AS5045B View Datasheet(PDF) - austriamicrosystems AG

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AS5045B

12-Bit Programmable Magnetic Position Sensor

austriamicrosystems AG 

AS5045B

Datasheet - Application Information

9.6.2 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 0.06 degrees rms (1 sigma)x1.

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:

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 lower-

ing 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 can be further 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 15,000 rpm and higher).

Note: Statistically, 1 sigma represents 68.27% of readings and 3 sigma represents 99.73% of readings.

9.6.3 High Speed Operation

Sampling Rate: The AS5045B samples the angular value at a rate of 10.42k samples per second. Consequently, the absolute outputs are

updated each 96µs. At a stationary position of the magnet, the sampling rate creates no additional error.

Absolute Mode: At a sampling rate of 10.4kHz, the number of samples (n) per turn for a magnet rotating at high speed can be calculated by

n = -----------6---0------------

rmp ⋅ 96μs

(EQ 3)

The upper speed limit is ~30,000 rpm. The only restriction at high speed is that there will be fewer samples per revolution as the speed

increases (see Table 7). Regardless of the rotational speed, the absolute angular value is always sampled at the highest resolution of 12-bit.

Incremental Mode: Incremental encoders are usually required to produce no missing pulses up to several thousand rpm. Therefore, the

AS5045B has a built-in interpolator, which ensures that there are no missing pulses at the incremental outputs for rotational speeds of up to

15,000 rpm, even at the highest resolution of 12 bits (4096 pulses per revolution).

9.6.4 Propagation Delays

The propagation delay is the delay between the time that the sample is taken until it is converted and available as angular data. This delay is

96µs.

Using the SSI interface for absolute data transmission, an additional delay must be considered, caused by the asynchronous sampling (0 … 1/

fsample) and the time it takes the external control unit to read and process the angular data from the chip (maximum clock rate = 1MHz, number

of bits per reading = 18).

Angular Error Caused by Propagation Delay. A rotating magnet will cause an angular error caused by the output propagation delay.

This error increases linearly with speed:

esampling = rpm * 6 * prop.delay

(EQ 4)

Where:

esampling = angular error [º]

rpm = rotating speed [rpm]

prop.delay = propagation delay [seconds]

Note: Since the propagation delay is known, it can be automatically compensated by the control unit processing the data from the AS5045B.

9.6.5 Internal Timing Tolerance

The AS5045B does not require an external ceramic resonator or quartz. All internal clock timings for the AS5045B 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:

- Absolute output; SSI interface: A new angular value is updated every 96µs (typ).

- PWM output: A new angular value is updated every 96µs (typ). The PWM pulse timings Ton and Toff also have the same tolerance as the

internal oscillator. If only the PWM pulse width Ton is used to measure the angle, the resulting value also has this timing tolerance. How-

ever, this tolerance can be cancelled by measuring both Ton and Toff and calculating the angle from the duty cycle (see Pulse Width Mod-

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