Mirjana Banjevic
Hall-effect devices make up by far the largest part of all magnetic sensors on the market today. The main reason is compatibility of Hall devices with modern semiconductor technology. In particular, integrated Hall sensor micro-systems fabricated in low-cost complementary metal oxide technology (CMOS) have dominated the market of magnetic sensors over the last decade. Recently, a new Hall device – the circular vertical Hall device (CVHD) – joined the family of CMOS compatible Hall devices. The CVHD can be biased and Hall voltage retrieved from it such that its output is a sine wave signal. The amplitude of the sine wave signal contains the information on the magnitude of the magnetic field, while the phase of the sine wave signal carries the information on the direction of the magnetic field. This is why a CHVD can be thought of as the first Hall device measuring an in-plane magnetic induction vector. It is due to this feature that the CVHD was first used as a sensing device in magnetic angular sensors. These CVHDs had a large number of contacts leading to a high angular accuracy, but limiting bandwidth of the sensor. This thesis was devoted to exploring and pushing the limits of CVHDs and integrated magnetic sensors based on CVHDs. The first aim of the thesis was to study the limits of bandwidth increase while keeping satisfying accuracy of angular sensors based on the CVHD. In order to increase the angular sensor's bandwidth, we optimized the CVHD, the front end of the sensor comprising the device and interface electronics, and finally the system level topology. The CVHD was ultimately miniaturized to the device containing only eight contacts (8CVHD). The smaller the number of contacts, the faster is obtained the sine wave signal from the CVHD. A novel symmetric Hall voltage retrieval facilitated by the geometry and including all contacts was used. It led to the residual offset voltage comparable with devices having a much larger number of contacts. It was shown that the increase of the sensor's bandwidth is not only limited by the device but also by the interface electronics. This is particularly so because the sensor system relies on the spinning current method for offset reduction. In this case, the interface electronics includes spinning switches and a preamplifier. The spinning current method introduces voltage spikes that decrease accuracy when increasing bandwidth. The modeling of the horizontal Hall device and the interface electronics containing spinning switches and preamplifier was presented. The model was extended for the specific case of the sensors based on the 8CVHD. Three solutions to tackling the challenge of increasing the spinning frequency, or equivalently sensor's bandwidth, while maintaining accuracy were proposed and discussed. The first one is a novel solution relying on the high input capacitance of the preamplifier which together with the sensing switch on-resistance filters out the voltage spikes. The second solution examines the partial guard band to remove a part of the spikes' energy without compromising the high spinning frequency. The third solution is based on the cancellation of the voltage spikes by the time-shifted clockwise and counterclockwise spinning current method on two pairs of devices. These solutions were employed in three different sensors based on the 8CVHD. On the system level, a novel concept of the angular sensor based on two 8CVHDs was used. The outputs of the 8CVHDs are separately processed in two channels and they act as mutual reference signals. The Hall voltage retrieval is done in the clockwise direction for the first device, and in the counter clockwise direction for the second one. In this way there is no need for the reference signal as in the known concept. The use of two devices doubles the sensitivity of the sensor. Finally, the optimized device, the solutions for the design of interface electronics, and the system level topology were combined in two implementations in 0.35 µm CMOS technology. The first implementation employed one 8CVHD per channel. The average angular error was found to be ± 4° with the sensor's bandwidth of about 300 kHz. The second implementation employed an array of 8CVHDs per channel. The average angular error was found to be ± 1.5° with the sensor's bandwidth of about 500 kHz. The second aim of the thesis was to investigate the feasibility of magnetic sensors based on the 8CVHD for some challenging applications. To this end a two-dimensional (2D) magnetometer and a magnetic sensor for use in open-loop current transducer were designed and experimentally characterized. The novel system level concept for the 2D CMOS integrated magnetometer based on the 8CVHD was proposed. It enables common biasing, signal retrieval, dynamic cancellation of offset and low frequency noise, and front-end signal conditioning electronics for both components of the measured in-plane magnetic field vector. Separation of two channels is postponed to the point where signal levels are high and less susceptible to nonidealities and mismatches of signal conditioning electronics. The magnetometer features bandwidth around 60 kHz, wide dynamic range 0-1.5 T, high spatial resolution, and a high measurement resolution of 300 µT over frequency range 0-30 kHz. The magnetic sensor for the open-loop current transducer is based on the high bandwidth angular sensor and a tangent converter circuit. The initial experimental verification showed the bandwidth around 100 kHz, along with a wide dynamic range and good linearity.
EPFL2011
