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.

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