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THE INTERNATIONAL JOURNAL OF METROLOGY




                              Innovative ADC Sets New
                       Temperature Calibration Standards

                        Implementation of a Commercial
  2006                  Quantum Hall Resistance System
 OCTOBER
NOVEMBER                       GPS Receiver Calibration
DECEMBER
                            Going Paperless and Wireless
       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

instrument of its type, thereby providing the highest possible reliability. The core technology is a wholly new type of ADC

converter) in the feedback loop is replaced by a 5-bit PWM (pulse-width modulation) DAC. The effect of this is to reduce

then determined by the electronic devices used in the ADC and measurement system, which are lower than existing
comparable instruments.




Introduction
                                                                    EMF generated by a thermocouple is measured in order
   Although we rightly focus on the thermodynamic                   to determine its temperature. Again, the demands on
challenges when making high accuracy temperature                    the electrical measurement rival those of the electrical
measurements, the electrical measurement requirements for           calibration laboratory; a gold-platinum thermocouple has
achieving low measurement uncertainty are comparable to
those of a good electrical laboratory. The main temperature
sensors used are PRTs (platinum resistance thermometers).
The SPRTs (standards PRTs) used in high accuracy                    full range of the thermocouple). This again represents a
calibration work comprise thin and lightly suspended coils          demanding electrical measurement objective.
of platinum wire, the resistance of which is measured to
determine their temperature.                                        The Need for a New ADC
                                                       for a
25                                                                    The ADC in a precision instrument is the "measurement
                                                                    engine." Its performance is the starting point for achieving
sense current is used with a 25 SPRT in order to minimise           low measurement uncertainty. Historically, AC bridges (or
self-heating, so the voltage uncertainty corresponding to           other transformer based measuring systems such as current
                                                                    comparators) have been used in temperature laboratories
accommodate the full temperature range of the SPRT).
This requirement represents a demanding electrical                  use a ratio-transformer as their ADC. In an AC resistance
measurement objective.                                              bridge, a common AC current is passed through the SPRT
  Thermocouples are commonly used in measurement                    (RX) and Reference Resistor (RS). The voltage generated
applications. Noble metal thermocouples are used for                across RS is scaled using the transformer and compared
high accuracy calibration work at higher temperatures               with the voltage across RX. A control circuit adjusts the




                       OCT 

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