AS5040 datasheet(18/28 Pages) AMSCO

18 / 28 page

AS5040 10-BIT PROGRAMMABLE MAGNETIC ROTARY ENCODER

Revision 1.6, 03-Oct-06

www.austriamicrosystems.com

Page 18 of 28

15.2 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 0.12

degrees rms (1 sigma)*1.

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 applying an averaging of

readings. An averaging of 4 readings will reduce the

transition noise by 6dB or 50%, e.g. from 0.12°rms

to 0.06°rms (1 sigma).

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.

Incremental mode:

In incremental mode, the transition noise influences

the period, width and phase shift of the output

signals A, B and Index. However, the algorithm used

to generate the incremental outputs guarantees no

missing or additional pulses even at high speeds (up

to 10,000 rpm and higher)

: statistically, 1 sigma represents 68.27% of readings,

3 sigma represents 99.73% of readings.

15.3 High Speed Operation

15.3.1

Sampling Rate

The AS5040 samples the angular value at a rate of

10.42k samples per second. Consequently, the

incremental, as well as the absolute outputs are updated

each 96µs.

At a stationary position of the magnet, this sampling rate

creates no additional error.

Absolute Mode:

With the given sampling rate of 10.4 kHz, the number of

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

can be calculated by:

rpm

⋅

In practice, there is no upper speed limit. The only

restriction is that there will be fewer samples per

revolution as the speed increases.

Regardless of the rotational speed, the absolute angular

value is always sampled at the highest resolution of

10 bit.

Likewise, for a given number of samples per revolution

(n), the maximum speed can be calculated by:

rpm

⋅

In absolute mode (serial interface and PWM output),

610 rpm is the maximum speed, where 1024 readings per

revolution can be obtained.

In incremental mode, the maximum error caused by the

sampling rate of the ADCs is 0/+96µs. It has a peak of

1LSB = 0.35° at 610 rpm.

At higher speeds this error is reduced again due to

interpolation and the output delay remains at 192µs

as the DSP requires two sampling periods (2x96µs) to

synthesize and redistribute any missing pulses.

Incremental Mode:

Incremental encoders are usually required to produce no

missing pulses up to several thousand rpm’s.

Therefore, the AS5040 has a built-in interpolator, which

ensures that there are no missing pulses at the

incremental outputs for rotational speeds of up to 10,000

rpm, even at the highest resolution of 10 bits (512 pulses

per revolution).

Absolute Output Mode

Incremental Output

Mode

610rpm = 1024 samples / turn

122rpm = 512 samples / turn

2441rpm = 256 samples / turn

etc…

no missing pulses

@ 10 bit resolution

(512ppr):

max. speed = 10,000 rpm

Table 7: Speed performance

15.4 Propagation Delays

The propagation delay is the delay between the time that

a sample is taken until it is converted and available as

angular data. This delay is 48µs for the absolute

interface and 192µs for the incremental interface.

AS5040 Datasheet(PDF) 18 Page - ams AG

AS5040 Datasheet(HTML) 18 Page - ams AG

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL