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AS5145

Data Sheet - A p p l i c a t i o n I n f o r m a t i o n

9.7.2 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 in fast mode (pin MODE = high) and 0.03 degrees rms (1 sigma)x1 in slow mode (pin

MODE = low or open).

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:

Absolute output; SSI interface: The transition noise of the absolute output can be reduced by the user by implementing averaging of read-

ings. An averaging of 4 readings will reduce the transition noise by 6dB or 50%, e.g. from 0.03ºrms to 0.015ºrms (1 sigma) in slow mode.

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 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).

Note:

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

9.7.3 High Speed Operation

Sampling Rate: The AS5145 samples the angular value at a rate of 2.61k (slow mode) or 10.42k (fast mode, selectable by pin MODE)

samples per second. Consequently, the absolute outputs are updated each 384µs (96µs in fast mode). At a stationary position of the mag-

net, the sampling rate creates no additional error.

Absolute Mode: At a sampling rate of 2.6kHz/10.4kHz, the number of samples (n) per turn for a magnet rotating at high speed can be cal-

culated by

(EQ 4)

(EQ 5)

The upper speed limit in slow mode is ~6.000rpm and ~30.000rpm in fast mode. 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 sam-

pled at the highest resolution of 12 bit.

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

AS5145 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 12 bits (4096 pulses per revolution).

9.7.4 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 in fast mode and 384µs in slow mode.

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).

This error increases linearly with speed:

(EQ 6)

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 AS5145.

60

rpm

384

()μs

⋅

-----------------------------------

60

rmp 96

μs

⋅

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