This ground connection in a USB cable and the safety grounds of the host and peripheral forms a ground loop which can cause the peripheral’s ground potential to move relative to the host’s and result in unreliable communication (see AN-375, AN-727]. The host device provides power and ground to the peripheral device. The directionality of data packets is established through the USB protocol, not by control signals. A host device controls the bus and communicates with a peripheral.

USB communication occurs over a single bidirectional differential pair (the D+ and D− signals in Figure 3). Digital isolators are non-optical isolators, which use CMOS interface ICs to transfer information through capacitive or magnetic coupling (Ott).Ĭonnecting two ac powered devices with a USB cable can cause a ground loop that disrupts communications through the bus. Digital isolation technology can be used to break ground loops without compromising the interface’s performance, and relatively few components in simple application circuits. Power dissipation in the LED and pull-up resistor can become significant as multiple optocouplers are used. Optocouplers’ have significant propagation delays making them only useful for low-speed signals. Optically isolating interfaces can become complex, expensive and require a significant amount of board space. This approach has limitations as the performance and complexity of the interface increase. This interference degrades the signal that is seen by Device #2 and hinders the transmission. These two ground connections create a large loop, which picks up a noise voltage from the magnetic field of a nearby interferer. There is a second low impedance path connection between the devices’ grounds through the safety grounds of their power supplies. For example, the ground connection could be the shield of a coaxial cable. The signal line is grounded at either device. Device #1 drives a single-ended signal that Device #2 receives. Hundreds of millivolts of noise can cause amps of current to flow through the ground loop.įigure 1 shows an example of how ground loop interference can occur in a generic data transmission path. Since grounds are low impedance, the noise currents are often large. Analogously, a distributed grounding system can also allow ground voltage noise from sources at one location to cause ground currents to flow in the ground loop. The 50 Hz/60 Hz magnetic field from ac power is a common source of the noise that ground loops pick up. These ground paths can act as a large loop antenna that causes noise to be picked up from the environment, inducing currents in the grounding system. Ground loops are, as their name suggests a physical loop in a system’s grounding scheme resulting from multiple ground paths between circuits. These are covered in greater detail in Ott, AN-375, AN-740, AN-770, and AN-727 (see the References section).

Though this discussion focuses on breaking ground loops as a motivation for isolating these interfaces, there are other important considerations, such as operator safety and the protection of electronics that necessitate isolation. Ground loops are discussed in the context of USB throughout this article, but other interfaces, such as RS-232, RS-485, and CAN, are also susceptible to ground loops (see AN-375, AN-740, AN-770). This article will explain how ground loops occur and discuss how galvanic isolation has been used to eliminate them. If this voltage is large enough, it can cause data errors at the receiver. A ground loop can be a source of interference that can induce a noise voltage between the grounds on either end of the transmission. Transmitting data over long distances is fraught with potential problems. This article explains how ground loops occur and discusses how galvanic isolation has been used to eliminate them. Breaking Ground Loops with Functional Isolation to Reduce Data Transmission Errors

Power ground isolator