle USB and Ethernet are common. Scalable FPGAs are used along with ASICs to enhance both analogue and digital processing. FPGAs and ASICs allow for more measurements and more computations. Some instruments have USB ports for USB memory sticks, so data can be captured and transferred to any computer,” says Green.

Some of the design considerations include the type of measurement and readings being displayed. Manjunath says, “Conventional DMMs that do not follow the true-RMS measurement are accurate as long as the AC signal is completely sinusoidal in shape. But today, due to increased use of electronic control equipment, we hardly find any sinusoidal waveform in industrial and commercial environments. This highly inaccurate way of measurement may lead to failure in design, operation and maintenance of electrical equipment and cause severe damage to equipment and personal safety. So a true-RMS multimeter is a must.”

“Use of surface-mount technology has made the design of equipment much more compact, reducing the size and weight of the equipment, while increasing the reliability of equipment considerably,” adds Waghchoure.

High accuracy, reliability and safety
When it comes to measurement of electrical quantities like voltage and current, there is always some risk associated due to mishandling or harsh environmental conditions. Siddique points out that many a time users need to use the equipment in high-voltage and high-current environments, which may be harmful for the instrument as well as the user.

“Reliability and safety, especially under tough conditions, are more important than ever before,” says Thakkar. He adds, “DMMs are designed with latest safety standards like the IEC category and voltage rating approved for the environment where the measurement is to be made. Choosing a meter with this rating which also has a UL, CSA, VDE or TÜV certification means the meter not only has been designed to IEC standards but has been independently tested to meet these standards.”

“Accuracy and resolution usually found only in bench-top DMMs of the past are now common in their handheld counterparts,” claims Thakkar. “Some equipment have basic DC voltage accuracy of up to 0.09 per cent and resolution of 6000 counts.” All the meters can be calibrated and have traceability to industry standards, just like their bench counterparts.

Manjunath adds, “Overload protection prevents damage to both the meter and the circuit while protecting the user. Also, some special high-energy fuses provide extra protection for the user and meter during current measurements and overloads.”

Need of the Indian market
Multimeters is a highly competitive market where several established local and global players have created a market for themselves in India.

Siddique says, “The Asia Pacific region is a very important market for multimeters with maximum demand coming from China and India. End users in these regions are highly cost-conscious demanding the best price-value combination.”

Vendors are offering unique features catering to various segments of the Indian market. Sai Venkat says, “With the requirements of flexible configurations, quick setup and affordability, the instruments have gained interest from the electronics industry, educators and students who continue to look for value-added instruments to achieve cost advantages in the ever-challenging world.”

Apart from affordability, ease-of-use, safety and reliable testing, there is also a need for communication in local language. Today, vendors offer a lot of new features and capabilities in handheld multimeters, such as visual and audible continuity indication, IP-54 certification (for water- and dust-resistance), PC connectivity and CAT III/IV safety protection along with user manuals in diverse Indian languages such as Hindi, Bengali, Tamil, Malayalam and Kannada.

Thakkar says, “When evaluating the suitability of several digital multimeters, the best approach is to choose a set of measurements and conditions that approximates your application. Choosing a DMM for the job requires not only looking at basic specifications but also the features, functions and the overall value represented by a meter’s design and the care taken in its production.”


The author is a senior technology journalist at EFY

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