Philips Magnetoresistive Sensor manual Using magnetoresistive sensors, KMZ10B

Philips Semiconductors

Magnetoresistive sensors for

magnetic field measurement

General

Using magnetoresistive sensors

The excellent properties of the KMZ magnetoresistive sensors, including their high sensitivity, low and stable offset, wide operating temperature and frequency ranges and ruggedness, make them highly suitable for use in a wide range of automotive, industrial and other applications. These are looked at in more detail in other chapters in this book; some general practical points about using MR sensors are briefly described below.

ANALOG APPLICATION CIRCUITRY

In many magnetoresistive sensor applications where analog signals are measured (in measuring angular position, linear position or current measurement, for example), a good application circuit should allow for sensor offset and sensitivity adjustment. Also, as the sensitivity of many magnetic field sensors has a drift with temperature, this also needs compensation. A basic circuit is shown in Fig.11.

In the first stage, the sensor signal is pre-amplified and offset is adjusted. After temperature effects are compensated, final amplification and sensitivity adjustment takes place in the last stage. This basic circuit can be extended with additional components to meet specific EMC requirements or can be modified to obtain customized output characteristics (e.g. a different output voltage range or a current output signal).

Philips magnetoresistive sensors have a linear sensitivity drift with temperature and so a temperature sensor with

linear characteristics is required for compensation. Philips KTY series are well suited for this purpose, as their positive Temperature Coefficient (TC) matches well with the negative TC of the MR sensor. The degree of compensation can be controlled with the two resistors R7 and R8 and special op-amps, with very low offset and temperature drift, should be used to ensure compensation is constant over large temperature ranges.

Please refer to part 2 of this book for more information on the KTY temperature sensors; see also the Section “Further information for advanced users” later in this chapter for a more detailed description of temperature compensation using these sensors.

USING MAGNETORESISTIVE SENSORS WITH A COMPENSATION

COIL

For general magnetic field or current measurements it is useful to apply the ‘null-field’ method, in which a magnetic field (generated by a current carrying coil), equal in magnitude but opposite in direction, is applied to the sensor. Using this ‘feedback’ method, the current through the coil is a direct measure of the unknown magnetic field amplitude and it has the advantage that the sensor is being operated at its zero point, where inaccuracies as result of tolerances, temperature drift and slight non-linearities in the sensor characteristics are insignificant. A detailed discussion of this method is covered in Chapter “Weak field measurement”.

handbook, full pagewidth		offset
	adjustment
		R1		R2
			500 kΩ
		100 kΩ
KMZ10B				R3
3			22 kΩ
2
4
1				R4
			14 kΩ

					R5		R7	R9
							2.4 kΩ	33 kΩ
				140 kΩ
			8			R6
2					KTY82-210
		op-amp 1
3					TLC2272
	4
								R10
							R8
							2.4 kΩ	33 kΩ

					sensitivity			VS = 5 V
					adjustment
			R12
			150 kΩ
	R11				IC1
22 kΩ		6
						7
				op-amp				VO = 0.2 V to 4.8 V
		5						(with resistive load
								greater than 10 kΩ)
							C1
							10 nF

MBH687

Fig.11 Basic application circuit with temperature compensation and offset adjustment.

2000 Sep 06 10