This Ph.D. dissertation describes the theory and realization of wide-bandwidth magnetic sensors for current measurements which breach the conventional offset and noise constraints in CMOS processes. These are achieved by several circuit-level and system-level innovations, including the use of three orthogonal ripple reduction loops (RRLs) in spinning-current Hall sensors, a multipath architecture with double Hall sensors, and a hybrid magnetic sensor system combining Hall sensors and pick-up coils. The prototypes with these techniques have advanced the bandwidth of state-of-the-art CMOS magnetic sensors by more than two orders of magnitude.@en

This paper presents a temperature-insensitive magnetic sensor system for contactless current measurements. To simultaneously achieve wide bandwidth and low noise, the proposed system employs a multi-path structure with a set of spinning current Hall sensors in its low-frequency path and a set of pick-up coils in its high-frequency path. The Hall sensors and pick-up coils are used in a differential sensing arrangement that naturally rejects common-mode magnetic field interference, e.g., due to the earth's magnetic field. A common-mode ac reference field can then be used to continuously stabilize the sensitivity of the Hall sensors, which, unlike that of the pick-up coils, is quite temperature dependent. In this design, the ripple reduction loops in the Hall sensor readout are implemented in a discrete-time manner, and so occupy 20% less area than a previous continuous-time implementation. Over a-45 °C to 105 °C temperature range, the proposed system reduces the Hall sensor drift from 22% to 1%, which corresponds to a temperature coefficient of 76 ppm/°C.

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This paper proposes a multipath multisensor architecture for CMOS magnetic sensors, which effectively extends their bandwidth without compromising either their offset or resolution. Two designs utilizing the proposed architecture were fabricated in a 0.18-μm standard CMOS process. In the first, the combination of spinning-current Hall sensors and nonspun Hall sensors achieves an offset of 40 μT and a resolution of 272 μTrms in a bandwidth of 400 kHz, which is 40 times more than previous low-offset CMOS Hall sensors. In the second, the combination of spinning-current Hall sensors and pickup coils achieves the same offset, with a resolution of 210 μTrms in a further extended bandwidth of 3 MHz, which is the widest bandwidth ever reported for a CMOS magnetic sensor.
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CMOS Hall sensors are widely used as magnetic sensors due to their linearity and ease of integration [1-3]. Being essentially n-well resistors, their resolution is determined by thermal noise and so decreases with bandwidth (Fig. 11.3.1), limiting their use in wide-band current-sensing applications, such as in switched-mode power supplies, electric motor control and short-circuit detection. In contrast, the differentiating characteristic of pick-up coils means that their resolution actually improves with bandwidth (Fig. 11.3.1). This paper describes a hybrid sensor in standard CMOS that achieves wide bandwidth and good resolution by employing a Hall sensor in a low-frequency (LF) path and a coil in a high-frequency (HF) path. The sensors' outputs are amplified by two transconductance amplifiers (gm,coil and gm,HS) and then combined by a transimpedance amplifier (TIA) (Fig. 11.3.1). The sensor achieves a resolution of 210μTrms in a 3MHz bandwidth; the latter represents an order of magnitude improvement over state-of-the-art CMOS magnetic sensors [1,4].@en

This paper presents a temperature-stable system for wide-bandwidth contactless current sensing. It employs a multipath combination of Hall sensors (low frequencies) and coils (high frequencies) to sense the magnetic field produced by a current-carrying conductor under a chip. To cancel the effect of earth's common-mode field, a differential sensing arrangement is used. This is further exploited to stabilize the Hall sensor's temperature drift with the help of a self-generated AC commonmode field. In a test chip fabricated in a 0.18 μm CMOS process, the stabilization scheme reduces the Hall sensor's sensitivity drift from 22% to 1% from -45°C to 105°C, corresponding to a temperature coefficient of 76 ppm/°C. The complete system has a bandwidth of 3 MHz, which represents a 10x improvement on previous low-drift CMOS magnetic sensors.@en

This paper presents an energy-efficient readout circuit for micro-machined resonant sensors. It operates by briefly exciting the sensor at a frequency close to its resonance frequency, after which resonance frequency and quality factor are determined from a single ring-down transient. The circuit employs an inverter-based trans-impedance amplifier to sense the ring-down current, with a programmable feedback network to enable the readout of different resonant sensors. An inverter-based comparator with dynamically-adjusted threshold levels tracks the ring-down envelope to measure quality factor, and detects zero crossings to measure resonance frequency. The excitation frequency is dynamically adjusted to accommodate large resonance frequency shifts. Experimental results obtained with a prototype fabricated in 0.35 μm standard CMOS technology and three different SiN resonators are in good agreement with conventional impedance analysis. The prototype achieves a frequency resolution better than 30 ppm while consuming less than 80 nJ/meas from a 1.8 V supply, which is 7.8x less than the state-of-the-art.@en

This paper presents a new ripple-reduction technique for spinning-current Hall sensors, which obviates the need for lowpass filtering to suppress the ripple caused by up-modulated sensor offset. A continuous-time ripple-free output is achieved by the use of three ripple reduction loops (RRLs), which continuously sense the offset ripple and then use this information to drive a feedback
loop that cancels sensor offset before amplification. Since no low-pass filter is involved, the bandwidth of the resulting system can be much higher than the spinning frequency. Moreover, since the front-end no longer has to process sensor offset, the requirements on its dynamic range can be significantly relaxed. A prototype system consisting of a Hall sensor readout system realized in a 0.18 m CMOS process was combined with three off-chip RRLs realized with off-chip electronics.At a spinning frequency of 1 kHz, the RRLs reduce the offset ripple by more than 40 dB to about 10 T, while also achieving low offset (25 T) and wide bandwidth (over 100 kHz).@en