Magnetic field feedback delta-sigma modulator sensor circuit

[0001] This application is related to U.S. provisional applications Ser. No. 60/888,622 and Ser. No. 60/888,622, both filed Feb. 7, 2007 and from which it claims benefits under 35 U.S.C. §119(e).

[0002] 1. Field of the Invention

[0003] The present invention relates generally to magnetic field sensors, and more specifically, to a converter circuit for providing a digital output from a semiconductor magnetic field sensor.

[0004] 2. Background of the Invention

[0005] Hall effect sensors and other semiconductor magnetic field sensors are widely used in applications in which it is desirable to provide a measurement of DC magnetic fields and relatively low frequency AC magnetic fields that are not otherwise easily sensed with coils or other antennas. Such applications include position and motion sensors for both linear and rotational motion, power supply and motor control applications in which the transformer or motor fields are detected, audio speaker applications in which the strength of the speaker's signal-induced field is detected, and lighting controllers for high-frequency energized lamps, such as sodium lamps.

[0006] Semiconductor magnetic field sensors typically vary widely in their output voltage vs. field strength sensor function, both with temperature and from device to device. Background magnetic fields may also contribute to measurement error, depending on the proximity of magnetic field sources and/or the strength of the field being measured with respect to normal background magnetic field strength.

[0007] Typical circuits for measuring the output of a semiconductor magnetic field sensor include circuits that connect a Hall effect or other sensor in the feedback path of an operational amplifier circuit, which is then provided to an analog-to-digital converter circuit or an analog comparator that detects the sensor output. Calibration in offset and gain provide for compensation for variation between devices, and temperature compensation may also be included in the amplifier circuit.

[0008] More recently, sensor implementations have been proposed that impose an AC magnetic field on the sensor, which can then be detected at the sensor output and used to calibrate the sensor circuit. One or more current loops is implemented around the sensor and stimulated with a signal having known characteristics, inducing an AC magnetic field at the sensor, which causes an AC voltage of the same frequency profile in the sensor output voltage. The sensor output can then be detected and the gain of the sensor circuit adjusted to calibrate the sensor. However, such a circuit has significant extra complexity, since a calibration source and detector are required, and the calibration must be either periodic, or out of the band of the desired detection range of the sensor circuit, which will typically require additional filtering circuitry.

[0009] Therefore, it would be desirable to provide a semiconductor magnetic field sensor measurement circuit that is calibrated via an induced magnetic field, without requiring much additional complexity in the measurement and calibration circuits.

[0010] The above stated objective of providing a semiconductor magnetic field sensor measurement circuit that is calibrated via an induced magnetic field, without requiring much additional complexity in the measurement and calibration circuits is accomplished in a circuit and method. The method is a method of operation of the circuit.

[0011] The circuit is a delta-sigma analog-to-digital converter having the semiconductor magnetic field sensor output terminal connected to an input of the delta-sigma modulator. The output of the quantizer of the delta-sigma modulator is magnetically coupled to the body of the semiconductor magnetic field sensor, providing a feedback path that causes a magnetic field to be imposed at the sensor that cancels the voltage produced at the output of the sensor, for frequencies in the band of the converter loop filter. Variations in the average value of the quantizer output thus represent variations in the detected magnetic field. The quantizer output is coupled to a current output digital-to-analog converter which provides a current that energizes a current loop that is inductively coupled to the sensor body.

[0012] The action of the magnetic field feedback described above provides automatic calibration of the measurement circuit and sensor, since the relationship between the quantizer output and the field produced by the current loop is well-defined. Therefore, variations in the external magnetic field that are canceled by the action of the feedback are also well-defined. A chopper amplifier can be coupled between the sensor voltage output and the loop filter of the converter to remove offset and prevent the introduction of 1/f noise in the measurement. Further reduction of sensor variation error and 1/f noise can be accomplished by rotating the bias and sensor output terminal positions by using a rotating switching circuit.

[0013] The foregoing and other objectives, features, and advantages of the invention will be apparent from the following, more particular, description of the preferred embodiment of the invention, as illustrated in the accompanying drawings.

