AAT003 Low-Resistance Angle Sensor
3
NVE Corporation 11409 Valley View Road, Eden Prairie, MN 55344-3617 Phone: (952) 829-9217 Fax: (952) 829-9189 www.nve.com ©NVE Corporation
Operation
Overview—Unique TMR technology
The heart of the unique sensor is an array of four Tunneling Magnetoresistance (TMR) elements in each quadrant. TMR
technology enables low power and miniaturization, making the sensors ideal for battery operation.
In a typical configuration, an external magnet provides a saturating magnetic field in the plane of the sensor, as illustrated below
for a bar magnet and a radially-magnetized disk magnet:
Figure 1. Sensor operation.
The device contains four sensing resistors at 90 degree intervals. The resistors are connected as two half-bridges, providing the
sine and cosine voltage outputs. For each half bridge, the resistance of one element increases and the other decreases as the field
rotates. Thus the bridge resistance, device resistance, and output impedances remain constant with rotation.
Transfer function
The half-bridge configuration provides a simple interface and can simplify external circuitry such as amplifiers and comparators.
Outputs are sinusoidal, centered around half the supply, and ratiometric with supply voltage. Mathematically, the outputs can be
expressed as:
V
SIN
= [V
CC-SIN
][(V
SIN-MAX
– V
SIN-MIN
) / 2)Sin θ + V
CC-SIN
/ 2 + V
OFFSET-SIN
]
V
COS
= [V
CC-COS
][(V
COS-MAX
– V
COS-MIN
) / 2)Cos θ + V
CC-COS
/ 2 + V
OFFSET-COS
]
Where:
θ is the magnetic field angle;
V
COS
and V
SIN
are the sensor output voltages (mV/V);
V
CC-SIN
and V
CC-COS
are the sensor supply voltages (normally tied together);
V
SIN-MAX
, V
COS-MAX
, V
SIN-MIN
, and V
COS-MIN
are the sensor output peak signal levels (mV/V); and
V
OFFSET-SIN
and V
OFFSET-COS
are the sensor offset voltages (mV/V),
defined as the average of the maximum and minimum outputs minus
half the supply voltage.
Wide range of magnets and magnet locations
The sensors operate with fields from 30 Oe to 200 Oe. This wide magnetic field range allows inexpensive magnets and operation
over a wide range of magnet spacing. Larger or stronger magnets require more distance to avoid oversaturating the sensor; smaller
or weaker magnets may require closer spacing. Low-cost radially-magnetized ferrite disk magnets can be used with these sensors
in production. Bar magnets are also used in some configurations.
Ideal for battery-powered applications
