Protect systems in harsh environments with isolated USB repeaters

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Universal Serial Bus (USB) is increasingly used in industrial applications, where it can support software downloading and configuration, aid in diagnostics and maintenance, and connect peripherals. However, USB has been defined as a consumer electronics interface and does not tolerate the noise disturbances, ground bounces, and ground potential differences found in industrial environments.

To overcome these limitations and meet the needs of a wide range of factory automation, medical equipment, and power applications, you can implement a galvanically isolated USB interface. However, you will need to ensure that your interface is EMI compliant and minimizes intersymbol interference (ISI).

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Galvanic isolation

Texas Instruments offers a variety of isolated USB solutions that incorporate a galvanic isolation barrier into a USB interface (Fig.1). Of the society ISOUSB211 and ISOUSB111 isolated USB repeaters each provide high voltage protection and enhanced signal integrity for USB 2.0 interface systems.

Both devices support Charge Downstream Port (CDP) advertising, role reversal, and auto-speed detection, allowing the same reference device to be used for host isolation , device, hub, or cable. Additionally, both are available in reinforced and basic insulation options, offering isolation voltages of 5700VRMS and 300VRMSrespectively, in accordance with UL 1577.

Available in an SSOP-28 package, the ISOUSB211 supports 480 Mb/s High Speed ​​(HS) mode in addition to 12 Mb/s Full Speed ​​(FS) and 1.5 Mb/s Low Speed ​​(LS) modes. The ISOUSB211 meets HS signal quality requirements without the need for an external crystal oscillator or clock input. Its incremental jitter is low enough to meet USB eye diagram specifications without retiming.

For applications that do not require high bandwidth, the ISOUSB111 supports both LS and FS modes. It is available in a 16-SSOP package which is 40% smaller than a competitive device. The USB repeater is also available in a 16-SOIC package compatible with competitor device pins.

EMC performance

Electromagnetic compatibility (EMC) performance is a key consideration when choosing a USB isolator for industrial applications. Figure 2 shows the transmit performance of the ISOUSB111 compared to a competitive device when used in a test setup with a 1m cable between a PC and a USB stick. At one point, the ISOUSB111 offered more than 20dB lower emissions than a competitive device, which failed to comply with CISPR 32 Class B.

picture 3 reveals the performance of the ISOUSB211 in a similar test setup. The ISOUSB211 meets the CISPR 32 Class B limit with a significant margin. Noise peaking around 80 MHz comes from an isolated power converter included on the ISOUSB211EVM evaluation module and not from the ISOUSB211 itself.

Pre-emphasis and equalization

A USB isolated repeater generates a new copy of its input signal across its isolation barrier, and input signal jitter will appear at the repeater output. The repeater itself includes on-off modulation circuitry which may contribute additional jitter. The total jitter on the repeater output is the square root of the input jitter and the jitter generated by the isolation circuit.

You can minimize unwanted effects by choosing a repeater that includes transmitter pre-emphasis and receiver equalization capabilities. These play a key role in improving signal integrity and reducing frequency dependent ISI caused by PCB trace losses.

To study the ISI, you can use an FR4 test circuit board having five tracks of differential pairs ranging from 6 inches to 4 feet in length, and with each differential pair having USB connectors on each end. Figure 4 shows the measured differential insertion loss for each pair of traces. Longer traces have higher DC losses, and for all lengths the insertion loss increases with frequency.

Figure 4 also illustrates ringing, which results from reflections due to impedance discontinuities along the traces. Insertion loss at high frequencies and reflections increase ISI, which occurs when signals deviate from their ideal square waveform. The transitions extend into adjacent intervals and the signal levels never quite reach the ideal high and low voltage levels.

To mitigate these effects, the ISOUSB211’s transmitter offers nine programmable pre-emphasis settings, and its receiver offers nine programmable EQ settings implemented with a Continuous Time Linear Equalizer (CTLE) architecture.

For a first-order approximation, the pre-emphasis/EQ function provides compensation by boosting high-frequency signals, peaking at 240 MHz, while keeping low frequencies unchanged. The result is reduced rise and fall times, allowing the compensated signal to approach the ideal. Figure 5 shows uncompensated, compensated and ideal waveforms.

In general, you can choose to use pre-emphasis or EQ, depending on your setup. For example, if you are using the FR4 breadboard with 6 in. at 4 ft. traces to make measurements, choose transmitter pre-emphasis if you place the test card between the USB repeater and the measuring instrument. On the other hand, use receiver equalization if you place the repeater between the breadboard and the instrument. Figure 6 shows each configuration with the repeater mounted on an ISOUSB211DPEVM evaluation module.

However, if you are using the repeater in a system that has differential trace loss greater than 2 dB, you should try to distribute the loss over the upstream and downstream sides of the repeater. Then use receiver EQ and transmitter pre-emphasis simultaneously to compensate for system loss.

Conclusion

USB isolators such as ISOUSB211 and ISOUSB111 from Texas Instruments can adapt the consumer-centric USB interface for industrial applications. TI devices easily meet EMI standards requirements, and HS versions offer pre-emphasis and equalization settings to compensate for PCB trace losses and other systems.

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