ADA4571 Datasheet, PDF (16/21 Pages) Analog Devices – Integrated AMR Angle Sensor and Signal Conditioner

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APPLICATIONS INFORMATION
The integrated AMR sensor is designed for applications with a
separate processing IC or electronic control unit (ECU) containing
an ADC with references connected to the supply voltage. With
the ADC input resolution related to VDD in the same way as the
AMR sensor output, the system is inherently ratiometric and the
signal dependency on supply voltage changes are minimized.
ANGLE CALCULATION
To calculate angle from the output of the AMR device, use the
trigonometric function arctangent2. The arctangent2 function
is a standard arctangent function with additional quadrant
information to extend the output from the magnetic angle range
of −90° to +90° to the magnetic angle range of −180° to +180°.
Because of the sensing range of AMR technology, this
calculated magnetic angle repeats over each pole of the magnet.
For a simple dipole magnet, the following equation reports
absolute angle over 180° mechanical:
arctan(^VSIN )

V_COS
2
CONNECTION TO ECU
Because of the limited driving capability of the ADA4571
output, minimize the length of printed circuit board (PCB)
traces between the ADA4571 and other IC. Shielding of the
signal lines is recommended. Match the load capacitors and
resistors for best angular accuracy. Add bandwidth limitation
filters related to the sampling frequency of the system in front
of the ADC inputs to reduce noise bandwidth.
In Figure 29, the load resistors on VCOS and VSIN are
representing the input load of the filter and the ADC. The
processor may be used for arctan and offset calculations, offset
storage, and additional calibration.
VTEMP Output Pin
A proportional to absolute temperature circuit provides a
voltage output at the VTEMP pin for temperature monitoring
or temperature calibration purposes. The output voltage is
ratiometric to the supply voltage enabling the interface with an
ADC that uses the supply voltage to generate the reference
voltage. This pin must be left open when not in use.
To achieve maximum accuracy from the VTEMP output
voltage, perform an initial calibration at a known, controlled
temperature. Then, use the following equation to extract
temperature information:
T_VTEMP

^VTEMP

V_DD

–


^VCAL
V_DD
TC _VTEMP

– T_CAL
 T_CO


where:
TVTEMP is the calculated temperature (°C) from the VTEMP
output voltage.
VTEMP is the VTEMP output voltage during operation.
VDD is the supply voltage.
VCAL is the VTEMP output voltage during calibration at a
controlled temperature.
TCAL is the controlled temperature during calibration.
TCO is the temperature coefficient of the internal circuit; see the
Specifications section for the exact value.
Gain Control Mode
Gain control (GC) enable mode can be activated by switching
the GC pin to the VDD pin. In this mode, the AMR bridge
sensor amplitude outputs are compensated to reduce
temperature variation. This results in higher and controlled
output voltage levels, boosting system dynamic range and
easing the system design task. If the GC pin is left floating, a
weak pull-up resistor ensures that the GC mode is enabled as a
default condition. The GC mode can also be used as a sensor
self diagnostic by comparing the sine and cosine amplitude
outputs when enabled and disabled, such as radius check. In the
event that the radius does not change, it indicates a gross failure
in the IC.
Power-Down Mode
Power-down mode can be activated by switching the PD pin to
the VDD pin. Within this mode, the device shuts down and its
output pins are set to high impedance to avoid current
consumption across the load resistors. The VTEMP output is
connected to ground through a pull-down resistor. Power-down
mode can be entered with GC = VDD or GC = GND. An internal
pull-down resistor ensures that the device remains active if the
PD pin is left floating.
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