The measurement of electric current strength is not always easy, especially when the measured signal requires further electronic conditioning. Simply connecting an ammeter to an electrical circuit and reading out the value is no longer enough. The current signal must be fed into a computer in which sensors convert current into a proportional voltage with minimal influence on the measured circuit. The basic sensor requirements are galvanic isolation and a high bandwidth, usually from DC up to at least 100 kHz. Conventional current measurement systems therefore tend to be physically large and technically complex.
Conventional sensors are physically large and technically complex; also they have disadvantages as stated above. Hence they are replaced by magnetoresistive current sensors. The magnetic field sensors are based on the magnetoresistive effect. These sensors can be easily fabricated by means of thin film technologies wit widths and lengths in the micrometer range. To reduce temperature dependence, they are usually configured as a half bridge or a full bridge. In one arm of the bridge, the barber poles are placed in opposite directions above the two magnetoresistors, so that in the presence of a magnetic field the value of the first resistor increases and the value of the second decreases.
The anisotropic magnetoresistive effect is known to be present in a whole family of ferromagnetic alloys. Most of these alloys are composed of iron, nickel, and chromium, and may be primary or ternary. They have in common a more or less strong anisotropy in their magnetic properties. Whenever these materials are exposed to a magnetic field during crystal formation, a preferred orientation in magnetization will result. The same happens when the materials are forced into shape that is a mechanical anisotropy is imposed.
It is found that changing the orientation of the magnetic moment in the wire caused a current passing through it to change correspondingly. The orientation could be changed by apply in an external magnetic field, and generally an increase in current was observed. This phenomenon is called anisotropic magnetoresistive effect.
The ferromagnetic materials can be deposited as thin films and structured into small strips that are typically 40mm thick,10mm wide, and 100mm long. In most general case, the electrical resistance of AMR material depends on the angle between the direction of the magnetization, and the direction of the current going through it. When the current and magnetic moment are parallel, the resistance of the strip is greatest; when they are at a 90 degree angle to each bother, it is smallest.
Magnetoresistive field sensors are usually configures as a half or full bridge. The barber poles are positioned such that in the presence of magnetic field the value of first resistor increases and that of second decreases.