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:
Angle
=
ArcTangent2
(
V
SIN
/
V
COS
)
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.
Rev. 0 | Page 7 of 13
Downloaded from
Downloaded from
Downloaded from
Downloaded from
Downloaded from
Downloaded from
Downloaded from
