AS5045 datasheet(21/24 Pages) AMSCO

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AS5045 12-BIT PROGRAMMABLE MAGNETIC ROTARY ENCODER

Revision 1.5

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Page 21 of 24

14.2

Transition Noise

Transition noise is defined as the jitter in the transition

between two steps.

Due to the nature of the measurement principle (Hall

sensors + Preamplifier + ADC), there is always a certain

degree of noise involved.

This transition noise voltage results in an angular

transition noise at the outputs. It is specified as 0.06

degrees rms (1 sigma)*1 in fast mode (pin MODE = high)

and 0.03 degrees rms (1 sigma)*1 in slow mode (pin

MODE = low or open).

This is the repeatability of an indicated angle at a given

mechanical position.

The transition noise has different implications on the type

of output that is used:

Absolute output; SSI interface:

The transition noise of the absolute output can be

reduced by the user by implementing averaging of

readings. An averaging of 4 readings will reduce the

transition noise by 6dB or 50%, e.g. from 0.03°rms

to 0.015°rms (1 sigma) in slow mode.

PWM interface:

If the PWM interface is used as an analog output by

adding a low pass filter, the transition noise can be

reduced by lowering the cutoff frequency of the

filter.

If the PWM interface is used as a digital interface

with a counter at the receiving side, the transition

noise may again be reduced by averaging of

readings.

: statistically, 1 sigma represents 68.27% of readings,

3 sigma represents 99.73% of readings.

14.3

High Speed Operation

14.3.1

Sampling Rate

The AS5045 samples the angular value at a rate of 2.61k

(slow mode) or 10.42k (fast mode, selectable by pin

MODE) samples per second. Consequently, the absolute

outputs are updated each 384µs (96µs in fast mode).

At a stationary position of the magnet, the sampling rate

creates no additional error.

Absolute Mode

At a sampling rate of 2.6kHz/10.4kHz, the number of

samples (n) per turn for a magnet rotating at high speed

can be calculated by

rpm

slow

384

mod

⋅

rpm

fast

mod

⋅

The upper speed limit in slow mode is ~6.000rpm and

~30.000rpm in fast mode. The only restriction at high

speed is that there will be fewer samples per revolution

as the speed increases (see Table 2).

Regardless of the rotational speed, the absolute angular

value is always sampled at the highest resolution of 12

bit.

14.4

Propagation Delays

The propagation delay is the delay between the time that

the sample is taken until it is converted and available as

angular data. This delay is 96µs in fast mode and 384µs

in slow mode.

Using the SSI interface for absolute data transmission,

an additional delay must be considered, caused by the

asynchronous sampling (0 … 1/fsample) and the time it

takes the external control unit to read and process the

angular data from the chip (maximum clock rate = 1MHz,

number of bits per reading = 18).

14.4.1

Angular Error Caused by Propagation Delay

A rotating magnet will cause an angular error caused by

the output propagation delay.

This error increases linearly with speed:

delay

prop

rpm

sampling

∗

where

esampling = angular error [°]

rpm = rotating speed [rpm]

prop.delay = propagation delay [seconds]

Note: since the propagation delay is known, it can be

automatically compensated by the control unit processing

the data from the AS5045.

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