Computer-based virtual measuring instruments are less expensive than traditional box-type measuring instruments and have seen rapid adoption in recent years. Like traditional box-type measuring instruments, they have a calibration validity period and therefore require periodic calibration to ensure measurement accuracy. This article introduces methods for internal and external calibration of computer-based measuring instruments. Computer-based measuring instruments offer great flexibility, leading to their increasing popularity. By controlling instrument functions, measurement systems that meet specific requirements can be developed. Cost is the primary consideration for any measurement system. Developing a computer-based measuring instrument is often several times cheaper than purchasing a standalone benchtop instrument. This is due to lower hardware costs, reusable software, and the fact that one test instrument can often replace several independent measuring instruments. Computer-based measuring instruments are closely linked to the computer industry and benefit from advancements in computer technology, including open communication standards, web servers, and simple interfaces for spreadsheets and word processing between instruments and desktop applications. These measuring instruments also benefit from the stability and decreasing price of computers, allowing for continuous performance improvements without additional cost. Achieving Precise Measurements Through Calibration Most measuring instruments provide information on the accuracy of their measurement circuitry in the form of accuracy tables. Accuracy tables help determine the overall uncertainty of a measuring instrument; however, these precise specifications only apply to successfully calibrated circuit boards. Therefore, you must use these specifications to verify the board's operation before and after measurement adjustments. The ability of a measuring instrument to accurately measure changes in physical quantities varies according to certain factors. Lifespan, temperature, humidity, exposure to external environments, and misuse all affect measurement accuracy. Calibration quantifies the uncertainty of a measurement by comparing the obtained test results with known standards. It verifies whether the measuring instrument is operating within its specified range. If the instrument's measured value exceeds the published uncertainty, the measurement circuitry must be adjusted to conform to the published specifications. Just as traditional measuring instruments require calibration over time, computer-based measuring instruments also need calibration. Users should choose computer-based test instruments with both internal calibration (also known as automatic calibration) and external calibration tools. Internal Calibration If you are using an instrument such as an oscilloscope, you have already completed internal calibration. In fact, most oscilloscopes have already completed internal calibration when you change the vertical range setting. Essentially, the instrument digitizes a high-precision on-board voltage source, compares its reading to a known value, and then stores the calibration factor in its own electrically erasable read-only memory (EROM). This on-board voltage source is also calibrated to well-known standards such as NIST. The main purpose of internal calibration is to compensate for changes in the operating environment, variations in internal calibration temperature, and other factors that may affect the measurement. Like traditional measuring instruments, computer-based measuring instruments should support internal calibration. Internal calibration of computer-based measuring instruments is initiated by calling a software function that calibrates the measurement circuitry. Because measurements can be performed immediately and there is no need to wait for this internal calibration whenever the vertical range is adjusted, software-controlled internal calibration saves testing time. Computer-based measuring instruments are installed in environments such as desktop computers, PXI/CompactPCI chassis, or VXI/VME chassis. Because computer-based measuring instruments are installed in various computer environments, designers should remember that they are susceptible to electromagnetic interference and power supply voltage variations, and must operate over a wide temperature range. Traditional measuring instruments, increasingly integrated with personal computers, face similar challenges. The most basic solutions for eliminating electromagnetic interference include: separating the ground planes of digital and analog signals, locally filtering power signals, and shielding sensitive components. To compensate for voltage source variations, DC-DC converters can be used to boost the power supply voltage, voltage regulators can be used to control the on-board power supply voltage, and large capacitors can be used to eliminate harmonics in the on-board power supply. On-board temperature sensors and internal calibration can be used to perform calibration at different temperatures in the operating environment. Information on these design techniques can be found in the NI white paper titled "Accurate Measurement with PC-Based Data Acquisition Hardware." Many users wonder if it's possible to move a computer-based measuring instrument from one computer to another without affecting calibration. The answer is yes; if the measuring instrument is designed according to the above standards, calibration remains valid. Computer-based measuring instruments, like traditional instruments, are typically calibrated in a metrology laboratory. The operating temperature of this laboratory may differ from that of the production workshop or design laboratory. Installing a computer-based measuring instrument on a new computer