Philips Semiconductors
Magnetoresistive sensors for
magnetic field measurement
General
WEAK FIELD MEASUREMENT
Contents
∙Fundamental measurement techniques
∙Application note AN00022: Electronic compass design using KMZ51 and KMZ52
∙Application circuit: signal conditioning unit for compass
∙Example 1: Earth geomagnetic field compensation in CRT’s
∙Example 2: Traffic detection
∙Example 3: Measurement of current.
Fundamental measurement techniques
Measurement of weak magnetic fields such as the earth’s geomagnetic field (which has a typical strength of between approximately 30 A/m and 50 A/m), or fields resulting from very small currents, requires a sensor with very high sensitivity. With their inherent high sensitivity, magnetoresistive sensors are extremely well suited to sensing very small fields.
Philips’ magnetoresistive sensors are by nature
To avoid this loss in sensitivity, magnetoresistive sensors can instead be stabilized by applying brief, strong
However, when measuring weak fields, second order effects such as sensor offset and temperature effects can greatly reduce both the sensitivity and accuracy of MR sensors. Compensation techniques are required to suppress these effects.
OFFSET COMPENSATION BY ‘FLIPPING’
Despite electrical trimming, MR sensors may have a maximum offset voltage of ±1.5 mV/V. In addition to this
static offset, an offset drift due to temperature variations of about 6 (μV/V)K−1 can be expected and assuming an ambient temperature up to 100 °C, the resulting offset can be of the order of 2 mV/V.
Taking these factors into account, with no external field a sensor with a typical sensitivity of 15 mV/V (kA/m)−1 can have an offset equivalent to a field of 130 A/m, which is itself about four times the strength of a typical weak field such as the earth’s geomagnetic field. Clearly, measures to compensate for the sensor offset value have to be implemented in weak field applications.
A technique called ‘flipping’ (patented by Philips) can be used to control the sensor. Comparable to the ‘chopping’ technique used in the amplification of small electrical signals, it not only stabilizes the sensor but also eliminates the described offset effects.
When the
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offset
H y
M x
MLC764
Fig.27 Butterfly curve including offset.
This reversible external magnetic field can be easily achieved with a coil wound around the sensor, consisting of current carrying wires, as described above. Depending on the direction of current pulses through this coil, positive and negative flipping fields in the x-direction (+Hx and −Hx) are generated (see Fig.28).
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