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Electronic Design

12-Bit DAS Provides 2500-V Isolation

<p>A remote data-acquisition system (DAS) requires some form of isolation if it must accommodate large ground-potential differences. One solution is to isolate the analog signal from the rest of the circuit, but that approach can be tedious.</p> <p> A more practical scheme, which includes a sample-and-hold amplifier on the analog-to-digital converter (ADC), isolates only the converter's I/O lines (<a href="/Files/29/11462/Figure_01.jpg">see the figure</a>). The resulting isolation barrier can withstand as much as 2500 V dc for one minute. Basically, the isolation barrier consists of two high-speed optocouplers (IC4 and IC5) and a transformer that conveys square-wave power from IC1, across the isolation barrier, where it powers the rest of the circuit. Obviously, the converter's serial-data outputs reduce the number of optocouplers required.</p> <p> In the operation of the circuit, IC1 generates out-of-phase square waves, at D1 and D2, which are rectified, filtered, and regulated to 5 V. The chip also includes two data drivers and two data receivers (compatible with SPI and Microwire serial-interface standards) that communicate with the microprocessor. The 12-bit sampling analog-to-digital converter (IC2) offers 5-V operation, a 0- to 5-V input range, and an on-board voltage reference. Transistors Q1 and Q2 boost current drive to the optocouplers.</p>

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