AS5311

Data Sheet

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Revision 1.01

14 - 23

7.4

Absolute Output Jitter and Hysteresis

Note that there is no hysteresis or additional filtering at the absolute output. This allows a determination of the

magnet’s absolute position within one pole pair down to submicron range.

Due to the intentionally omitted hysteresis and due to noise (e.g. from weak magnetic fields), the absolute output

may jitter when the magnet is stationary over the chip.

In order to get a stable 12-bit absolute reading, two common methods may be implemented to reduce the jitter.

7.4.1

Adding a Digital Hysteresis

The hysteresis feature of the incremental outputs is described in 7.2. An equivalent function can be implemented

in the software of the external microcontroller. The hysteresis should be larger than the peak-to-peak noise

(=jitter) of the absolute output in order to mask it and create a stable output reading.

Remark: the 2-bit hysteresis on the incremental output (=3.9µm) is equivalent to a hysteresis of 8LSB on the

absolute output.

7.4.2

Implementing Digital Filtering

Another useful alternative or additional method to reduce jitter is digital filtering. This can be accomplished

simply by averaging, for example a moving average calculation in the external microcontroller. Averaging 4

readings results in 6dB (=50%) noise and jitter reduction. An average of 16 readings reduces the jitter by a

factor of 4.

Averaging causes additional latency of the processed data. Therefore it may be useful to adjust the depth of

averaging depending on speed of travel. For example using a larger depth when the magnet is stationary and

reducing the depth when the magnet is in motion.

7.5

Z-axis Range Indication (“Red/Yellow/Green” Indicator)

The AS5311 provides several options of detecting the magnet distance by indicating the strength of the

magnetic field. Signal indicators MagINCn and MagDECn are available both as hardware pins (pins 1 and 2) and

as status bits in the serial data stream (see Figure 8). Additionally the LIN status bit indicates the non-

recommended “red” range. The MAGnitude register provides additional information about the strength of the

magnetic field (see Figure 9).

The digital status bits MagINC, MagDec, LIN and the hardware pins MagINCn, MagDECn have the following

function:

Table 5: Magnetic Field Strength Red-Yellow-Green Indicators

Status Bits

MAG

Hardware Pins

Mag

INC

Mag

DEC

LIN

M11..

M0

Mag

INCn

Mag

DECn

Description

0

0

0

3F hex

Off

Off

No distance change

Magnetic input field OK ( GREEN range, ~10…40mT peak

amplitude)

0

1

0

3F hex

Off

Off

Distance increase; this state is a dynamic state and only

active while the magnet is moving away from the chip.

Magnitude register may change but regulates back to 3F hex.

1

0

0

3F hex

Off

Off

Distance decrease; this state is a dynamic state and only

active while the magnet is moving towards the chip.

Magnitude register may change but regulates back to 3F hex.

1

1

0

20 hex-

5F hex

On

Off

YELLOW range: magnetic field is ~3.4….54.5mT.

The AS5311 may still be operated in this range, but with

slightly reduced accuracy.

1

1

1

<20 hex

>5F hex

On

On

RED range: magnetic field is <3.4mT (MAG <20) or >54.5mT

(MAG >5F).

It is still possible to operate the AS5311 in the red range, but

not recommended.

All other combinations

n/a

n/a

Not available