AD7769

–13–

REV. A

Figure 23. Typical Dedicated Servo Control Loop Using

the AD7769

Figure 24. Typical Relationship Between Input Voltage and

Output Current for Transconductance Amplifier

APPLICATIONS

The AD7769 analog I/O port is used to convert servo related

signals between the analog and digital domains. The input

structure of the two-channel ADC makes it very easy to convert

the typical output signals provided by a servo demodulator.

In a magnetic disk drive employing a dedicated servo surface,

the servo demodulator produces two, positive-only, quadrature

signals, generally sinusoidal or triangular, from the all-bit pat-

terns read from the servo surface. The quadrature signals have

± V

AD7769 ADC allows sequential conversion of these quadrature

signals without introducing significant phase delay errors. These

converted signals provide the servo microcontroller with posi-

tion and track crossing information from which velocity infor-

mation can be derived. In optical disk drives, analogous servo

signals can be derived from the quad photodiode detector to

provide position and focus information for the microcontroller.

The two DACs in the AD7769 accept servo data from the

microcontroller to position the head assembly. The DACs pro-

± V

which are ideal for driving voice coil motors. In magnetic disk

drives, a single voice coil motor is used to position the head as-

sembly and one DAC is usually sufficient to drive the motor in

both the seek and track modes. In the seek mode, the DAC can

be used to generate directly the desired analog velocity trajec-

tory which the head must travel in order to achieve minimum

access times. Alternatively, the DAC can generate a servo error

value (computed by the microcontroller) between the actual

head velocity and the desired head velocity. In the track mode,

the DAC can be used to provide a position error signal to keep

the head over the track or to detent the head off track, for such

purposes as thermal compensation and soft error retries. The

second DAC in the AD7769 may be employed in this fine posi-

tioning loop. Alternatively, the second DAC can be used to con-

trol the speed of the spindle motor via a pulse width modulator.

In optical disk drives two voice coil motors are used, requiring

both DACs of the AD7769–one for the focus servo loop and

one for the radial positioning servo loop.

A typical servo control loop using the AD7769 is shown in Fig-

ure 23. In this dedicated servo drive, the servo demodulator con-

verts the servo information bit patterns from the disk into the

standard N and Q (normal and quadrature) servo signals. The

power transconductance amplifier. One input to this amplifier is

power stage is determined by external sense resistors.