Integrated signal and power isolation provides robust and compact measurement and control

Baoxing Chen -July 10, 2013

I. Introduction

Robust and accurate measurements and controls are important for industrial instrumentation and process control. Various industrial sensors such as RTD or thermocouples usually require input isolation, not only to prevent ground loops that can compromise measurement accuracy, but also to prevent voltage transients that can cause permanent damage to the instrument. Besides small differential signal of interest, these sensors can carry significant common mode potential that will reduce data acquisition accuracy or large voltage transients that can pose as a safety hazard. Similarly isolated analog outputs are required to operate actuators such as hydraulic actuators, solenoids or motor starters safely and accurately. 

Besides isolation for analog inputs and outputs from system ground, isolation is also needed between inputs and between outputs to handle different common mode potentials for various sensors and actuators and prevent interference among them. Digital isolators have been successful in improving analog system performance and reducing system size by replacing traditional analog isolators or opto-couplers which are bulky and inaccurate. High resolution ADC placed right at the sense node converts the precise analog sensing signals into digital streams before they are transmitted across isolator barrier to the system microcontrollers using highly integrated multi-channel digital isolators. Similarly, the digital commands from the system micro-processor are transmitted through isolation barrier using digital isolators first and then are converted to accurate analog current or voltage controls using high resolution DACs.

The digital isolators eliminate the signal distortion and degradation from analog isolators or traditional opto-couplers, however, the DACs to interface with controls or the ADCs to interface with these sensors would require some isolated power to operate. While discrete isolated DC/DC converters can be used to provide the isolated power from system side, they are bulky and difficult to design as they require many discrete components. The outputs of these discrete DC/DC converters are not well regulated either and they become very inefficient at low loads, typical for low power high resolution ADCs or DACs. To enable robust and compact analog measurement and control systems, small, integrated isolated power together with digital isolation is needed. isoPower®, an extension to iCoupler® technology, industrial leading digital isolators, provides isolated signal and power in a shrink small outline package (SSOP).

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