A 20-Bit (1 ppm) Linear Slope-Integrating A/D Converter

By combining an ‘‘inferior’’, 20 year old A/D conversion technique with a microprocessor, a developmental A/D converter achieves 1 part-per-million (20-bit) linearity. The absolute accuracy of the converter is primarily limited by the voltage reference available. The precision achieved by the unlikely combination of technologies surpasses conventional approaches by more than an order of magnitude. The approach used points the way towards a generation of ‘‘smart’’ converters, which would feature medium to high resolution (12 bits and above) with high accuracy over extended temperature range. The conversion technique employed, while slow speed, suits transducer based measurement systems which require high resolution over widely varying conditions of time and temperature. In addition, extensions of the basic converter have achieved 15-bit digitization of signal inputs of only 30 mV full-scale with no sacrifice in linearity or stability. This offers the prospect of an ‘‘instrumentation converter’’ which could interface directly with low level analog signals.