as5040 austriamicrosystems, as5040 Datasheet - Page 23

• Manufacturer Part Number: as5040
• Description: 10-bit Programmable Magnetic Rotary Encoder
• Manufacturer: austriamicrosystems

AS5040 10-BIT PROGRAMMABLE MAGNETIC ROTARY ENCODER
16.2 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)
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:
*1
3 sigma represents 99.73% of readings.
16.3 High Speed Operation
16.3.1
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.
Revision 1.8
: statistically, 1 sigma represents 68.27% of readings,
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 30,000 rpm and higher)
Sampling Rate
*1
.
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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:
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:
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 30,000
rpm, even at the highest resolution of 10 bits (512 pulses
per revolution).
n
rpm
=
610rpm = 1024 samples / turn
2441rpm = 256 samples / turn
122rpm = 512 samples / turn
Absolute Output Mode
rpm
=
n
60
⋅
⋅
96
60
96
μ
μ
etc…
s
s
Table 7: Speed performance
max. speed = 30,000 rpm
Incremental Output
@ 10 bit resolution
no missing pulses
(512ppr):
Mode
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