How to use design convenience resources to simplify product design from the perspective of electromagnetic flowmeter design

According to a survey conducted by a leading domestic electronic technology media, analog and mixed-signal circuit design is one of the most challenging design tasks for engineers. For engineers engaged in weak signal acquisition, such survey results are by no means surprising. Xiao Ma, a core design engineer of a domestic metering equipment provider, has a deep understanding of this. He experienced the selection, verification and prototype design of the electromagnetic flowmeter last year. The experience gained in the process may be a good reference for more engineers engaged in similar work.

Flowmeters are products that are not familiar to the public, but they are widely used in the industrial field. The global market for this product is worth billions of dollars. However, traditional mechanical flowmeters have long been popular, and price competition is fierce, and profit margins are shrinking. Differentiation and customized production have become an important breakthrough point in the fierce competition among manufacturers. How to effectively obtain the actual needs of users and improve traditional products is a big challenge for manufacturers in the differentiation and customization of production processes. More and more users are accepting and using new flow measurement technologies. Among them, electromagnetic flowmeters are widely adopted by flowmeter manufacturers because they have no pressure loss and are not affected by viscosity, fluid density, temperature, pressure or conductivity. They are very suitable for high-precision measurement of pulp, mud, sewage, etc.

Xiao Ma's company is one of them. The company planned to design a new electromagnetic flowmeter at the beginning of last year. As a technical backbone, Xiao Ma was responsible for the entire process of the project from project establishment to design, but he has always been better at digital circuit system design, and the design of the team's analog front-end circuit has given him a headache. However, during the process, he promptly used some of ADI's design resources, which effectively helped him greatly shorten the R&D process and has roughly achieved the original design goals of the system.

Simplify analog circuit component selection and evaluation using tool kits

As a mature product, the system design of electromagnetic flowmeter is not simple, involving power supply, electromagnetic excitation, signal conditioning, analog-to-digital conversion, processor, display, keyboard, logic I/O and multiple communication mechanisms, etc. At the beginning of the project, Xiao Ma first encountered the thorny problem of key component selection. In addition to the system processor, which was basically finalized as a 32-bit AMR core MCU that he was very familiar with, the front-end operational amplifiers, instrumentation amplifiers, high-precision analog-to-digital converters, digital-to-analog converters, and digital-to-analog signal isolation circuits were all undecided.

Xiao Ma, who had successfully used a 14-bit ADC from ADI, first locked on the 16-bit analog-to-digital converter products provided by ADI for high-precision data converters. As ADI occupies nearly half of the global data converter market (47%, according to data from market research firm Databeans in 2010), it can provide design engineers with a variety of analog-to-digital converters. More importantly, ADI also provides convenient design resources for the reasonable use of these converters, including application notes, design manuals, FAQs, and various tutorials.

In the last product design, Xiao Ma successfully completed simulation, selection, evaluation, and solved common problems through the VisualAnalog design software toolkit and ADC evaluation board. In the process, he also referred to the small tutorials and videos on the use of tools and reasonable settings of the evaluation board provided by ADI, paving the way for subsequent designs. Xiao Ma was impressed by the rich design resources. In fact, ADI provides a convenient evaluation kit for each high-performance analog-to-digital converter, which can help engineers easily complete performance evaluation and selection.

Figure 1: The AD7682 hardware and software evaluation platform provides comprehensive performance evaluation.

Among the several 16-bit analog-to-digital converters provided by ADI, Xiaoma's team initially decided to choose AD7682 after comprehensively comparing key characteristics such as conversion rate and noise. In addition to the fact that the unipolar single-ended and differential inputs and single power supply support for bipolar inputs are key features required for their project, the ADC was chosen mainly because of its competitive price and low power consumption (the latter is especially important for battery-powered electromagnetic flowmeters), which are also important competitive advantages that domestic electromagnetic flowmeter manufacturers need to pay attention to. The complete evaluation kit (evaluation board and evaluation software platform) provided by ADI for AD7682 provides great convenience for the subsequent verification of ADC performance characteristics. Based on this kit, Xiaoma's team easily completed a comprehensive performance evaluation of AD7682 in electromagnetic flowmeter applications.

