Philips Magnetoresistive Sensor manual Weak Field Measurement, Contents

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 bi-stable (refer to Appendix 2). ‘Standard’ techniques used to stabilize such sensors, including the application of a strong field in the x-direction (Hx) from a permanent stabilization magnet, are unsuitable as they reduce the sensor’s sensitivity to fields in the measurement, or y-direction (Hy). (Refer to Appendix 2, Fig. A2.2).

To avoid this loss in sensitivity, magnetoresistive sensors can instead be stabilized by applying brief, strong non-permanent field pulses of very short duration (a few μs). This magnetic field, which can be easily generated by simply winding a coil around the sensor, has the same stabilizing effect as a permanent magnet, but as it is only present for a very short duration, after the pulse there is no loss of sensitivity. Modern magnetoresistive sensors specifically designed for weak field applications incorporate this coil on the silicon.

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 bi-stable sensor is placed in a controlled, reversible external magnetic field, the polarity of the premagnetization (Mx) of the sensor strips can be switched or flipped between the two output characteristics (see Fig.27).

VO M x

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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