as5030 austriamicrosystems, as5030 Datasheet - Page 20

• Manufacturer Part Number: as5030
• Description: 8-bit High-speed Absolute Magnetic Rotary Encoder
• Manufacturer: austriamicrosystems

AS5030 8-bit Programmable Magnetic Rotary Encoder
7
The AS5030 is using a fast tracking ADC (TADC) to determine the angle of the magnet. The TADC has a tracking rate of 1.15µs
(typ).
Once the TADC is synchronized with the angle, it sets the LOCK bit in the status register (see 5.2.1). In worst case, usually at
start-up, the TADC requires a maximum of 127 steps (127 * 1.15µS = 146.05µs) to lock. Once it is locked, it requires only one
cycle (1.15µs) to track the moving magnet.
The AS5030 can operate in locked mode at rotational speeds up to 30,000 rpm.
In Low Power Mode or Ultra Low Power Mode, the position of the TADC is frozen. It will continue from the frozen position once
it is powered up again. If the magnet has moved during the power down phase, several cycles will be required before the TADC
is locked again. The tracking time to lock in with the new magnet angle can be roughly calculated as:
t
OldPos =
NewPos =
7.1
The Propagation delay is the time required from reading the magnetic field by the Hall sensors to calculating the angle and
making it available on the serial or PWM interface. While the propagation delay is usually negligible on low speeds it is an
important parameter at high speeds.
The longer the propagation delay, the larger becomes the angle error for a rotating magnet as the magnet is moving while the
angle is calculated. The position error increases linearly with speed.
The main factors contributing to the propagation delay are:
7.1.1
For high speed applications, fast ADC’s are essential. The ADC sampling rate directly influences the propagation delay. The
fast tracking ADC used in the AS5030 with a tracking rate of only 1.15µs (typ.) is a perfect fit for both high speed and high
performance.
7.1.2
A commonplace practice for systems using analog-to-digital converters is to filter the input signal by an anti-aliasing filter. The
filter characteristic must be chosen carefully to balance propagation delay and noise.
The lowpass filter in the AS5030 has a cut-off frequency of typ. 23.8kHz and the overall propagation delay in the analog signal
path is typ. 15.6µs.
7.1.3
Aside from the chip-internal propagation delay, the time required to read and process the angle data must also be considered.
Due to its nature, a PWM signal is not very usable at high speeds, as you get only one reading per PWM period. Increasing the
PWM frequency may improve the situation but causes problems for the receiving controller to resolve the PWM steps. The
frequency on the AS5030 PWM output is typ. 1.95kHz with a resolution of 2µs/step.
A more suitable approach for high speed absolute angle measurement is using the serial interface. With a clock rate of up to
6MHz, a complete set of data (21bits) can be read in >3.5µs
7.2
The total propagation delay of the AS5030 is the delay in the analog signal path and the tracking rate of the ADC:
15.6µs + 1.15µs = 16.75µs.
If only the SIN-/COS-outputs are used, the propagation delay is the analog signal path delay only (typ. 15.6µs).
Position Error over speed
The angle error over speed caused by the propagation delay is calculated as:
Δφ
In addition, the anti-aliasing filter causes an angle error calculated as:
Δφ
Rev. 1.8
t
LOCK
LOCK
pd
lpf
High Speed Operation
= rpm * 6 * 16.75E
= ArcTan [ rpm / ( 60*f
=
=
Propagation Delay
Total propagation delay of the AS5030
ADC Sampling Rate
Chip internal lowpass filtering
Digital readout rate
. 1
15
μ
s
∗
time required to acquire the new angle after power up from one of the reduced power modes [µs]
Angle position when one of the reduced power modes is activated [°]
Angle position after resuming from reduced power mode [°]
NewPos
-6
0
in degrees.
)]
−
OldPos
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