MA780 – 8-BIT TO 12-BIT LOW-POWER ANGLE SENSOR

MA780 Rev. 1.2

MonolithicPower.com

7

8/30/2021

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OPERATION

Sensor Front End

The magnetic field is detected by the integrated

Hall devices located in the center of the package.

The angle is measured using the SpinaxisTM

method, which directly digitizes the direction of

the

field

without

complex

arctangent

computation or feedback loop-based circuits

(interpolators).

The SpinaxisTM method is based on phase

detection,

and

this

method

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 for Time-to-Digital Converter

Figure 2: Phase Detection Method

The front-end output delivers a digital number

that is 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 undergoes digital filtering

treatment to achieve the final effective resolution.

The filter transfer function can be calculated with

Equation (1):

1

1+

τs

H(s) =

(1)

Where

τ is the filter time constant, which varies

depending on the chosen filter window. Table 14

on page 22 shows the values of

τ for different

filter window sizes.

To reduce power consumption, the front end and

digital filtering function are disabled in idle mode,

but the SPI block works continuously to provide

the angle.

Sensor

– Magnet Mounting

The sensitive volume of the MA780 is confined

to a region less than 100µm wide in the center of

the package (see Figure 3). This area contains

multiple integrated Hall devices. 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 measured magnetic

field is the in-plane components (i.e. the X and Y

components) in the middle point of the package.

X

Y

Z

Figure 3: Space Coordinates (Field is Sensed on

the XY Plane)

By default, when looking at the top of the

package, the angle increases when the magnetic

field rotates clockwise. Figure 4 shows the zero

angle of a sensor that has not been configured,

where the cross indicates the sensitive point.

Both the rotation direction and the zero angle

can be configured.

0.90±0.10

3.00±0.10

0.45±0.05

1.50±0.05

Positive Direction

3.00±0.10

Side View

Top View

90°±5°

B

Zero Angle

Figure 4: Detection Point and Default Positive

Direction

This type of detection provides flexibility for the

angular encoder design. The sensor requires

that in the center of the package, the field in the

XY plane is below 30mT.

The MA780 can work with fields smaller than

30mT,

but

the

linearity

and

resolution

performance may deviate from the specifications

under other conditions.