[0017] The present invention encompasses a semiconductor magnetic field sensor circuit and method that uses the noise-shaping loop of a delta-sigma modulator analog-to-digital converter (ADC) to provide a calibration signal to the semiconductor magnetic field sensor. The resulting operation self-calibrates the circuit to compensate for the offset voltage and low frequency variation of the sensor output signal. The ADC output provides the digital output of the sensor circuit, yielding a compact integrated solution with high degree of noise immunity.

[0018] Referring now to FIG. 1, a semiconductor magnetic field sensor circuit in accordance with an embodiment of the present invention is shown. A Hall effect sensor H, which may alternatively be another type of magnetic field sensor such as a magneto-resistive device, is provided a bias current I_B, and has output terminals connected to differential inputs of amplifier A, which may be a chopper amplifier to reduce offset and 1/f noise that would otherwise be introduced by amplifier A. A loop filter 10 filters the output of amplifier A and a quantizer 12 quantizes the output of loop filter 10 to provide an analog-to-digital conversion. The output of quantizer 12 is converted to a differential current by a digital-to-analog converter (DAC) 16. The outputs of DAC 16 are connected to a magnetic feedback current loop L, which has an axis aligned with the magnetic field sensing axis of Hall effect sensor H, e.g., the plane of the semiconductor body of Hall effect sensor H is parallel to the plane of the windings of loop L.

[0019] As quantizer 12 generates a noise-shaping waveform, a magnetic field is induced by loop L that causes a corresponding variation in the output voltage of Hall effect sensor H. Thus, the circuit of FIG. 1 provides a delta-sigma modulator loop that includes the inductive coupling of current loop L to Hall effect sensor H, as well as the sensor function from the field present at Hall effect sensor H to the output terminals of Hall effect sensor H. Since the magnetic field characteristics of current loop L are substantially linear and well-known, provided that ferromagnetic or paramagnetic materials are not within proximity of current loop L, the closed-loop converter has a well defined transfer function from changes in magnetic field strength at sensor H to changes in the output of quantizer 12. The action of the feedback provided by DAC 16 and current loop L provides that the average value of the output of quantizer reflects only the external magnetic field at Hall effect sensor H, as the magnetic field introduced by current loop L and any offset from Hall effect sensor H are canceled by the action of the delta-sigma modulator loop. The output of quantizer 12 is decimated and filtered by digital decimation filter 14 to yield a digital number corresponding to the external field strength at sensor H.

[0020] Referring now to FIG. 2, a semiconductor magnetic field sensor circuit in accordance with another embodiment of the present invention is shown. A Hall effect sensor H′, which may alternatively be another type of magnetic field sensor such as a magneto-resistive device, has four terminals connected to a switch S1, which is operated by a clock signal that alternates which opposing pair of Hall effect sensor H terminals receive bias current I_B and which (opposing) pair of terminals provide the output voltage to the differential inputs of a chopper amplifier A2. The alternation of terminals of Hall effect sensor H is known as rotating or “spinning” the Hall effect bias current. The action provided by switch S1 further reduces 1/f noise and offset from Hall effect sensor H, as the bias current I_B is effectively shifted (chopped) to the chopping frequency f_c that also controls chopper amplifier A2. The operation of switch S1 and chopper amplifier A2 thus shifts the field detection frequency by the chopping frequency, so that offset and low-frequency 1/f noise and other artifacts are removed from the measurement. The remainder of the circuit of FIG. 2 operates as described above with reference to the circuit of FIG. 1.

[0021] Referring now to FIG. 3, an integrated circuit die 20 in accordance with an embodiment of the present invention is shown connected to semiconductor magnetic field sensor H′ and magnetic feedback loop L. The circuits shown in FIG. 1 or FIG. 2 may be integrated on integrated circuit die 20. In the depicted embodiment, die 20 has four input terminals S1-S4 corresponding to the sensor-side terminals of switch S1 of FIG. 2. Terminals F− and F+ are provided from the output of DAC 16 for providing feedback signals to feedback loop L. An output terminal OUT provides a digital output signal from digital decimation filter 14.

[0022] While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.