This instrument, having sixteen thermopile detectors, required a low-noise, low-offset multiplexer and preamplifier to achieve its target measurement accuracy. We were able to design a system having an offset voltage of about 1 nanovolt, and an offset current of about 0.1 picoamp. No trimming was required, and the offsets remain at these levels for the life of the instrument. The system included the multiplexers and their drivers, preamplifier, A/D converter and digital signal processing in hardware.