Magnetic field sensors including a magnetic field sensing element, or transducer, such as a Hall Effect element or a magnetoresistive element, are used in a variety of applications to detect aspects of movement of a ferromagnetic article, or target, such as proximity, speed, and direction. Applications using these sensors include, but are not limited to, a magnetic switch or “proximity detector” that senses the proximity of a ferromagnetic article, a proximity detector that senses passing ferromagnetic articles (for example, magnetic domains of a ring magnet or gear teeth), a magnetic field sensor that senses a magnetic field density of a magnetic field, and a current sensor that senses a magnetic field generated by a current flowing in a current conductor. Magnetic field sensors are widely used in automobile control systems, for example, to detect ignition timing from a position of an engine crankshaft and/or camshaft, and to detect a position and/or rotation of an automobile wheel for anti-lock braking systems.
Magnets, in the form of a permanent magnet, or magnetically permeable structures, sometimes referred to as concentrators or flux guides, are often used in connection with magnetic field sensors. In applications in which the ferromagnetic target is magnetic, a magnetically permeable concentrator or magnetic flux guide can be used to focus the magnetic field generated by the target on the magnetic field transducer in order to increase the sensitivity of the sensor and, allow use of a smaller magnetic target, or allow the magnetic target to be sensed from a greater distance (i.e., a larger airgap). In other applications in which the ferromagnetic target is not magnetic, a permanent magnet, sometimes referred to as a back bias magnet, may be used to generate the magnetic field that is then altered by movement of the target.
During manufacturing or during use in application, magnetic field sensors (and other parts) sometimes present failures. These failures may be due to manufacturing defects, design defects, latent failures, or a combination of both. Magnetic field sensors can also develop faults after they are installed in a working environment.
To reduce the occurrence of defective parts entering the field, parts are often tested during or after the manufacturing process. Some parts have been designed to include self-test capabilities, i.e. internal circuitry included in the part that can be used by the part to test itself during manufacturing. These self-tests may include built-in self-tests (i.e. “BIST” tests). During a so-called logical-BIST (i.e. an “LBIST” test), for example, the internal registers within the part are connected together in a scan chain so that the output of one leads to the input of the next. Data is fed through the scan chain and the result is compared to an expected value. If the result does not match the expected value, it may indicate there is a defect in the part. Certain parts may also include other types of BIST or self-test circuitry including tests that check the integrity of analog circuits.
Circuits or parts that perform testing on themselves cannot always achieve a desired level of test coverage. For example, it may be difficult or costly to design test circuits that test certain other circuits within the part. As another example, once a part is installed in its operating environment, certain types of self-tests may interrupt operation or degrade performance of the part if performed while the part is under operation.