AD7569/AD7669

–17–

REV. B

Figure 25. Peak-Reading A/D Converter

head (or motor) is monitored. The closed-loop system allows an

error between the desired position and the actual position to be

monitored and corrected. The correction is achieved by adjust-

ing the ratio of the phase currents in the motor windings until

the required head position is reached.

The AD7669 is ideally suited for the closed-loop microstepping

technique with its on-chip dual DACs for positioning the disk

drive head, and onboard ADC for monitoring the position of the

head. A generalized circuit for a closed-loop microstepping sys-

tem is shown in Figure 26. The DAC waveforms are shown in

Figure 27, along with the direction information for clockwise ro-

tation supplied by the controller.

Figure 26. Typical Closed-Loop Microstepping Circuit with

the AD7669

The AD7669 is used in the unipolar 0 V to +2.5 V configura-

tion. This allows the circuit of Figure 26 to be completely uni-

polar (+5 V, +12 V supplies); no negative power supplies are

required. The power output stage is a dual H-Bridge device

such as the UDN-2998W from Sprague Electric. The phase

currents in both windings are detected by means of the small

The voltage developed across these resistors is amplified and

compared with the respective DAC output voltage. The com-

parators in turn chop the phase winding current. The ADC

completes the feedback path by converting information from a

suitable transducer for analysis by the controller.

PEAK DETECTION—AD7569

The circuit of Figure 25 shows a peak-reading A/D converter,

which is useful in such applications as monitoring flow rates,

temperature, pressure, etc. The circuit ensures that a peak will

not be missed while at the same time does not require the mi-

croprocessor to frequently monitor the data. The peak value is

stored in the A/D converter and can be read at any time.

The gain on the AD524 is adjusted to yield a 0 V to +2.5 V out-

put. When the input signal exceeds the current stored value, the

output of the TL311 goes low, triggering the Q output of the

74121. This low-going pulse starts a conversion on the AD7569

ADC, and at the end of conversion latches the result into the

DAC. This pulse must be at least 120 ns greater than the con-

version time of the ADC. The Q output is used to drive the

strobe input of the TL311, resetting the TL311 output high in

readiness for another conversion.

The additional gates on the RD and WR inputs are to allow the

data to be read by the microprocessor while at the same time

ensuring that the DAC is not updated when the microprocessor

reads the data. It may be necessary to monitor the AD7569

BUSY

line to ensure that a processor READ is not attempted

while the AD7569 is in the middle of a conversion. The READ

pulse width from the processor must be less than 1

µs to ensure

correct data is read from the ADC. A low-going pulse on the

RESET

line resets the DAC output to 0 V and starts a new “peak-

detection” period. This RESET pulse must also be less than 1

µs.

DISK DRIVE APPLICATION—AD7669

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

original full-step accuracy specification due to load torque effects.

To ensure that the increased resolution is usable, it is necessary

to use a closed-loop system where the position of the disk drive