AD7769 Datasheet, PDF (15/16 Pages) Analog Devices – LC2MOS Analog I/O Port

AD7769
Figure 28. Generating a Software Programmable V_SWING
(DAC)
For example, with a fixed input swing voltage of 2.5 V, the pro-
grammable span via DAC B is as follows:
D_B = 0: V_SWING (DAC) = 2.08
D_B = 1/2: V_SWING (DAC) = 2.5 V = V_SWING
D_B ≈ 1: V_SWING (DAC) = 3.125 V
The AD7769 is specified for a V_SWING (DAC) voltage range
from 2 V to 3 V, although in practice this range can be extended
while still maintaining monotonic operation.
Closed Loop Microstepping
Microstepping is a popular technique in low density disk drives
(both floppy and hard disk) that allows higher positional resolu-
tion of the disk drive head over that obtainable from a full-step
driven stepper motor. Typically, a two-phase stepper motor has
its phase currents driven with a sine-cosine relationship. These
cosinusoidal signals are generated by two DACs driven with the
appropriate data. The resolution of the DACs determines the
number of microsteps into which each full step can be divided.
For example, with a 1.8° full-step motor and a 4-bit DAC, a
microstep size of 0.11° (1.8°/2ⁿ) is obtainable.
The microstepping technique improves the positioning resolu-
tion possible in any control application. However, the positional
accuracy can be significantly worse than that offered by the orig-
inal full-step accuracy specification due to load torque effects.
To ensure that the increased resolution is usable, it is therefore
necessary to use a closed-loop system where the position of the
disk drive head (or motor) is monitored. The closed-loop system
allows an error between the desired position and the actual posi-
tion to be monitored and corrected. The correction is achieved
by adjusting the ratio of the phase currents in the motor wind-
ings until the required head position is reached.
The AD7769 is ideally suited for the closed-loop microstepping
technique with its dual DACs for positioning the disk drive
head and dual channel ADC for monitoring the position of the
head. A typical circuit for a closed-loop microstepping system is
shown in Figure 29. The DAC waveforms are shown in Figure
30 along with the direction information of clockwise rotation
supplied by the controller.
Figure 29. Typical Closed-Loop Microstepping Circuit
with the AD7769
Figure 30. Typical Control Waveforms for the Microstep-
ping Circuit of Figure 29
A typical transducer would be a moire-fringe transducer which
consists of two gratings, one fixed and one moveable. The rela-
tive positions of these two gratings will modulate the amount of
light from a LED which can pass through. In order to derive
head direction information the stationary grating has two sets of
bars, with a 90° phase relationship, and two photo-transistors.
The quadrature sinusoidal output waveforms (N & Q) can be
converted directly by the AD7769.
REV. A
–15–