MA800 – 8-BIT, DIGITAL ANGLE SENSOR

MA800 Rev. 1.02

www.MonolithicPower.com

9

6/15/2018

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OPERATION

Sensor Front-End

The magnetic field is detected with integrated

Hall devices located in the center of the

package. The angle is measured using the

SpinaxisTM method, which digitizes the direction

of the field directly without complex arctangent

computations or feedback loop-based circuits

(interpolators).

The SpinaxisTM method is based on phase

detection and generates a sinusoidal signal with

a phase that represents the angle of the

magnetic field. The angle is then obtained by a

time-to-digital converter, which measures the

time

between

the

zero

crossing

of

the

sinusoidal signal and the edge of a constant

waveform (see Figure 2). The time-to-digital is

output

from

the

front-end

to

the

digital

conditioning block.

Top: Sine Waveform

Bottom: Clock of the Time-to-Digital Converter

Figure 2: Phase Detection Method

The output of the front-end delivers a digital

number

proportional

to

the

angle

of

the

magnetic field at the rate of 1MHz in a

straightforward and open-loop manner.

Digital Filtering

The front-end signal is further treated to

achieve the final effective resolution. This

treatment does not add any latency in steady

conditions. The filter transfer function can be

calculated with Equation (1):

2

1 2 s

H(s)

(1

s)







(1)

Where

τ is the filter time constant related to the

cutoff frequency by

τ = 0.38/Fcutoff. See the

General Characteristics table on page 5 for the

value of Fcutoff.

Sensor – Magnet Mounting

The sensitive volume of the MA800 is confined

in a region less than 100µm wide and has

multiple integrated Hall devices. This volume is

located both horizontally and vertically within

50µm of the center of the QFN package. The

sensor detects the angle of the magnetic field

projected in a plane parallel to the package’s

upper surface. This means that the only

relevant

magnetic

field

is

the

in-plane

component (X and Y components) in the middle

point of the package.

By default, when looking at the top of the

package,

the

angle

increases

when

the

magnetic field rotates clockwise. Figure 3

shows the zero angle of the unprogrammed

sensor, where the cross indicates the sensitive

point. Both the rotation direction and the zero

angle can be programmed.

Figure 3: Detection Point and Default Positive

Direction

This type of detection provides flexibility for the

design of an angular encoder. The sensor only

requires the magnetic vector to lie essentially

within the sensor plane with a field amplitude of

at least 30mT. Note that the MA800 can work

with fields smaller than 30mT, but the linearity

and resolution performance may deviate from

the specifications. The most straightforward

mounting method is to place the MA800 sensor

on the rotation axis of a permanent magnet (i.e.:

a diametrically magnetized cylinder) (see Figure

4). The recommended magnet is a Neodymium

alloy (N35) cylinder with dimensions Ø5x3mm

inserted into an aluminum shaft with a 1.5mm

air gap between the magnet and the sensor

(surface of package). For good linearity, the

sensor is positioned with a precision of 0.5mm.