A Spinning Current Circuit for Hall Measurements Down to the Nanotesla Range

The spinning current modulation technique is shown not only to be able to suppress all the offset and low-frequency noise contributions from the Hall sensing element and its preamplifier, but it may also suppress the parasitic signals, such as pickup noise and thermal electromotive force contributed by the interconnects in a hybrid Hall plate/driving electronics system with remote Hall sensor head. The selection process of an adequate modulation sequence achieving this goal is detailed. The optimization of the elements involved in the analog circuitry, including the analog switches, differential amplifiers, filters, interconnects, and so on, for given measurement conditions is discussed. A printed circuit board aimed at optimizing the operating conditions for a broad spectrum of Hall sensor types and applications, using pluggable modules for adapting the gain and transfer function, is presented. The modulation sequence, modulation frequency, and fine-tuning of the delays involved in the spinning sequence are adjustable by software. With this circuit board and the optimized spinning sequence, we show that the parasitics originating from the interconnection in a hybrid Hall microsystem with remote sensor 1.5 m away can indeed be suppressed. We demonstrate an offset reduction by about four decades, from 630 μT down to less than 100 nT for a system with +/-35-mT full scale. The residual noise has a white spectrum down to at least 60 mHz, close to the Johnson-Nyquist thermal noise. A detectivity of 30 nT down to the sub-Hz range is demonstrated.