AD7769
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
the form of V
BIAS
±
V
SWING
. The very fast conversion time of the
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-
vide positive-only output signals of the form V
BIAS
±
V
SWING
,
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
voice coil motor current, I
L
, is bidirectional and is supplied by the
power transconductance amplifier. One input to this amplifier is
held at V
BIAS
(DAC), while the other input is driven from a DAC
output, V
OUT
A/B. Typical input/output waveforms for this power
stage are shown in Figure 24. The transconductance, G
O
, of the
power stage is determined by external sense resistors.
Figure 23. Typical Dedicated Servo Control Loop Using
the AD7769
Figure 24. Typical Relationship Between Input Voltage and
Output Current for Transconductance Amplifier
REV. A
–13–
AD [ ANALOG DEVICES ]
