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ADM8840
–
7
–
REV. PrG
2/03
PRELIMINARY TECHNICAL DATA
SERIAL INTERFACE
The COM Driver section of the ADM8840 uses a serial
interface to input data and transfer it into the DACs. Figure
3, below, shows the operation of the serial interface. The data
is transmitted along the serial DATA line, along with a serial
clock signal, SCLK. This data is read into a Shift Register.
When the 8 bits are sucessfully stored in the Shift Register a
low-to-high transition on the CS/LDAC input causes the
latch to load the 8-bits of data into the relevent DAC.
This function is also shown in the waveforms in Figure 4
below. A falling edge on the CS/LDAC input initiates the
data read into the shift register. The first bit of the datastream
is the DAC Select Bit (DAC_SEL) which determines which
internal DAC the data will be written to. A ‘1’ selects DAC
1 which sets the Amplitude of the output and a ‘0’ selects
Figure 4. Serial Interface Waveforms
t
4
D7
t
3
D2
D3
D4
D5
D6
t
5
t
7
t
6
CS /
LDAC
DATA
SCLK
t
1
t
2
D0
D1
DAC
SEL
SHIFT
REGISTER
SCLK
DATA
DAC1
LATCH
CS/LDAC
8 DATABITS
DAC2
LATCH
DAC
SELECT
DAC_SEL BIT
DAC 1 OUT
DAC2 OUT
DAC1
(AMPLITUDE)
DAC2
(CENTRE VOLTAGE)
Figure 3. Serial Interface Diagram
DAC 2 which sets the Centre Voltage of the output. The
individual data bits are then read in one by one on the DATA
line. After the DAC_SEL bit and the 8 data bits have been
read there is a pause to ensure the shift register outputs are
stable. Then a rising edge on the CS/LDAC input loads the
8 bits on the shift register outputs into the relevent DAC (and
the DAC outputs will change accordingly). Note that if CS/
LDAC goes high before all 8 data bits are read in then
incorrect data will be loaded into the DACs. All bits on the
DATA line are read in on each rising edge of the SCLK
signal.
When the ADM8840 comes out of shutdown the DACs
are preset with default values generating a COM_OUT
Amplitude of 6V with a Centre voltage of 1.5V.