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AS5311

Datasheet - D e t a il e d D es c r i pt io n

Data D11:D0 is valid, when the status bits have the following configurations:

*MagInc=MagDec=1 is only recommended in YELLOW mode (see Table 13).

7.4 Absolute Output Jitter and Hysteresis

Note:

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 Incremental Output Hysteresis. 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.

Note:

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. The signal indicator pins

MagINCn and MagDECn are available as hardware pins (pins 2 and 3) and display the “Red/Yellow/Green Range”.

Additionally, the serial data stream (see Figure 6) offers the MagINC, MagDEC and LIN status bits. 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 7). For Z-

axis Range Indication only M11:M4 of the magnitude are used.

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

Table 12. Status Bit Outputs

OCF

COF

LIN

MagINC

MagDEC

Parity

10

0

00

Even checksum of bits 1:17

01

10

1*

1*

Table 13. Magnetic Field Strength Red-Yellow-Green Indicators

Status Bits

MAG

Hardware Pins

1

MagINC MagDEC

LIN

M11…M4 MagINCn MagDECn

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.