Key factors in selecting test and measurement instruments

Accuracy, resolution, and measurement speed are some of the important factors that engineers must consider when deciding how to collect and measure data. Another important factor is the environment. Is the measurement being made in a factory area with a lot of electrical noise, or in a lab with minimal electrical noise? Also, is the sensor located far away and difficult to access? In many applications, environmental factors can play a role in determining the structure.

Measurement requirements

1. Sensitivity: The smallest change in the measured signal that can be detected;

2. Accuracy: the degree of agreement between the measured value and the primary standard;

3. Resolution: the smallest part of the signal being observed;

4. Measurement speed: maximum sampling rate;

5. Bandwidth: The highest frequency signal component of the sampled signal;

6. Data storage requirements; Number of channels to be measured;

7. Number of I/Os: analog and digital;

8. Trigger: timing control and switch control;

9. Anti-interference degree: normal noise suppression ratio and its analog suppression ratio;

10. Signal conditioning; Isolation;

11. Network/bus protocol requirements, such as Ethernet and IEEE-488 (GPIB);

12. Display; easy to set up and use;

13. Calculation and analysis of collected data;

14. Size, weight and portability;

15. Power requirements; system integration issues;

16. System cost and cost per channel.

After considering these factors and identifying your requirements, you can consider the correct system architecture for your application.

Choice of instrument structure

There are four main types of test equipment structures:

1. Stand-alone instruments. The most accurate and sensitive stand-alone instruments are desktop instruments. They are traditional instruments with many new and improved features, such as graphic display, key selection function, menu programming, etc. Portable digital multimeters with self-contained power supply are used for field measurements, but generally speaking, they do not have the performance, sensitivity and accuracy of desktop instruments.

2. Computer-connected instruments. This is a subset of stand-alone instruments. These instruments are used when the number or type of measurements exceeds the capabilities of stand-alone instruments, a terminal display is required, or flexible software control is desired. Many instruments offer stand-alone capabilities as well as computer control modes for complex test and measurement systems. The external data communication bus that connects the instrument to the PC controller can use one of several standard protocols.

3. Distributed instruments. Currently provided by some manufacturers. This type of test system consists of some independent instruments connected together through a communication network. This structure consists of some miniaturized instruments, which can be placed anywhere in the factory in principle, and transmit the fully processed signals to the computer through the communication network. Many instruments meet the requirements of laboratory measurement level. Because they are located near the test signal, the cable induced noise is minimized, thereby reducing measurement errors. The biggest advantage of the distributed instrument network layout is that it eliminates the cable connection from each test point to the PC, simplifying the installation.

There can be a small local display near the instrument that can be used to read out data and find faults. You can also rely entirely on the display on the controlling computer. The data communication protocol used by distributed instruments is similar to the protocol used by computer-connected instruments.

4. PC-based test instruments. The main attributes here are measurement speed and the ability to acquire large amounts of data. There are two basic configurations. The most common is that the analog test signal is connected to a PC plug-in board that is located in a computer bus slot or on the PC parallel port. The other configuration consists of many boards mounted in a chassis, which is mounted on a rack at a considerable distance from the PC. The chassis contains measurement boards, multiplexer boards, A/D converter boards, and signal conditioning boards, which network the fully processed digital signals to the PC. The chassis system effectively expands the scale of the measurement system to a much larger number of channels than using the several available board slots in the PC.

Check parameters

An important parameter that determines the sensitivity of a digital instrument is resolution and sensitivity, where sensitivity is equal to the range divided by the resolution. Therefore, for a specific instrument measurement range, the greater the resolution, the better the sensitivity.

Sampling rate (or measurement speed) is another parameter, but this comes at the expense of sensitivity.

Also consider the desired instrument accuracy (how close the measured value is to the primary standard). Accuracy is expressed in several ways, depending on the specific instrument. But typically it is expressed as a percentage, PPM, or number of digits. Benchtop stand-alone instruments offer the highest accuracy, sensitivity, and resolution. They are suitable as a calibration source for transfer standards.