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2000 Sep 06 28
Philips Semiconductors
Magnetoresistive sensors for
magnetic field measurement
General
The optimal method of compensating for temperature
dependent sensitivity differences in MR measurements of
weak fields uses electro-magnetic feedback. As can be
seen from the sensor characteristics in Figs 31 and 32,
sensor output is completely independent of temperature
changes at the point where no external field is applied
(the null-point). By using an electro-magnetic feedback
set-up, it is possible to ensure the sensor is always
operated at this point.
To achieve this, a second compensation coil is wrapped
around the sensor perpendicular to the flipping coil, so that
the magnetic field produced by this coil is in the same
plane as the field being measured.
Should the measured magnetic field vary, the sensor’s
output voltage will change, but the change will be different
at different ambient temperatures. This voltage change is
converted into a current by an integral controller and
supplied to the compensation coil, which then itself
produces a magnetic field proportional to the output
voltage change caused by the change in measured field.
The magnetic field produced by the compensation coil is in
the opposite direction to the measured field, so when it is
added to the measured field, it compensates exactly for
the change in the output signal, regardless of its actual,
temperature-dependent value. This principle is called
current compensation and because the sensor is always
used at its ‘zero’ point, compensation current is
independent of the actual sensitivity of the sensor or
sensitivity drift with temperature.
Information on the measured magnetic signal is effectively
given by the current fed to the compensating coil. If the
field factor of the compensation coil is known, this
simplifies calculation of the compensating field from the
compensating current and therefore the calculation of the
measured magnetic field. If this field factor is not precisely
known, then the resistor performing the current/voltage
conversion must be trimmed. Figure 34 shows a block
diagram of a compensated sensor set-up including the
flipping circuit.
Fig.33 Magnetic field directions and the flipping and compensation coils.
handbook, full pagewidth
flipping coil
sensor KMZ10A1
compensation coil
compensation field
flipping field
earth's field
MLC757