Electronic Design

Magnetic Sensor Helps ASIC Measure Changing Fields

Is there a more accurate way to measure changing magnetic fields in applications like automotive navigation systems, medical imaging, land-mine detection, and industrial controls? Fluxgate-technology-based magnetometers served some of these applications adequately in the past. Now, however, sensing accuracy under dynamic conditions must improve without raising cost. Doing so will broaden the range of applications for these devices.

In an effort to upgrade existing methods, American Microsystems Inc. (AMI) in Pocatello, Idaho, and Precision Navigation Inc. (PNI), based in Santa Rosa, Calif., pooled their resources. Together, they developed a solution that marries a unique magnetic-sensing technique with submicron CMOS ASIC technology. According to AMI, the new magnetic-sensor ASIC solution is more sensitive than both the emerging giant magnetoresistive (MR) and traditional fluxgate sensors.

In applications like automotive navigation systems, the sensor must detect small changes in a magnetic field. At the same time, it should cancel out any magnetic noise present in the vehicle. The sensor must also handle any external static that may interfere with a passing vehicle's magnetic field. Ultimately, the challenge is to suppress all the spurious magnetic noise and accurately interpret the true magnetic signals.

"Existing sensors, such as MR, fluxgate, and Hall-effect types have two major drawbacks," notes AMI mixed-signal design engineer Tim White. "These sensors require high current pulses and present huge overhead in analog processing. Together, AMI and PNI have addressed the issue by merging a novel magnetic-sensing technique with clever low-power ASIC design that eliminates a lot of complex analog processing. And, unlike traditional magnetic sensors that require high-current pulses, the AMI/PNI solution's power demands are substantially lower. Since there is no need to process low-amplitude voltages or currents via high-gain amplifiers, integrators, and analog filters, its drive-current requirements are only a few milliamps at a 5-V supply voltage."

A newer version of the ASIC can work with supply voltages as low as 2.5 V and a supply current of only 1 mA. It also incorporates a third coil for 3D measurement capability. In fact, it is being aimed at portable devices like wristwatches and other battery-operated products (see the figure). While the 5-V part uses AMI's 1-µm CMOS process, the 2.5-V version has been implemented in 0.8-µm CMOS.

In this scheme, the ASIC interfaces with a set of proprietary nonlinear coils developed by PNI. Changes in coil inductance are prompted by minor variations in the magnetic field. The unique core material contained in this magneto-inductive sensor accounts for such a property. For wrist-watch or similar applications where the compass module must be really tiny, PNI has developed magneto-inductive micro-coils with dimensions as small as 0.2 by 0.036 by 0.036 in.

As a result, the coils are surrounded by a precise state of flux. They're configured to share identical analog circuitry and can be driven by the same ASIC chip developed by AMI using PNI's algorithm. To measure both polarities, the coils are situated perpendicular to each other. This magnetic flux is converted into frequency by an LR oscillator on the ASIC chip. The on-chip state machine changes the frequency into 16 bits of information, revealing the magnetic field's direction and strength.

"Measuring frequency cycles for the same period of time in both polarities of the coil has the effect of making a fully differential measurement," says White. This approach has the effect of canceling the magnetic noise in the system. In addition, all the offsets and magnetic noise are corrected by the on-chip digital state machine. It does not require any nonvolatile memory for autocalibration. As a result, it ensures ±0.4-µT (microTesla) accuracy. The on-chip registers also permit the user to adjust the sensitivity of the circuit.

For more information, point your browser at www.precisionnav.com.

TAGS: Automotive
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