The Universal Serial Bus (USB) is quickly becoming the standard interface for most PC peripherals. It is replacing RS232 and parallel printer ports due to its superior speed, flexibility, and support for hot-plugging of devices. Industrial and medical device manufacturers are also eager to use this bus, but adoption has been slow because there is no good way to provide the necessary isolation for machine connections that control dangerous voltages or for low-leak, defibrillation-proof connections in medical applications. .

The ADuM4160 is primarily designed to be used as an isolation component for USB peripherals. But in some cases, it can also be used to implement isolated cable functions. To do this, several issues must be resolved first. The buffers upstream and downstream of the ADuM4160 are identical and capable of driving the USB cable, but the downstream buffer must also be able to adjust its speed depending on the full-speed or low-speed peripheral connected to it. The upstream connection must behave like a peripheral and the downstream connection must behave like a host.

Unlike applications building dedicated peripheral interfaces, where the speed is known and does not change, host applications must improvise based on detecting whether a low-speed or full-speed device is connected. The ADuM4160 is hardwired through a pin to determine a single speed; therefore, when the peripheral plugged into the downstream side is the correct speed, it works correctly; when the connected peripheral is not the correct speed, it does not work. The best way to solve this problem is to target the application circuit shown in Figure 1 to isolate a peripheral that has implemented the USB interface. Since no 100% efficient power converter can be used to transmit the bus voltage across the isolation, it is impossible to obtain a fully compliant bus-powered cable. Additionally, the converter's quiescent current does not meet the USB standard's standby current requirements, and the ADuM4160 is coupled with a hub controller.

The upstream side of the hub controller can be thought of as a standard fixed-speed peripheral port that is easily isolated using the ADuM4160, while the downstream ports are all handled by the hub controller. But in many cases, while not considered fully compliant with the USB standard, a single-speed cable is acceptable from a practical perspective, especially if a custom connector is used so that it is not confused with a compatible device. The hub chip can be removed so the design becomes very small and simple.

The ADuM4160 provides an economical and simple way to implement isolation buffers for industrial and medical peripherals. The challenge in utilizing this device is that it must be paired with a small isolated DC-DC converter such as the ADuM5000 to build a bus-powered cable isolator using this isolation buffer. As with isolating any device, the services that the ADuM4160 provides the following features: ADuM5000 .

1. Isolate the USB D+ and D− lines of the cable directly upstream.
2. Implement automated control schemes for control data flow that do not require external control lines.
3. Provide medical grade isolation.
4. Supports full-speed or low-speed signaling rates.
5. Supports isolated power supply via cable.

The goal of the application circuit shown in Figure 1 is to isolate a peripheral that has implemented a USB interface. Since no 100% efficient power converter can be used to transmit the bus voltage across the isolation, it is impossible to obtain a fully compliant bus-powered cable. Additionally, the converter's quiescent current does not meet the USB standard's standby current requirements, and the ADuM4160 has speed detection limitations. What is possible is a fixed-speed or switch-controlled speed cable that can deliver moderate power to downstream peripherals. However, this is a custom application and does not fully comply with the USB standard.

The power used by the upstream USB connector is derived from the 5 V VBUS voltage provided by the USB cable. The bus voltage can also drive the ADuM5000 , which is used to generate the VBUS2 voltage for use by the downstream side of the ADuM4160 and provide up to 100 mA power to peripherals. The ADuM5000 was chosen for its high isolation voltage and small size. It can provide sufficient operating power for small bus-powered devices such as mice, keyboards and memory sticks. Because this device uses a chip-scale microtransformer and has a very high internal switching frequency, the design must use ferrite beads on the cable and follow the recommendations in application note AN-0971 to minimize the impact of electromagnetic radiation. In order for a system to pass EMI/RFI testing, special layout, decoupling, and grounding techniques must be used. For guidance, please refer to Tutorial MT-031 and Tutorial MT-101. For the complete layout and Gerber files for the ADuM4160 USB Cable Isolator, please visit: http://www.analog.com/CN0159-DesignSupport .

