Hubert Blanchard

Highly sensitive Hall sensor in CMOS technology

Abstract We present a highly sensitive Hall device fabricated in a standard CMOS technology and combined with integrated flux concentrators acting as magnetic amplifiers. The active area of the Hall plate is in a buried n-well with a shape optimized by removing the parts less sensitive to the magnetic field. The effect of the shape of the concentrators is studied. This results in the design of elliptical shape integrated concentrators for the optimization of the sensitivity, and of the measurement range, as well as for the decrease of the overall chip size. The CMOS sensor combined with the optimized concentrators has a sensitivity of 2.1 V/T with a 4 V bias, the lowest detectable field is 0.2 μT in a frequency range of 10 −3 –10 Hz and the linearity is better than 1% in a ±16 mT measurement range.

High performance micromachined Sm2Co17 polymer bonded magnets

Abstract A new technique has been developed to fabricate thick permanent magnets in large batches with a remanence of up to 0.34 T and an energy product of over 22 kJ/m 3 . To our knowledge, these are the best values reached in micromachined magnets. To illustrate the potential of this new technology, a new type of rotational speed microsensor has been fabricated. A 1 mVpp signal is obtained at 0.25 mm from a gearwheel.

Compensation of the temperature-dependent offset drift of a Hall sensor

Abstract We present a method to cancel the offset of a Hall sensor and compensate its temperature-dependent drift. We adapt and improve a compensation technique based on a correction using the input voltage. A more accurate compensation for the temperature drift is obtained by an offset calibration at two different temperatures. To decrease the calibration time, we propose a procedure for fast heating of the sensor. It consists of forcing a current pulse through a p-n junction of the sensor. Since the resistance of a forward-biased diode is small, this principle is compatible with low-voltage applications. After correction, the resulting offset is less than two percent of the initial offset over the temperature range −10 to +60°C. The corresponding residual equivalent offset is lower than 250 μT.

Cylindrical Hall device

We present a new magnetic sensor based on the Hall effect, sensitive to a cylindrical magnetic field. Such a field is found around a wire carrying a current or under the gap between magnetic concentrators. Compared to traditional (plate-like) Hall devices, the geometry of the cylindrical Hall device allows lower bias voltage and lower offset to be achieved. The combination of this new device with integrated magnetic concentrators results in a very high detectivity magnetic sensor.

High accuracy analog Hall probe

The Hall probe we developed is a miniaturized analog system, consisting of two silicon vertical Hall devices, current stabilizers, amplifiers and error-correcting circuits. The output signal is 1 V/T, with an error less than 10/sup -4/ in the ranges /spl plusmn/2 T, 15/spl deg/C to 35/spl deg/C, and DC to 10 kHz. The probe features high long-term stability, low offset drift and low noise.