ADA4571R-EBZ User Guide
UG-739
Fourier Transform
The fourth set of figures are labeled
Fourier Transform
. These
figures show the spectral analysis of the two independent
output signals. The fundamental frequency corresponds to the
motor speed. Harmonics from the sensor are also depicted in
this plot. The y-axis of this plot shows the magnitude of the
frequency components. This value is the root mean squared
(RMS), magnitude in 12-bit code of that specific frequency.
The x-axis is shown in hertz. The fundamental frequency
shown depicts the electrical frequency. This will be two times
the mechanical frequency. An example calculation for motor
speed in RPM from a 100 Hz fundamental electrical frequency
is shown as,
100 Hz × (1 electrical cycle)/(2 mechanical cycles) ×
(60 seconds)/(1 minute) = 3,000 RPM
Figure 11. Spectral Output for Sine and Cosine Channels
Due to the layout of the AMR bridge odd harmonics which are
usually present in the output of sensors, such as third and fifth,
are suppressed.
Spectral analysis of the sensor can be useful for debuging
purposes. Even harmonics appear in this plot when there is
gross misalignment between the center of the AMR sensor
and the magnetic stimulus.
Radius Plot
The fifth set of figures are labeled
Radius Check
(see
Figure 12). This figure plots the sine channel on the y-axis and
the cosine channel on the x-axis. Both axes are shown in 12-bit
codes. Due to the sinusoidal nature of the two channels and the
90° phase delay between the sine and cosine channels the plot is
circular in nature.
The radius of this plot is constant throughout the entire rotation
of the magnetic stimulus. The exact radius of this plot is
inversely proportional to the temperature of the
ADA4571
. At
lower device temperatures, the radius increases while at higher
device temperatures the radius decreases; when held at a
constant temperature the radius will also be constant.
Figure 12. Radius Plot of Output Waveforms
The temperature dependent variation in output amplitude of
the AMR bridge is due to the reduced change in resistance of
the AMR film at higher temperatures. The
ADA4571
provides
an internal regulated voltage to the AMR bridge supply. By
enabling the temperature compensation mode of the
ADA4571
,
this regulated supply voltage varies with temperature. At higher
temperatures, the regulator provides a higher bridge supply
voltage, thus increasing the output amplitude of the device.
Using this mode, the output amplitude and, therefore, radius
in this plot will be more consistent over the wide temperature
range of −40°C to 150°C.
This mode is enabled by default by an internal pull-up resistor
on the GC pin. Moving Jumper P3 to the correct position as
outlined in Figure 14 and Figure 15 will disable the temper-
ature compensation mode of the device. There is an internal
temperature sensor on the
ADA4571
that is used to adjust the
bridge supply voltage. The internal temperature sensor voltage
is available to the end user and can be monitored on the
daughter board or the motherboard as outlined in Figure 14
and Figure 15.
Rev. 0 | Page 9 of 13
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