UG-739

ADA4571R-EBZ User Guide

Angular Error (Linearity)

There are two different motors that may be supplied with your

demo board: a brushless dc motor or a brushed dc motor. The

brushed dc motor will introduce extra errors into the system

due to cogging of the motor during rotation. To fully evaluate

the performance of the sensor, an externally applied magnetic

stimulus that has a smoother rotational velocity should be used

with the supplied LabVIEW GUI.

The third set of figures are labeled Angular Error (Degrees)

(see Figure 6). This figure shows the linearity of the ArcTangent

plot discussed in the Arc Tangent2 (Magnetic Angle) section.

There is no encoder attached to the magnetic stimulus on the

board and therefore the program cannot compare the calculated

angle to the actual position of the magnet. Therefore, this plot is

calculating the linearity of both the ADA4571 and the brushless

dc motor providing the magnetic stimulus.

Due to the nonidealities of the brushless dc motor providing

the magnetic stimulus, the reported angular error will always

be higher than the actual error of the ADA4571. The motor

provides a more constant rate of speed at lower rotational

velocities and therefore the reported linearity will be lower

at slower speeds. These extra errors are not indicative of the

bandwidth of the ADA4571 as the part can handle motor

rotational speeds as high as 50,000 RPM.

Figure 9 shows the Uncorrected Angular Error (Degrees) plot.

When powered by the host PC USB supply this waveform

reports higher error than when powered by an external supply.

This increased error is because the internal 2.5 V reference is

subtracted in hardware from each ADC channel resulting in a

higher inherent offset of the sensor if the supply voltage is not

exactly 5 V. When powered by an external power source the

ADA4571 supply is regulated to 5 V and, therefore, the ADC

introduces no extra offset or errors to the system.

Figure 9. Uncorrected Linearity Error Plot

Figure 10 shows the Offset Corrected Error (Degrees) plot.

Large spikes appear in this plot due to the nonidealities of the

magnetic stimulus. When the motor commutes, there is a kick

from the excitation coils resulting in a higher reported linearity

error at these points. The sensor measures the position of the

magnetic stimulus and so these kicks during commuting will

show up as an error from an ideal linear plot. This nonideality

from the magnetic stimulus is more prevalent at higher speeds

but is not a result of the sensor. The included motor is very

smooth at lower speeds and, therefore, these spikes will not

appear in the error plot.

Figure 10. Offset Corrected Linearity Error Plot

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