Philips Magnetoresistive Sensor

2000 Sep 06 13

Philips Semiconductors

Magnetoresistive sensors for

magnetic ﬁeld measurement General

The following general recommendations for operating the

KMZ10 can be applied:

•To ensure stable operation, avoid operating the sensor

in an environment where it is likely to be subjected to

negative external fields (‘−Hx’). Preferably, apply a

positive auxiliary field (‘Hx’) of sufficient magnitude to

prevent any likelihood of flipping within he intended

operating range (i.e. the range of ‘Hy’).

•Beforeusing the sensor for the first time, apply a positive

auxiliaryfield of at least 3 kA/m;this will effectively erase

the sensor’s magnetic ‘history’ and will ensure that no

residual hysteresis remains (refer to Fig.6).

•Use the minimum auxiliary field that will ensure stable

operation, because the larger the auxiliary field, the

lowerthe sensitivity, but the actual value will depend on

the value of Hd. For the KMZ10B sensor, a minimum

auxiliaryfield of approximately 1 kA/mis recommended;

to guarantee stable operation for all values of Hd, the

sensorshould be operated in an auxiliary field of 3 kA/m.

These recommendations (particularly the first one) define

a kind of Safe Operating ARea (SOAR) for the sensors.

This is illustrated in Fig.15, which is an example (for the

KMZ10B sensor) of the SOAR graphs to be found in our

data sheets.

The greater the auxiliary field, the greater the disturbing

field that can be tolerated before flipping occurs.

Forauxiliary fields above 3 kA/m, the SOAR graph shows

that the sensor is completely stable, regardless of the

magnitudeof the disturbing field. It can also be seen from

this graph that the SOAR can be extended for low values

of ‘Hy’. In Fig.15, (for the KMZ10B sensor), the extension

for Hy< 1kA/m is shown.

TEMPERATURECOMPENSATION

With magnetoresistive sensors, temperature drift is

negative. Two circuits manufactured in SMD-technology

which include temperature compensation are briefly

described below.

Thefirst circuit is the basic application circuit already given

(see Fig.11). It provides average (sensor-to-sensor)

compensationof sensitivity drift with temperature using the

KTY82-210 silicon temperature sensor. It also includes

offset adjustment (via R1); gain adjustment is performed

with a second op-amp stage. The temperature sensor is

partof the amplifier’s feedback loop and thus increases the

amplification with increasing temperature.

The temperature dependant amplification A and the

temperature coefficient TCA of the first op-amp stage are

approximately:

for R8=R

Tisthe temperature dependent resistance of the KTY82.

The values are taken for a certain reference temperature.

This is usually 25 °C, but in other applications a different

reference temperature may be more suitable.

Figure 16 shows an example with a commonly-used

instrumentation amplifier. The circuit can be divided into

two stages: a differential amplifier stage that produces a

symmetrical output signal derived from the

magnetoresistive sensor, and an output stage that also

provides a reference to ground for the amplification stage.

To compensate for the negative sensor drift, as with the

above circuit the amplification is again given an equal but

positive temperature coefficient, by means of a

KTY81-110 silicon temperature sensor in the feedback

loop of the differential amplifier.

Fig.15 SOAR of a KMZ10B sensor as a function of

auxiliary field ‘Hx’ and disturbing field ‘Hd’

opposing ‘Hx’ (area I).

handbook, halfpage

012 4

MLC133

(kA/m)

H (kA/m)

SOAR

AR5

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=12RT

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TCATCKTY

1R7

2RT

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