AD7769
CIRCUIT DESCRIPTION
Analog Inputs and Outputs
The AD7769 provides the analog-to-digital and digital-to-analog
conversion functions required between the microcontroller and
the servo power amplifier in digital servo systems. It is intended
primarily for closed loop head positioning in Winchester disk
drives, but may also be used for microstepping in drives with
stepper motor head positioning or other servo applications. The
AD7769 contains a high speed, 8-bit, sampling ADC with two
input channels and two 8-bit DACs with output buffer amplifi-
ers. A unique feature of the AD7769 is the input and output sig-
nal conditioning circuitry that allows the analog input and
output voltages to be referred to a point other than analog
ground. The input range and offset of the ADC, the output
swing and offset of the DACs may be adjusted independently by
the application of ground-referenced, positive control voltages,
V
BIAS
(ADC), V
SWING
(ADC), V
BIAS
(DAC) and V
SWING
(DAC).
Thus, for example, the peak-to-peak output swing of the DACs
could be set to 3 V above and 3 V below a bias voltage of 5 V.
Figures 5 and 6 show the transfer functions of the ADC
and DACs and their relationship to V
BIAS
and V
SWING.
The
midpoint code of the ADC, 80 Hex (10000000 Binary), occurs
at an input voltage equal to V
BIAS
. The input FSR of the ADC is
equal to 2 V
SWING
, so that the Plus Full-Scale code transition
(FE to FF Hex) occurs at a voltage equal to V
BIAS
+ V
SWING
–1.5 LSBs and the Minus Full-Scale code transition (01 to 00
Hex) occurs at a voltage V
BIAS
– V
SWING
+0.5 LSBs. The
transfer function of the DACs bears a similar relationship to
V
BIAS
and V
SWING
. The DAC output voltage for code 80 Hex
(10000000 binary) is equal to V
BIAS
, while FF Hex (11111111
binary) gives an output voltage of V
BIAS
+ V
SWING
–1 LSB
(Plus Full-Scale) and 00 Hex gives an output voltage of V
BIAS
–
V
SWING
(Minus Full-Scale).
The ability to refer input and output signals to some voltage
other than ground is of particular importance in disk drive ap-
plications. Typically, only +5 V digital and +12 V analog supply
voltages are available, and the analog signals are often referred
to a voltage around half the analog supply.
Driving the Analog Inputs and Reference Inputs
The analog inputs, V
IN
A and V
IN
B, must be driven from low
output impedance sources, such as from op amps. In addition,
V
BIAS
(ADC) must be driven from a similar type low impedance
source (e.g., voltage reference).
Op amps are not required to drive the V
SWING
(ADC), V
BIAS
(DAC) and V
SWING
(DAC) inputs as these are high impedance
inputs (200 nA typical input current) that feed into on-chip
buffer amplifiers. The reference voltages for these inputs can be
derived using suitable resistor divider networks.
The analog reference available in the disk drive system can be
used to set the bias voltage of the AD7769, and could also be at-
tenuated to provide the reference for the input and output swing
as shown in Figure 7. The same bias voltage would generally
(though not necessarily) be used for the ADC and the DACs,
though the input and output ranges might be different.
Figure 5. ADC Transfer Function
Figure 7. Typical Analog Connections to the AD7769
ADC Conversion Cycle
Figure 6. DAC Transfer Function
Figure 8 shows the operating waveforms for a conversion cycle.
On the rising edge of
WR,
the conversion cycle starts with the
acquisition and tracking of the selected ADC channel, V
IN
A or
V
IN
B. The analog input voltage is held 50 ns (typically) after the
fourth falling edge of the input CLK following a conversion
start. If t
D
in Figure 8 is greater than 150 ns, then the falling
edge of the input CLK will be seen as the first falling clock edge.
If t
D
is less than 150 ns, the first falling clock edge to be recog-
nized will not occur until one cycle later.
–8–
REV. A
AD [ ANALOG DEVICES ]
