as5040 austriamicrosystems, as5040 Datasheet - Page 24

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

AS5040 10-BIT PROGRAMMABLE MAGNETIC ROTARY ENCODER
16.4 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.
Using the SSI interface for absolute data transmission,
an additional delay must be considered, caused by the
asynchronous sampling (t= 0…1/f
the external control unit to read and process the data.
16.4.1
caused by the output delay.
This error increases linearly with speed:
where: e
Note: since the propagation delay is known, it can be
automatically compensated by the control unit that is
processing the data from the AS5040, thus reducing the
angular error caused by speed.
16.5 Internal Timing Tolerance
The AS5040 does not require an external ceramic
resonator or quartz. All internal clock timings for the
AS5040 are generated by an on-chip RC oscillator. This
oscillator is factory trimmed to ±5% accuracy at room
temperature (±10% over full temperature range). This
tolerance influences the ADC sampling rate and the
pulse width of the PWM output:
Revision 1.8
e
A rotating magnet will therefore cause an angular error
sampling
Absolute output; SSI interface:
A new angular value is updated every 100µs (typ.)
Incremental outputs:
the incremental outputs are updated every
100µs (typ.)
PWM output:
A new angular value is updated every 100µs (typ.).
The PWM pulse timings T
same tolerance as the internal oscillator.
If only the PWM pulse width T
the angle, the resulting value also has this timing
tolerance.
However, this tolerance can be cancelled by
measuring both T
angle from the duty cycle (see section 7):
rpm
prop.delay = propagation delay [seconds]
=
sampling
Angular Error Caused by Propagation Delay
rpm
∗
6
*
= angular error [°]
= rotating speed [rpm]
prop
on
and T
.
delay
off
on
and T
and calculating the
s
on
) and the time it takes
is used to measure
off
also have the
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16.6 Temperature
16.6.1
One of the major benefits of the AS5040 compared to
linear Hall sensors is that it is much less sensitive to
temperature. While linear Hall sensors require a
compensation of the magnet’s temperature coefficient,
the AS5040 automatically compensates for the varying
magnetic field strength over temperature. The magnet’s
temperature drift does not need to be considered, as the
AS5040 operates with magnetic field strengths from
±45…±75mT.
Example:
A NdFeB magnet has a field strength of
75mT @ –40°C and a temperature coefficient of
-0.12% per Kelvin. The temperature change is from
–40° to +125° = 165K.
The magnetic field change is: 165 x -0.12% = -19.8%,
which corresponds to
75mT at –40°C and 60mT at 125°C .
The AS5040 can compensate for this temperature related
field strength change automatically, no user adjustment
is required.
16.6.2
The influence of temperature in the absolute accuracy is
very low. While the accuracy is ≤ ±0.5° at room
temperature, it may increase to ≤±0.9° due to increasing
noise at high temperatures.
16.6.3
The internal RC oscillator is factory trimmed to ±5%.
Over temperature, this tolerance may increase to ±10%.
Generally, the timing tolerance has no influence in the
accuracy or resolution of the system, as it is used mainly
for internal clock generation.
The only concern to the user is the width of the PWM
output pulse, which relates directly to the timing
tolerance of the internal oscillator. This influence
however can be cancelled by measuring the complete
PWM duty cycle (see 16.5).
Position
Magnetic Temperature Coefficient
Accuracy over Temperature
Timing Tolerance over Temperature
=
(
t
t
on
on
⋅
+
1025
t
off
)
−
1
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