ADuM4160 isolated cable applications have multiple power supply, bus speed, and ESD/EOS protection options that must be determined. Peripherals operate at one of three speeds: low speed (1.5 Mbps), full speed (12 Mbps), and high speed (480 Mbps). The ADuM4160 does not support high-speed operation and blocks the handshake used to negotiate that speed. Overspeed mode starts with a full-speed configuration, and the peripheral requests high-speed support through a process called high-speed chirping. The ADuM4160 ignores this high-speed tuning; therefore, the high-speed operation request is never passed to the host and the peripheral automatically continues to run at full speed. This application circuit consists of a switch and a single channel isolator that allows the user to select the cable speed: full speed or low speed by setting the SPU and SPD pins. This feature is optional if single speed operation is sufficient.

The VBUSx pin provides power. The 3.3 V signal voltage is generated at the VDDx pin by an internal 3.3 V regulator. The ADuM4160 supports other power supply configurations, see other circuit notes for details. In the circuit shown in Figure 1, both the upstream and downstream sides of the ADuM4160 are set up to derive power from the VBUSx line and the internal regulator.

The ADuM4160 also offers an option to delay the application of an upstream pull-up resistor under peripheral control. This feature is controlled by PIN entry. In this application, the PIN input is shorted to high, so the upstream pull-up resistor is used whenever peripheral power is applied.

This circuit also uses protection devices. These devices are selected from manufacturers that offer a variety of different devices, and the specific device chosen allows them to be replaced with a 0 Ω short-circuit resistor to remove it from the circuit. Designers should carefully consider protection device selection, ranging from situations where no external protection is required to a full suite of transient suppressor and filter components. The components included in this application circuit show what type of protection can be used.

When the circuit is working, packet detection occurs and data is transferred from one side of the isolation to the other. Figures 3 and 4 and the data shown illustrate typical full-speed processing in the form of time domain data and eye diagrams respectively. In real-time data, characteristics to note are that the packet starts in the passive idle state, which transitions to the driven J state, and that the end of the packet at the end of processing shows a single-ended 0 state, followed by the idle J state. It is this automatic control flow and handling of these special logic states that enables the ADuM4160 chip and is unique on the market.

The cable is designed to be fully isolated from the upstream data connection and can withstand transient voltages up to 2.5 kV. Future isoPower® modules will support medical grade cable isolation up to 5 kV. The downstream port is powered by the upstream VBUS1 line, and the power available to the application circuit is limited to 500 mA (5 V), which is the maximum power available for a standard USB port and is sufficient to run the ADuM5000 with a 100 mA external load. Low-speed, full-speed, and high-speed peripherals can be connected to downstream ports, but the cable's full-speed and low-speed modes must be manually switched. This design relies on the ADuM5000's internal short circuit for safety.

The data shown in Figure 3 and Figure 4 is generated during the USB-IF certification process. Figure 3 shows a test packet being transmitted from the ADuM4160 upstream port to the host. Note the pre-idle state, where the passive resistor network remains in the idle J state. The center of the pack is a mix of Jack and King. On the right side of the packet is the EOP (End of Packet) flag, which is a single-ended 0, followed by a driven J state, and then transitions to the idle J state.

Upstream full-speed signal quality test reference document—USB 2.0 specification Section 7.1.11, Section 7.1.2.1. Upstream full-speed rise time test reference document—USB 2.0 specification Section 7.1.11, Section 7.1.2.2. Upstream full-speed drop time test reference document—USB 2.0 specification section 7.1.11, section 7.1.2.2.

Figure 3 shows the full-speed eye diagram of the ADuM4160, which shows that it can provide a fully open eye diagram and stay away from the forbidden area. Similar data were obtained for the low-speed evaluation.

For the complete design support package for this circuit note, see www.analog.com/CN0159-DesignSupport .