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[v2-00] 2017-May-03

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AS5304/AS5306 − Application Information

Resolution and Maximum Rotating Speed

When using the AS5304/AS5306 in an off-axis rotary

application, a multi-pole ring magnet must be used. Resolution,

diameter and maximum speed depend on the number of pole

pairs on the ring.

Resolution

The angular resolution increases linearly with the number of

pole pairs. One pole pair has a resolution (= interpolation factor)

of 160 steps or 40 quadrature pulses.

Resolution [steps] = [interpolation factor] x [number of pole

pairs]

Resolution [bit] = log (resolution[steps]) / log (2)

Example: Multi-pole ring with 22 pole pairs

Resolution = 160x22 = 3520 steps per revolution

= 40x22 = 880 quadrature pulses / revolution

= 11.78 bits per revolution = 0.1023° per step

Multi-Pole Ring Diameter

The length of a pole pair across the median of the multi-pole

ring must remain fixed at either 4mm (AS5304) or 2.4mm

(AS5306). Hence, with increasing pole pair count, the diameter

increases linearly with the number of pole pairs on the

magnetic ring.

Magnetic ring diameter = [pole length] * [number of pole pairs] / π

for AS5304: d = 4.0mm * number of pole pairs / π

for AS5306: d = 2.4mm * number of pole pairs / π

Example: (same as above) Multi-pole ring with 22 pole pairs for

AS5304

Ring diameter = 4 * 22 / 3.14 = 28.01mm (this number

represents the median diameter of the ring, this is where the

Hall elements of the AS5304/AS5306 should be placed; (see

Figure 25).

For the AS5306, the same ring would have a diameter of:

2.4 * 22 / 3.14 = 16.8mm

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