Analog Angle Article

RS485 transceiver module isolates power and data, solves loop problems

Milpitas, Calif.—Galvanic isolation (meaning there is no ohmic signal path connecting the two sides of the interface) is the solution to many line and loop problems, as well as a way to avoid potential operational and safety problems. The LTM2881 isolated RS485 uModule from Linear Technology Corp. (also suitable for its simpler sibling standard, RS422) provides fully isolated signal and power for this still-vibrant communications interface standard, which remains in use in legacy applications, and retains new-design popularity due to its low cost, ease of troubleshooting, and simple protocol compare to IP connectivity.

Block diagram of the LTM2881 isolated RS485 transceiver module
(Click on image to enlarge)

The LT2881 uses the basic magnetic induction principle of transformers to achieve isolation (other techniques used for achieving this objective include capacitive, optical, magnetoresistive, and RF isolation). It isolates the logic-level interface and the line transceiver to a 2500 Vrms level, while also providing common-mode transient immunity of greater than 30 kV/μsec. The integral isolated power supply (designed for low EMI) also provides an isolated 5 V rail at up to 1 W for external circuitry, far greater than available alternatives which LTC claims provide only 50 to 100 mW.

Typical applications for this isolated transceiver include breaking ground loops, managing common-mode voltages, and minimizing problems due to open, unterminated line taps. Along with ±15 kV ESD protection (human body model), it offers operation (both half or full duplex) to 20 Mbps, or to 250 kbps with low-EMI operation of its supply.

Its driver maintains a high-input impedance and offers skew-rate control to minimize EMI, and it retains the high impedance even in device shutdown mode, where the LTM2881 dissipates just 5 μA (maximum). The receiver structure allows up to 256 nodes, and defaults to a logic “1” if the receiver inputs go open, short, or are terminated but not driven. The device offers a user-selectable, integrated 120 &#937 termination on the RS485 line side to minimize signal reflections and maintain signal integrity, and a 1.62-to-5.5 V logic-side supply pin.

Supply options include 3.3 V (LTM2881-3) or 5 V (LTM2881-5) operation. Thermal shutdown protection and short-circuit protection are also included in the design.—Bill Schweber

Availability, packaging, and price : The LTM2881 uModule is housed in a surface-mount LGA or BGA package measuring 11.25 × 15 mm and × 2.8 mm high. It is priced at $8.48 each in 1000-piece lots, and is available now.

For more information : contact Linear Technology Corp.

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