IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 60, NO. 12, DECEMBER 2011 3907 Design Considerations for a CCC Bridge With Complete Digital Control Jonathan M. Williams, Gert Rietveld, Member, IEEE, Ernest Houtzager, and Theodoor J. B. M. Janssen Abstract—The design considerations for a completely digitally controlled cryogenic current comparator resistance ratio bridge, including the feedback loops for voltage and current balance, are described. The numerical algorithms have been modeled and optimized, and the overall system performance is illustrated with example measurement data. It is shown that the final performance of the bridge is limited only by the accuracy of the current com- parator, the noise sources associated with the current and voltage null detectors, and the resistors under test. An explanation of how the effect of leakage resistance in the system is minimized is also given. Index Terms—Cryogenic current comparator (CCC), digital feedback loops, digital measurement, noise measurement, resis- tance measurement. I. I NTRODUCTION T HE CRYOGENIC current comparator (CCC) bridge has become a key component of top-level resistance metrol- ogy. CCC bridges are used by many national metrology labora- tories for quantized Hall effect realizations of the SI ohm and for scaling the resistance unit across different decade values [1]–[4]. The improvement of CCC bridges has focused, in recent years, on automation and ease of operation with digital electronics playing an increasing role [5]–[8]. The negative feedback loops for the current and voltage balance units in CCC bridges were originally implemented using analog electronics. However, a system with digital control of all aspects has now been realized [9], with the advantage that predetermined constants can be loaded into the storage registers in the hardware for a variety of measurement configurations covering different resistance ratios. Furthermore, the large inte- gral term in the feedback loop required for high accuracy of the current ratio in a CCC bridge is more easily achieved with dig- ital signal processing than with analog components. A digital feedback loop also facilitates optimization of the gain settings and analysis of the noise reduction and overall performance. Manuscript received November 26, 2010; revised January 24, 2011; accepted March 17, 2011. Date of publication June 2, 2011; date of current version November 9, 2011. This work was supported in part by the U.K. Department for Business Innovation and Skills and in part by the Dutch Ministry of Economic Affairs. The Associate Editor coordinating the review process for this paper was Thomas Lipe. J. M. Williams is with the National Physical Laboratory, TW11 0LW Teddington, U.K., and also with the School of Engineering and Mathemat- ical Sciences, City University, EC1V 0HB London, U.K. (e-mail: jonathan. williams@npl.co.uk). G. Rietveld and E. Houtzager are with Van Swinden Laboratorium, 2629 JA Delft, The Netherlands. T. J. B. M. Janssen is with the National Physical Laboratory, TW11 0LW Teddington, U.K. Digital Object Identifier 10.1109/TIM.2011.2149330 Fig. 1. Diagram of the CCC bridge showing the measurement circuit and principle of the current and voltage balance. This paper gives more details on the optimization of param- eters such as sampling rate, number of bits, and antialiasing filter bandwidth than was presented in [9]. The choice of these parameters and their effect on measurement uncertainty are described together with uncertainty components arising from leakage resistance associated with the electronics and its power sources. II. PRINCIPLE OF OPERATION We follow the standard design of the CCC bridge with two current sources to energize the resistors being compared, with each resistor connected in series with a ratio winding of the CCC (see Fig. 1). The two current sources I 1 and I 2 , the current balance unit, and the voltage balance unit are in separate modules, each with its own isolated power supply. These modules are controlled by a personal computer (PC) using an optical fiber interface [9]. Flux null in the CCC, with turns N 1 and N 2 , is achieved with a digital feedback loop operating in the current balance module, which sends numerical corrections to the slave current source over the optical fiber interface. Voltage null is achieved with a second feedback loop operating in the voltage balance module, which generates a feedback current through an auxiliary CCC winding with N 3 turns. The measurement currents are simultaneously reversed at regular intervals, typically 10–20 s, to eliminate static offsets 0018-9456/$26.00 © 2011 IEEE