AD7769

–15–

REV. A

(DAC)

For example, with a fixed input swing voltage of 2.5 V, the pro-

grammable span via DAC B is as follows:

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

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.