0 Introduction With the development of microelectronics and computer technology, computers have been introduced into electronic measuring instruments, forming computer-based automatic testing systems. Computers are low-voltage devices, therefore, automatic testing systems are highly susceptible to interference. Grounding interference, in particular, can cause errors in system test data, and in severe cases, can lead to "program overload," rendering the entire testing system malfunction. Therefore, good grounding technology is essential in the design of computer-based testing systems. 1 Grounding Overview Using the earth as a zero-potential reference is based on the earth's extremely high capacitance. However, the earth is not a good conductor; its conductivity is between that of a conductor and an insulator, making it a semiconductor. When two different potential points within an electronic system are grounded separately, current flows through the grounding loop, creating common-mode interference and causing adverse effects. Therefore, the earth is only considered a zero-potential reference when grounded to a single point. The term "grounding" usually refers to single-point grounding. Proper grounding is crucial to prevent interference with weak signals within the system. While the earth can serve as a zero-potential reference, automatic testing systems also require a potential reference surface (simulating ground potential). Except for lines operating at high potential, a reference point and other metal bodies in the line must be kept at a stable potential. Generally, the outer casing or mounting metal plate is used as the potential reference surface and connected to the ground. The purpose of grounding is: (1) to prevent the casing from becoming energized when high-voltage components and wiring break down, and to prevent personnel from being electrocuted. It is also called "protective ground"; (2) to reduce electromagnetic interference caused by common impedance or other couplings, so that the automatic test system is stable and reliable. This is called "measurement ground". 2 Types of ground wires Computer-based automatic test systems have many types of ground wires. The following types are summarized: (1) Digital ground, also called logic ground. It is the zero potential of the digital circuit in the test system; (2) Analog ground, which is the zero potential of the input signal of the amplifier, sample/hold circuit (S/H) and A/D converter; (3) AC ground, the ground wire of the AC 50Hz power supply, which is a noise ground; (4) DC ground, which refers to the ground wire of the DC power supply; (5) Signal ground, which refers to the ground of the sensor; (6) Power ground, which refers to the zero potential of the high current network components; (7) Shielding ground, which is designed to prevent electrostatic induction and electromagnetic induction, also known as the chassis ground. Whether the grounding problem is handled correctly or not will directly affect the normal operation of the system. Grounding includes two aspects: whether the grounding point is correct and whether the grounding is reliable. The former is used to prevent crosstalk between different parts of the control test system, and the latter is to make each grounding point well connected to prevent voltage drop on the grounding wire. Here are some grounding techniques for your reference. 3. Grounding Technology 3.1 Single-Point Grounding For low-frequency systems such as audio and ultrasonic systems, the parasitic inductance of wiring and components has little impact, so single-point grounding is the best approach. From an economic and interference prevention perspective, circuits can be categorized into three types based on the power used: high-level interference circuits; medium-level general circuits; and low-level sensitive circuits. A star-connected grounding method is used, with each ground wire individually connected to the main grounding point of the test system before being grounded. The wiring length should be kept as short as possible to reduce the impact of parasitic inductance and capacitance. When the frequency is between 1 and 10 MHz, the ground wire length should not exceed 1/20 of the wavelength; otherwise, multi-point grounding should be used. A single-point grounding system is shown in Figure 1. 3.2 Multi-Point Grounding To prevent interference introduced by common impedance coupling, single-point grounding is generally preferred. In high-frequency circuits, the inductance of the grounding wire, the parasitic capacitance between them, and the additional loops formed by shielding can disrupt single-point grounding. Therefore, it is necessary to connect each grounding point to a grounding conductor surface to create a single low-impedance path to weaken the impact of electromagnetic interference. Therefore, when the frequency is higher than 10 MHz, multi-point grounding should be used. 3.3 Connection Technology of Digital Ground and Analog Ground Digital ground mainly refers to the ground terminal of digital logic circuits such as TTL or CMOS chips, I/O interface chips, and CPU chips, as well as the digital ground of A/D and D/A converters. Analog ground refers to the ground terminal of analog signals in amplifiers, S/H sample-and-hold circuits, and A/D and D/A converters. In computer-based test systems, digital ground and analog ground must be grounded separately. Even if there are two types of ground on the same chip, they must be grounded separately and then connected at only one point. Otherwise, the digital loop will return to the digital power supply through the ground line of the analog circuit, interfering with the analog signal. For example, the grounding of computer data sampling in a test system is shown in Figure 2. 3.4 Grounding Technology of Automatic Test Systems In a complete computer-based automatic test system, there are generally three types of ground: one is the low-level circuit ground, such as digital ground and analog ground; another is the ground of high-voltage components such as relays, motors, and electromagnetic switches (temporarily referred to as noise ground); and the third is the outer casing ground of the machine housing and instrument cabinet (referred to as metal ground). If the instrument or equipment uses AC power, the power ground should be connected to the metal ground. During system connection, these three ground wires should be grounded at a single point, as shown in Figure 3. 4 Conclusion In a computer-based automatic testing system, grounding issues are often complex because poor grounding manifests in various ways, with different interference phenomena that are difficult to grasp. The testing system needs to employ good grounding techniques to minimize the length of the grounding wires and avoid crossovers as much as possible, thereby reducing ground loops and coupling between ground current and the circuit. Furthermore, the grounding plane must be carefully protected to prevent damage and vibration, avoiding the formation of high impedance in the grounding system. Ultimately, this effectively reduces ground interference. References: 1. Pan Xinmin et al. Microcomputer Control Technology. Beijing: People's Posts and Telecommunications Press, 1999.6 2. Bai Tongyun et al. Electromagnetic Interference and Compatibility. Changsha: National University of Defense Technology Press, 1991.2