High-Rate Optically Isolated Analog Multiplexer Using Amplitude-to-Width Conversion

The operating principle and the performances of a new type of isolated analog multiplexer are described. It has a maximum sampling rate of 80 kHz, 2.5-kV isolation, dc CMRR higher than 140 dB, linearity error lower than 0.6 percent, and thermal drift in the order of 100 μV/°C. Each acquisition channel has a floating pulsewidth modulator driving an optoelectronic coupler. This last controls a generator which injects a reference current into the virtual ground of an integrator common to all the channels. The resulting output analog voltage is digitized by a fast ADC. The analog information through the optical couplers is carried by a pulse of fixed amplitude and variable width. This circumvents the linearity and stability limitations of optical isolators. The described multiplexer has been devoted to analog data logging in electricity generating plants. Its sampling rate and accuracy performances have been optimized in view of this particular application. The system is susceptible of a factor 10 increase in speed with a limited degradation of other characteristics.