is no different from moving a traditional oscilloscope from one environment to another. Because instruments are designed for various operating environments, all calibrations remain valid, and the aforementioned white paper includes test results explaining why. When selecting measuring instruments, ensure that computer-based instruments support internal calibration. For ease of use, the internal calibration process should be automated, meaning it should not require adjustments to voltage dividers or jumpers in the device. For example, internal calibration of NI's 12-bit data acquisition products can be completed automatically across the entire range in approximately 10-20 seconds. External Calibration After a period of time, typically a year, the on-board voltage source used for internal calibration needs to be calibrated to a known standard. This on-board power supply calibration process is an example of external calibration. External calibration uses high-precision external standards. During external calibration, the on-board calibration constants are adjusted against the external standard. Similar to internal calibration, external calibration does not require adjusting voltage dividers or moving jumpers. External calibration is typically maintained by a metrology laboratory or other traceable institution. Once external calibration is complete, the new calibration constants are stored in a protected area of ​​the measuring instrument's memory and are inaccessible to the user, thus protecting the integrity of the calibration from accidental adjustments. Regardless of the type of measuring instrument, manufacturers must provide the corresponding calibration procedures and the calibration software necessary for external calibration on computer-based measuring instrument devices. Manual and Automated Calibration Procedures To meet engineers' needs for calibrating computer-based measuring instruments, both manual and automated calibration schemes are available. Automated calibration systems are fast, require no human intervention, and provide detailed calibration records for compliance with standards like ISO-9000. Manual calibration procedures provide comprehensive information for users who want to directly embed calibration functionality into their measuring systems, avoiding the hassle of transporting computer-based measuring instruments back to a metrology laboratory. Manual calibration procedures explain how to perform external calibration of the measuring instrument; these procedures are typically sold as part of the measuring instrument's maintenance manual or can be downloaded from the manufacturer's website. The disadvantage of manual calibration procedures is that they are time-consuming and labor-intensive, not due to the difficulty of adjusting the measuring instrument, but because of the lengthy measurement verification process. To meet the requirements of calibration guidelines, the performance of the measuring instrument must be verified before and after calibration. Only in this way can it be determined whether the measuring instrument is operating within the specifications before and after calibration. For example, external calibration on an E-series data acquisition device requires measurements of gain, dynamic range, and polarity, necessitating hundreds of measurements. Products like Fluke MET/CAL and the NICalibration Executive tool include descriptions of how to automate this process in metrology laboratories, significantly reducing the time required for external calibration. Communicating with external standards via GPIB, the calibration software can read external voltage values ​​from instrument settings. These values ​​are then used to verify and calibrate the adjusted instrument. At the end of the calibration process, the instrument specifications are automatically read from the configuration file, generating a detailed calibration report (Figure 1). Automated calibration software allows for simultaneous measurements of several instruments in the time required to calibrate a traditional measuring instrument, especially when a voltmeter is not present in the device under test. These tools are designed to meet the stringent calibration requirements of metrology laboratories. Large companies with their own metrology laboratories typically offer manual procedures and calibration software products. For companies without metrology laboratories, data acquisition and measurement instrument companies usually have to collaborate with metrology service companies worldwide. There are generally two types of external calibration certificates. One is a basic calibration certificate, usually generated after product manufacturing and provided by the measuring instrument manufacturer. This type of certificate allows NIST or local standards verification bodies to trace the instrument's origin and information about the measurement environment conditions during the calibration validity period. Certified metrology laboratories typically provide detailed calibration certifications. These certifications, in addition to providing the same information included in the basic certification, refine the data before and after each measurement. Detailed calibration certifications should comply with specific guidelines such as ANSI-Z540-1, which are primarily used in the United States, or more commonly, ISO Guideline 25. These guidelines ensure continuity of calibration, and most ISO-9000 certified companies comply with them. Summary Computer-based measuring instruments, like traditional instruments, require periodic internal and external calibrations to ensure the required accuracy. A well-designed computer-based measuring system should provide users with tools and services for automated internal calibration, as well as methods for automated external calibration.