ADA4571R-EBZ User Guide

UG-739

Raw Waveforms

The first figure is labeled Raw Waveforms (see Figure 11).

This figure shows the sampled ADC raw data as the motor is

spinning. The amplitude is shown in 12-bit code. The dual

channel AD7866 simultaneously samples the sine and cosine

channels at 100,000 samples per second. The internal 2.5 V

reference of the AD7866 has been subtracted in hardware as

the readout of the ADC is in twos complement. Therefore, the

two signals are centered around zero in the raw waveform plot.

The magnetic stimulus is sampled over many rotations and

then cropped to three mechanical revolutions or six electrical

revolutions. It is important that these two channels are

simultaneously sampled or extra errors will be introduced

from the phase delay between the sampling of the individual

channels.

Figure 7. Raw Sine and Cosine Waveform

There are two different figures shown depicting these sine and

cosine waveforms. The uncorrected figure has not had any post

processing applied and shows the absolute raw data from the

sensor. For the offset corrected figure, the full three mechanical

revolutions are averaged independently for both the sine and

cosine channels giving the offset for the channel. These offset

values are then subtracted from their respective channel to give

the end waveform centered perfectly about zero.

Note that when powering the board through a USB instead of

by an external power supply there will be some variation in the

supply voltage of the ADA4571. Due to the readout of the

AD7866 being in twos complement form with respect to the

internal 2.5 V reference, the offset of these waveforms will

be higher than the inherent offset of the ADA4571.

Arc Tangent2 (Magnetic Angle)

The second set of figures are labeled ArcTangent (Magnetic

Angle) (see Figure 8). These figures depict the calculated

electrical angle of the ADA4571. This calculation is performed

by the following equation:

Because the two AMR bridges are deposited at a 45° relative

angle, the sine and cosine outputs are 90° out of phase. This

constant 90° phase difference results in the calculated angle

over a constant rotary speed as a straight line.

Normally the ArcTangent function will have a range of (−90°

to +90°). However, if quadrant information is kept and cor-

rected for after the calculation the second and third quadrant

can be distinguished from the first and fourth. The function

ArcTangent2 has this quadrant information and therefore the

range is extended to (−180° to +180°). The program then zeros

this waveform and therefore shows the angle between (0° and

360°).

Figure 8. ArcTangent Calculation Plot

The electrical angle period will repeat for every sinusoidal

period of the sine and cosine outputs from the ADA4571. Just

like the raw waveform data, three mechanical revolutions or six

electrical revolutions are used for calculations. This results in

six periods of angle information shown.

The Uncorrected ArcTangent (Magnetic Angle) plot will

have a slight bowed shape to it due to the sensor offset in the

raw waveform. The Offset Corrected ArcTangent (Magnetic

Angle) plot shows the angle information after the offset is

nulled from each individual channel. This offset correction to

the raw waveforms before the ArcTangent calculation results

in a much more linear output plot.

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