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Figure 11: A typical application assembly
with the ADMT4000 positioned opposite
a dipole magnet mounted on a rotating
shaft. (Source: Analog Devices)
The ADMT4000 also includes a quadrant
sensor, which derives quadrant-sensing
information from the GMR sensor and
combines it with the output of the angle
sensor. The quadrant sensor extends the
measurement range of the angle sensor
(which has a limit of 180°) to a full 360°. In
addition, the ADMT4000 includes GMR
signal-conditioning circuitry in the form of
an analog front end (AFE) and a multiplexer
(MUX) that connects AMR and GMR
signals to an analog-to-digital converter
(ADC). The ADC connects to a control
logic block, which in turn connects to
Serial Peripheral Interface (SPI) and Single
Edge Nibble Transmission (SENT) digital
outputs (Figure 12), the latter of which is
commonly used in automotive applications
for low-pin-count communication.
Magnet Selection and
Stray Field Immunity
Magnetic field strength is another design
consideration. The ADMT4000 features
a magnetic window with a range of 16mT
to 31mT, which can be applied using a
diametrically magnetized dipole magnet
with end-of-shaft mounting for enhanced
field uniformity. At less than 16mT, there
may not be sufficient magnetic energy to
Figure 12: ADMT4000 functional block diagram. (Source: Analog Devices)
move the domains out of the DWG and
into the spiral. Similarly, if a field higher
than 31mT is applied, it can inadvertently
saturate the device and reset the sensor.
The sensor can be deliberately reset by
using a high magnetic field of 55mT or
more, exposing it to an external magnet,
bringing the system magnet closer to
the sensor, or applying a short current
pulse to a specially designed coil located
close to the sensor. A manual reset can
also be performed by rotating the sensor
(i.e., magnet) clockwise by 46 turns and
then counterclockwise for 46 turns.
To support evaluation of the various
electrical and magnetic aspects of an
ADMT4000-based system, Analog
Devices offers the EVAL-ADMT4000SD1Z
evaluation kit, which provides a flexible
hardware platform along with a graphical
user interface. The kit, featuring an
end-of-shaft magnet configuration
(Figure 13), demonstrates turn count,
angular measurement, system-level
calibration, and device configuration.
Additionally, it is fitted with a coil to enable
magnetic reset of the turn-count sensor.
Figure 13: The EVAL-ADMT4000SD1Z
evaluation kit with end-of-shaft magnet
configuration. (Source: Analog Devices)
