AD8303AR View Datasheet(PDF) - Analog Devices
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AD8303AR Datasheet PDF : 16 Pages
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AD8303
SDI
CLK
en
CS
D
12-BIT SHIFT
REGISTER
12
CLK Q11–Q0
AD8303
MSB
12
DAC REGISTER A
RESET LOAD
DAC A
VOUTA
MSB
12
DAC REGISTER B
RESET LOAD
DAC B
VOUTB
MSB RS LDA
LDB
SHDN
Figure 27. AD8303 Digital Section Functional Block Diagram
DIGITAL INTERFACE
The AD8303 has a double-buffered serial data input. The
serial-input register is separate from the two DAC registers,
which allows preloading of a new data value into the serial
register without disturbing the present DAC values. A
functional block diagram of the digital section is shown in
Figure 27, while Table I contains the truth table for the control
logic inputs.
Three pins control the serial data input. Data at the Serial Data
Input (SDI) is clocked into the shift register on the rising edge
of CLK. Data is entered in MSB-first format. Twelve clock
pulses are required to load the 12-bit DAC value. If additional
bits are clocked into the shift register, for example when a µC
sends two 8-bit bytes, the MSBs are ignored (Figure 28). The
CLK pin is only enabled when Chip Select (CS) is low. If only
one AD8303 is connected to a serial data bus, then CS can be
tied (hardwired) to ground.
BYTE 1
BYTE 2
MSB
LSB MSB
LSB
B15 B14 B13 B12 B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0
X X X X D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
D11–D0: 12-BIT DAC VALUE
X = DON'T CARE
THE MSB OF BYTE 1 IS THE FIRST BIT THAT IS LOADED INTO THE DAC
Figure 28. Typical AD8303-Microprocessor Serial Data
Input Format
Separate Load pins (LDA and LDB) are provided to control the
flow of data from the shift register to the DAC registers. After
the new value is loaded in the serial-input register, it can be
asynchronously transferred to either DAC register by strobing
the appropriate Load pin (LDA or LDB). The Load pins are
level sensitive, so they should be returned high before any new
data is loaded into the serial-input register.
RESET (RS) AND MSB PINS
The RS pin forces both of the DAC registers to a known state,
based on the logic level on the MSB pin. If MSB is a logic zero,
then forcing RS low will set the DAC latches to all zeros and the
DAC output voltage will be zero volts. If MSB is a logic one, then
RS will force the DAC latches to one-half scale (800H) and the
DAC outputs will be 1.024 V. The half-scale reset is useful for
systems where the DAC output is referenced to a “false
ground” (see the Generating Bipolar Outputs with a Single
Supply section of this data sheet for more information).
The reset function is useful for setting the DAC outputs to zero
at power-up or after a power supply interruption. Test systems
and motor controllers are two of many applications which
benefit from powering up to a known state. The reset pulse can
be generated by the microprocessor’s power-on RESET signal,
by an output from the microprocessor (Figure 33), or by an
external resistor and capacitor (Figure 34).
RS and MSB have level-sensitive thresholds. The RS input
overrides other logic inputs (specifically, LDA and LDB).
However, LDA and LDB should be set high before RS goes
high. If LDA or LDB are kept low, then the contents of the shift
register will be transferred to the DAC register as soon as RS
goes high.
–10–
REV. 0
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