At the same time, in the selection of operational amplifiers and instrumentation amplifiers, Xiao Ma also selected ADI products as the main alternative. In the last design, he still remembered the handy use of Multisim. The free downloadable simulation software NI Multisim ADI version, developed by ADI and NI, can help design engineers simulate circuit design and select the most suitable amplifier chip for their projects. NI Multisim ADI version simulation software uses industry-standard SPICE technology to provide an intuitive simulation environment for testing and simulating analog circuits, including selecting and evaluating more than 400 ADI amplifier product models. By simulating the designed virtual circuit prototype, you can detect errors in the design process earlier, thus avoiding expensive and time-consuming repeated prototype design. In fact, ADI has also added relevant specialized simulation modules (such as simulation modules for key characteristics such as amplifier stability) in Multisim for electromagnetic flowmeter design for easy use.

The AD8667 low-noise amplifier with extremely low leakage current designed for battery-powered applications and the AD8220 instrumentation amplifier launched by ADI to replace the classic AD620 amplifier were selected as amplifier chips for their design. In terms of the choice of DAC, Xiao Ma's team used the AD5422 16-bit DAC from ADI that is suitable for industrial environments and has many integrated diagnostic features.

After deciding to use ADI's analog-to-digital converter and amplifier chips, the Xiaoma team, which had several successful sample application experiences, applied for five samples each on the same day through the ADI free sample application link, and also purchased evaluation boards suitable for the relevant samples. Since these were the most commonly used analog-to-digital converter and amplifier products applied for, ADI's local stocking of popular components allowed the Xiaoma team to obtain the key components needed for the project three days later. [page]

Lab Circuits and Reference Designs Overcome Design Challenges

In view of the fact that analog, mixed-signal and RF designs are usually very challenging for engineers, ADI creatively launched the "laboratory circuit concept" in 2008: based on resources collected from different regions, customer opinions, and demand feedback, ADI combines the technical capabilities and experience of internal experts to design practical circuits required by customers, and introduces the circuit design ideas, design methods, design suggestions and test results in detailed text and pictures to help design engineers cope with today's analog, mixed-signal and RF design challenges and achieve system integration easily and quickly.

The front-end design of electromagnetic flowmeter is a typical analog and mixed signal circuit design, which usually encounters the following challenges: the maximum measurement range of electromagnetic flowmeter products may be as wide as 1500:1, and the range used to reflect the flow rate is 0.01m/s to 15m/s; isolation is required between system power supply, central logic unit, communication and I/O, and the isolation level requirements range from 1kV to 2.5kV; portable electromagnetic flowmeters require ultra-low power microcontrollers, amplifiers and ADC devices... Design engineers must consider many different system requirements, including accuracy, bandwidth and electromagnetic excitation frequency, in order to design a reasonable electromagnetic flowmeter system. In addition, the temperature environment of industrial sites is very complex and sometimes even causes extremely harsh effects. EMC immunity (such as ESD, EFT and surge, etc.) is also a major challenge in the design of electromagnetic flowmeters. In order to be able to work over a wide temperature range, low temperature drift coefficient and low power consumption are crucial for electromagnetic flowmeters. In addition, the space inside the electromagnetic flowmeter is limited and the system density is very high, so the device size must be reduced.

In Xiaoma's project, they made full use of ADI's laboratory circuits to efficiently overcome the above technical challenges. Among the many laboratory circuits provided by ADI, five laboratory circuits, such as "Complete high-speed, high common-mode rejection ratio (CMRR) precision analog front end suitable for process control applications" and "Using single-chip voltage and current output DAC AD5422 and digital isolator ADuM1401 to build a 16-bit fully isolated output module", provided very high reference value for Xiaoma's electromagnetic flowmeter design.

