An engineer's guide to signal and power isolation in communications cabling
When communication lines between equipment are required, problems can arise: if there is a potential difference between local grounds, DC or 50/60Hz current can flow along data line returns or screens disrupting signals or even fusing wires. Also, the cable can act as an antenna, introducing EMI into equipment causing data corruption or damage.
Figure 1: Discrete implementation of RS-485 data and power isolation
Isolation solves the problem
A common solution is to introduce galvanic isolation at one or both ends of the cable connection so there is no electrical continuity between signal returns or ground systems. Ethernet, for example, includes transformers, often built in to connectors, performing this function with no active driver circuit on the transformer output. Other protocols such as RS-485, RS-232 or USB do not include isolation in their basic specification so if there are concerns about ground loops, safety and EMI, isolation has to be provided by transformers or opto-isolators. This then poses the challenge that an active driver is required on the cable side of the isolation, requiring power rails which themselves must be isolated. The application is so common, though, that custom ICs are available with data and power isolation built in or the function can be achieved with a data isolator and separate dedicated DC-DC converter (Figure 1). This is commonly done for RS-485 and RS-232 with some USB applications seen as well.
Isolation grade is important
The grade of isolation needed for data and power is often a cause of confusion: in typical normal operation, voltages across the barrier are close to zero but with occasional transients due to EMI from other equipment or perhaps lightning strikes. Because these transients are poorly defined, the isolation rating of DC-DCs used, for example, is typically high at 3kV. This value is often a ‘momentary’ rating, though, and manufacturers will warn that continuous high voltages should not be applied and the DC-DC barrier should not be regarded as providing any ‘safety’ isolation. There are cases where the communication line isolation does need to provide protection against electric shock, in which case data isolators and DC-DC converters need to hold the relevant safety approvals for isolation to high continuous system voltages. A typical example is in the medical arena where data isolation may need one or two Measures Of Patient Protection (MOPPs) to allow for the case where a patient becomes ‘live’ from other faulty equipment and a lethal current could pass through the patient and data line to equipment earth (Figure 2).
Figure 2: Data and power need isolation in some medical applications
Isolated power has other uses
When isolated power for the data drivers in cabling is necessary, the possibility arises for the power also to be routed down spare wires to the remote equipment. Often commercial DC-DC converters have ratings around 1W and therefore have spare capacity with data drivers typically only needing 200mW or so. The excess power available may be sufficient to power a remote sensor, for example, saving the cost and inconvenience of providing a local power source from a battery or line-powered ‘adapter’ at the sensor (Figure 3).
Isolation of USB lines in industrial, audio and medical applications is often seen for EMI or safety reasons, but of course a basic characteristic of USB is that 5V is also normally provided at different levels, from 0.5A/2.5W upwards to 5A/100W with the latest ‘USB Type CTM’ arrangements. USB 5V is also monitored for current draw with a complex ‘enumeration’ procedure followed to allow the source to allocate power between different peripherals. Clearly, if isolated power is needed, a dedicated DC-DC converter is needed capable of higher power levels and which can match the requirements of standard USB host controllers.
Figure 3: RS-485 sensors can be powered from 'spare' isolated power
Available integrated solutions
Figure 4: Murata's NMUSB202MC series dual channel
isolated, powered USB interface
A range of integrated, isolated data and power solutions are available from Murata. Its NM485D6S5MC is an RS-485 full-duplex transceiver with an embedded DC-DC converter that powers the driver and provides +/-6V and regulated +5V for external use up to a maximum of 0.8W combined. The module, in a footprint of just 18.16 x 14.70mm is recognised for reinforced/250Vrms isolation to UL60950 and one MOPP, according to ANSI/AAMI ES60601-1 for medical applications. The Murata NMTTLD6S5MC is a TTL isolator with two bi-directional channels capable of a data rate of 25Mbps with the same auxiliary isolated power available as the RS-485 module, +/-6V and regulated +5V at 0.8W combined. It features UL60950 basic/250Vrms isolation and one MOOP medical recognition (Measure of Operator Protection).
The Murata NMUSB202MC (Figure 4, right) is an example of a dual isolated USB 2.0 interface, which can provide a full 5V/0.5A from each port with UL60950 reinforced/250Vrms isolation with one MOPP and two MOOP medical recognition. Data rates are ‘full speed’ (12Mbps) and ‘low speed’ (1.5Mbps) with automatic switching between the two. The surface-mount part is 23.88 x 23.11mm and finds applications where it is built in to equipment with its primary side power obtained from a system +5V. The NMUSB202MC acts as a fully featured USB hub with enumeration and power surge notification.
Isolating data interfaces gives many benefits and when isolated power is also made available, potential savings are made by avoiding the need for remote power supplies. Murata has a range of isolator products that meet the need.
Philip Lechner Technical Specialist, Power Avnet Abacus
Philip Lechner studied electronics and telecommunications in Amsterdam before beginning his career in electronic component sales. His career in electronics distribution has spanned 33 years, including 13 years as a field sales engineer covering power, semiconductors and IP&E, and 20 years in product management for power supplies and DC-DC converters for Avnet Abacus.
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