Shield Techniques in Harsh Environments with Remoted USB Repeaters
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The Common Serial Bus (USB) more and more finds use in industrial purposes, the place it may well help software program add and configuration, help with diagnostics and upkeep, and join peripheral units. Nevertheless, USB was outlined as a consumer-electronics interface, and it doesn’t inherently tolerate the noise disturbances, floor bounce, and floor potential variations present in industrial environments.
To beat such limitations and meet the wants of a broad spectrum of factory-automation, medical-equipment, and power-delivery purposes, you may implement a galvanically remoted USB interface. Nevertheless, you have to to make sure that your interface complies with EMI requirements and minimizes inter-symbol interference (ISI).
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Galvanic Isolation
Texas Devices gives quite a lot of remoted USB options that combine a galvanic isolation barrier right into a USB interface (Fig. 1). The corporate’s ISOUSB211 and ISOUSB111 remoted USB repeaters every present high-voltage safety and enhanced sign integrity for USB 2.0 interface methods.
Each units help charging-downstream-port (CDP) promoting, role-reversal, and computerized pace detection, which permits the identical part-number gadget for use for host, peripheral, hub, or cable isolation. As well as, each are available bolstered and fundamental isolation choices, providing isolation voltages of 5,700 VRMS and 300 VRMS, respectively, in accordance with UL 1577.
Out there in an SSOP-28 package deal, the ISOUSB211 helps the 480-Mb/s Excessive Pace (HS) mode along with the 12-Mb/s Full Pace (FS) and 1.5-Mb/s Low Pace (LS) modes. The ISOUSB211 meets HS signal-quality necessities with out the necessity for an exterior crystal oscillator or clock enter. Its incremental jitter is low sufficient to fulfill USB eye-diagram specs with out retiming.
For purposes not requiring excessive bandwidths, the ISOUSB111 helps LS and FS modes. It’s out there in a 16-SSOP package deal that’s 40% smaller in measurement than a aggressive gadget. The USB repeater additionally is obtainable in a 16-SOIC package deal that’s pin-compatible with the aggressive gadget.
EMC Efficiency
Electromagnetic-compatibility (EMC) efficiency is a key consideration when selecting a USB isolator for industrial purposes. Determine 2 exhibits the emissions efficiency of the ISOUSB111 in contrast with that of a aggressive gadget when utilized in a take a look at setup with a 1-m cable between a PC and a USB drive. At one level, the ISOUSB111 gives emissions greater than 20 dB decrease than that of a aggressive gadget, which fails to attain compliance with the CISPR 32 Class B commonplace.
Determine 3 reveals the efficiency of the ISOUSB211 in the same take a look at setup. The ISOUSB211 meets the CISPR 32 Class B restrict with important margin. The noise peaking round 80 MHz is coming from an remoted energy converter included on the ISOUSB211EVM analysis module and never from ISOUSB211 itself.
Pre-emphasis and Equalization
A USB remoted repeater generates a recent copy of its enter sign throughout its isolation barrier, and input-signal jitter will seem on the repeater’s output. The repeater itself consists of on-off keying circuitry that may contribute extra jitter. Complete jitter on the repeater’s output is the root-sum-squared worth of the enter jitter and the jitter generated by the isolation circuitry.
You may decrease the adversarial results by selecting a repeater that features transmitter pre-emphasis and receiver equalizer capabilities. These play a key position in bettering sign integrity and lowering the frequency-dependent ISI attributable to PCB hint losses.
To research ISI, you should use an FR4 take a look at printed circuit board having 5 differential-pair traces starting from 6 in. to 4 ft. in size, and with every differential pair having USB connectors at every finish. Determine 4 exhibits the measured differential insertion loss for every pair of traces. Longer traces have increased dc losses, and for all the lengths, the insertion loss will increase proportionally with frequency.
Determine 4 additionally illustrates ringing, which ends from reflections as a result of impedance discontinuities alongside the traces. The insertion loss at excessive frequencies and the reflections enhance ISI, which happens when indicators deviate from their superb square-wave format. Transitions increase into adjoining intervals and sign ranges by no means absolutely attain the perfect high- and low-voltage ranges.
To mitigate these results, the ISOUSB211’s transmitter gives 9 programmable pre-emphasis settings, and its receiver gives 9 programmable equalizer settings carried out with a continuous-time linear equalizer (CTLE) structure.
To a first-order approximation, the pre-emphasis/equalizer operate gives compensation by amplifying high-frequency indicators, peaking at 240 MHz, whereas retaining low frequencies unchanged. The result’s diminished rise and fall occasions, permitting the compensated sign to get nearer to the perfect. Determine 5 exhibits uncompensated, compensated, and superb waveforms.
Normally, you may select to make use of both pre-emphasis or equalization, relying in your setup. For instance, for those who’re utilizing the FR4 take a look at board with 6-in. to 4-ft. traces to make measurements, go for transmitter pre-emphasis for those who’re finding the take a look at board between the USB repeater and the measurement instrument. Alternatively, use receiver equalization for those who’re finding the repeater between the take a look at board and the instrument. Determine 6 exhibits every configuration with the repeater mounted on an ISOUSB211DPEVM analysis module.
Nevertheless, for those who’re utilizing the repeater in a system that has differential hint loss better than 2 dB, you need to attempt to distribute the loss on each the upstream and downstream sides of the repeater. Then, use each receiver equalization and transmitter pre-emphasis concurrently to compensate for the system loss.
Conclusion
USB isolators similar to Texas Devices’ ISOUSB211 and ISOUSB111 can adapt the consumer-centric USB interface for industrial purposes. The TI units simply meet EMI requirements necessities, and the HS variations supply pre-emphasis and equalization settings to compensate for PCB-trace and different system losses.
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