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Better Accuracy in Temperature
Calibration and Measurement through a
New Type of Analog-to-Digital Converter
Paul Bramley
Metrosol Limited, UK
John Pickering
Metron Designs Limited, UK
This article describes a new reversal-switched DC precision thermometer that achieves performance levels previously
attained only with AC resistance bridges. The microK is designed for use in secondary temperature calibration and high
accuracy temperature measurement applications (to 0.4mK uncertainty) and is the only precision thermometer that
can work with both resistance thermometers and thermocouples at sub mK uncertainties. The use of a touch-screen
interface and the elimination of switches, relays and potentiometers from the design make this the first solid-state
instrument of its type, thereby providing the highest possible reliability. The core technology is a wholly new type of ADC
(analog-to-digital converter) based on the - (sigma-delta) technique but in which the single-bit DAC (digital-to-analog
converter) in the feedback loop is replaced by a 5-bit PWM (pulse-width modulation) DAC. The effect of this is to reduce
the quantization noise from the - ADC technique by more than two orders of magnitude. The noise performance is
then determined by the electronic devices used in the ADC and measurement system, which are lower than existing
comparable instruments.
Introduction (above ~1000