AS5040 datasheet(19/28 Pages) AMSCO

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AS5040 10-BIT PROGRAMMABLE MAGNETIC ROTARY ENCODER

Revision 1.6, 03-Oct-06

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Page 19 of 28

Using the SSI interface for absolute data transmission,

an additional delay must be considered, caused by the

asynchronous sampling (t= 0…1/fs) and the time it takes

the external control unit to read and process the data.

15.4.1

Angular Error Caused by Propagation Delay

A rotating magnet will therefore cause an angular error

caused by the output delay.

This error increases linearly with speed:

delay

prop

rpm

sampling

∗

where: esampling

= angular error [°]

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 that is

processing the data from the AS5040, thus reducing the

angular error caused by speed.

15.5 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:

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 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.

However, this tolerance can be cancelled by

measuring both Ton and Toff and calculating the

angle from the duty cycle (see section 6):

() 1

1025 −

⋅

off

Position

15.6 Temperature

15.6.1

Magnetic Temperature Coefficient

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.

15.6.2

Accuracy over Temperature

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

Timing Tolerance over Temperature

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).

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