Low Noise Programmable Current Source for the NIST-3 and NIST-4 Watt Balance D. Haddad, B. Waltrip, and R.L. Steiner. National Institute of Standards and Technology 100 Bureau Drive Gaithersburg, MD 20899 USA
[email protected] [email protected] Abstract — We give a brief description of a low noise, high resolution programmable current source that was designed for the NIST-3 watt balance to determine the Planck constant h and will be used for the NIST-4 watt balance. The current source is used to control the induction coil position in the magnetic field. It features bipolar operation to ±20 mA, high resistance isolation to ground, and low power consumption. Index Terms — programmable current source, watt balance, Planck constant, GPIB to fiber optic interface, CPLD
monitors, and from 60 Hz power line interference. The PCS should be galvanically isolated, with resistance to ground of at least 10 Ω. It should be able to output up to ±20 mA at a compliance voltage of 18 V, the current noise should be less than 100 pA/√Hz at 1 Hz when the nominal current is 10 mA. The present state of the art for current source noise characteristics is an order of magnitude less than this [2], but these designs are neither bipolar nor programmable. The current source must also be able to drive a highly inductive load.
I. DESCRIPTION OF FORCE MODE IN NIST-3 WATT BALANCE The current NIST-3 watt balance described in [1] compares mechanical power to electrical power expressed in SI units. The result links a mass standard, m, to the Planck constant h. The experiment operates in a vacuum and is placed inside a radiofrequency shielded room. An induction coil suspended from a balance is placed in a background radial magnetic field. In the force mode, a current, I, in the induction coil creates a magnetic force to balance the gravitational acceleration, g, on a reference kilogram mass, m, by means of a wheel balance. The programmable current source operates in a feedback loop to generate the necessary current I flowing through the induction coil to maintain the position of the balance to within a few nanometers. The position of the balance is continuously monitored using a laser interferometer having a resolution of 0.6 nm. A tare mass m/2 is placed on – the counter mass side and balanced with negative current I , and then the reference mass m is placed on and balanced with + positive current I . In normal operation of the watt balance, the PtIr reference mass of 1 kg is used and 0.5 kg is placed on the counter mass. The current I is ramped in the induction coil having an inductance of 9 H and a resistance of 750Ω from zero to ±10 mA to create the necessary electromagnetic force to balance ±5 N, depending on whether the PtIr mass is on or off the mass pan. The current I is determined by measuring the voltage drop of 1 V across a 100Ω resistance. The performance of the current feedback is related to the noise, resolution, and short term stability of the programmable current source.
II. DESIGN CHOICES We chose to control the PCS using a fiber optic cable and to power it using batteries. The programmable current source consists of two parts, a GPIB (IEEE-488) to fiber optic interface, and a fiber optically controlled PCS. A. GPIB to fiber optic interface The GPIB to fiber optic interface connects the desktop computer to the PCS via a fiber optic cable. It converts the data stream over the GPIB bus from the desktop computer to an optical signal. We chose the GPIB bus because it remains relatively popular and requires no clock that could generate unwanted interference. B. Fiber optically controlled programmable current source A simplified diagram of the fiber optically controlled PCS is shown in Fig. 1. The PCS consists of an optical receiver, timing circuitry, a complex programmable logic device (CPLD), two digital to analog converters (DACs), a summing circuit, and a transimpedance amplifier. The timing circuitry is asynchronously triggered by the received optical data and distributes the serial data and necessary clocks to the CPLD. The CPLD logic receives the serial data and reformats it to control two 20-bit DACs. Two DACs are used for coarse and fine operation to provide adequate resolution to span the required current range and compensate for drifts. The summing circuit combines the signals from the coarse and fine DACs and the transimpedance amplifier converts the voltage to a current. Since the PCS is battery-powered, the components were carefully chosen for low power consumption.
II. REQUIRED SPECIFICATIONS The programmable current source (PCS) should be well isolated from digital noise sources, such as computers and
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The PCS is comparable to the one described in [3] but with only one optical fiber, higher output current, and greater resolution. The DACs were chosen for high resolution (20 bits), low temperature coefficient (