Under common voltage, the output range of electromagnetic flowmeter sensor may be as small as tens of microvolts, and the output impedance is often higher than the megohm range. The front-end precision operational amplifier or instrumentation amplifier requires ultra-high input impedance, extremely low leakage current and excellent common-mode rejection ratio. The laboratory circuit "Complete high-speed, high common-mode rejection ratio precision analog front end for process control applications" helps to solve these challenges of analog front-end circuit design. The optimized high-performance analog front end can not only provide the high precision required by the electromagnetic flowmeter, but also provide a high common-mode rejection ratio. High precision, high input impedance and high CMR are provided by the instrumentation amplifier AD8226, which can provide excellent CMR performance required for small and large signal inputs.

Figure 2: An optimized high-performance analog front-end lab circuit provides the high accuracy and high common-mode rejection ratio required for electromagnetic flow meters.

Figure 3: Laboratory circuit of a 16-bit industrial control output module suitable for electromagnetic flowmeters based on the AD5422 and ADuM1401 building blocks.

Since Xiao Ma's team used ADI's 16-bit DAC AD5422, and in the laboratory circuit referring to "Using single-chip voltage and current output DAC AD5422 and digital isolator ADuM1401 to build a 16-bit fully isolated output module", because the laboratory circuit combined with the good circuit design concept and comprehensive performance achieved by the digital isolator ADuM1401, they abandoned the optical isolation devices that had been used before in the selection of isolators, and used ADI's iCoupler digital isolator ADUM1401 for the first time. In their circuit, ADuM1401 not only provides all the signal isolation required between the microcontroller and the DAC, but also its own isolated power supply guarantees isolation performance. High integration is also very important for electromagnetic flowmeters with limited space. The laboratory circuit also contains standard external protection functions, and has been tested and verified to be fully compliant with the IEC 61000 standard.

In fact, the specific circuit design of each product often has different changes. In ADI's laboratory circuits, more targeted suggestions are usually made foreseeably for various changes such as different parameter requirements, device selection and device matching that may be encountered, which greatly improves practicality. In addition, the performance of any high-speed circuit is highly dependent on proper PCB layout, including power bypassing, controlled impedance lines, component layout, signal routing, power and ground layers, etc. ADI laboratory circuits usually provide details on PCB layout, related reference tutorials, and a complete design support package for this circuit note (circuit schematics, PCB files, BOM lists, etc.)

ADI is about to launch a complete system-level reference design for electromagnetic flowmeters. If this reference design is adopted, their design cycle can be further shortened by 1/3 or even half (this series of reference solutions can currently provide users with complete design documents including schematics of a full set of electromagnetic flowmeters, detailed PCB files, related software modules, test and measurement data, BOM lists and other resources). In fact, ADI provides system-level reference design solutions that integrate ADI's leading analog, hybrid, RF and digital technologies for many popular applications.

summary:

While writing this article, I learned that ADI recently won the "Outstanding FAE Support" award from a well-known domestic media. Yes, in addition to the world's leading technology solutions, excellent localized technical support has always been the service that ADI emphasizes, because ADI has been serving local Chinese design engineers for a long time and has deeply realized that unlike European and American countries, Chinese engineers are relatively young and they are eager to advance in technology and get more technical support from manufacturers.

However, the current demand of Chinese enterprises for design services from semiconductor solution providers is not only to provide advanced solutions and more effective on-site technical support, but also to provide design tool kits, verification models, evaluation boards and comprehensive tools and documentation support such as reference designs, laboratory circuits and application notes required for each stage of design. It has become the demand of more engineers and manufacturers. It is for this reason that ADI upgraded its popular laboratory circuits and online sample application procedures last year, continuously enriched design materials such as laboratory circuits, application notes, and complete solutions, and provided young engineers with a full range of online and offline technical support. In addition to field application engineers, the technical team of ADI's Asian Technical Support Center will also handle and answer various technical questions from customers through telephone and email, and provide design reference information. A highly active ADI Chinese Technical Forum has also been opened. If design engineers can properly use these resources, they will be able to accelerate the growth of their technical capabilities, overcome various technical challenges often faced in the design process, and thus shorten the product development cycle.