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.
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 (see
the figure). 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.
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.
