as5140 austriamicrosystems, as5140 Datasheet - Page 18

• Manufacturer Part Number: as5140
• Description: 10-bit 360? Programmable Magnetic Rotary Encoder For High Ambient Temperatures Up To 150?c
• Manufacturer: austriamicrosystems

AS5140H 10-BIT PROGRAMMABLE MAGNETIC ROTARY ENCODER
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.
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.
15.4.1
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 AS5140H, thus reducing the
angular error caused by speed.
15.5 Internal Timing Tolerance
The AS5140H does not require an external ceramic
resonator or quartz. All internal clock timings for the
AS5140H 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 1.1, 02-03-2007
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 6):
rpm
prop.delay
=
sampling
Angular Error Caused by Propagation Delay
rpm
= rotating speed [rpm]
∗
6
*
prop
on
= angular error [°]
= propagation delay [seconds]
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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15.6 Temperature
15.6.1
One of the major benefits of the AS5140H 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 AS5140H automatically compensates for the varying
magnetic field strength over temperature. The magnet’s
temperature drift does not need to be considered, as the
AS5140H 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 +150° = 190K.
The magnetic field change is: 190 x -0.12% = -22.8%,
which corresponds to
75mT at –40°C and 57.9mT at 150°C .
The AS5140H can compensate for this temperature
related field strength change automatically, no user
adjustment is required.
15.6.2
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.
15.6.3
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 15.5).
Position
Magnetic Temperature Coefficient
Accuracy over Temperature
Timing Tolerance over Temperature
=
(
t
t
on
on
⋅
+
1025
t
off
)
−
1
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