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Philips Semiconductors
Electronic Compass Design using
KMZ51 and KMZ52
5.3 Sensitivity Difference (∆S) Compensation
At a given temperature, ∆S can be compensated by adjusting the SCU for equal output voltage swings Vy,pp
and Vx,pp during compass rotation. The output voltage swings can be equalized by adjusting the amplification
of one SCU channel. An alternative software solution would be to correct one output voltage mathematically: As
the ratio Vy,pp/Vx,pp equals the ratio of sensitivities Sy/Sx=(S+∆S)/S, Vy can be corrected as follows:
=
V
V
, y
corrected
, y
When determining ∆S by measuring SCU output voltage swings, soft iron interference effects have to be
avoided carefully, as these would also appear as difference in output voltage swings (refer to section 7).
To maintain high accuracy over temperature, the temperature drift of sensor sensitivity should be compensated
during operation. Figure 13 shows the output characteristic Vo vs. H of a typical MR sensor element at different
temperatures. All these curves indicate, that the sensor output is significantly dependent on temperature except
at its zero point: a field strength of zero always causes an output voltage of exactly zero, independent of
temperature. Of course this is only true disregarding sensor offset. However, offset can be eliminated perfectly
by means of the flipping technique, as described above.
Thus, temperature drift can be compensated by operating the sensors at their zero point. Therefore, the earth
field component at each sensor has to be compensated by an opposing field of equal strength. If now
magnitude and polarity of this compensation field are known, the measurement task is fulfilled, even though the
sensor delivers zero output signal. This method is referred to as "electro-magnetic feedback".
V
, x
pp
⋅
measured
V
, y
pp
15
V O
(mV/V)
10
5
0
5
10
15
3
2
Figure 13
Typical MR sensor output characteristic (KMZ10)
(7)
T
amb
operating range
1
0
1
19
Application Note
o
=
25 C
o
25 C
o
75 C
o
125 C
2
3
H
(kA/m)
y
AN00022
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