AD7008
When amplitude modulation is not required, the IQ multipliers
can be bypassed (CR = 2). The sine output is directly sent to
the 10-bit DAC.
Digital-to-Analog Converter
The AD7008 includes a high impedance current source 10-bit
DAC, capable of driving a wide range of loads at different
speeds. Full-scale output current can be adjusted, for optimum
power and external load requirements, through the use of a
single external resistor (R
SET
).
The DAC can be configured for single or differential-ended
operation.
IOUT
can be tied directly to AGND for single-ended
operation or through a load resistor to develop an output volt-
age. The load resistor can be any value required as long as the
full-scale voltage developed across it does not exceed 1 volt.
Since full-scale current is controlled by R
SET
, adjustments to
R
SET
can balance changes made to the load resistor.
DSP and MPU Interfacing
TC3–TC0. At some time after the second falling edge of
WR,
the LOAD signal may go high. As long as the load signal is high
5 ns (see setup time) before the rising edge of the CLOCK sig-
nal, data will be transferred to the destination register.
The limiting factor of this technique is the
WR
period which is
30 ns. Thus the CLOCK may run up to 33 MSPS using this
technique and the effective update rate would be one half or
16.5 MHz. See timing Figure 10 for timing details.
DATA
HI WORD
LOW WORD
WR
CLOCK
TC
The AD7008 contains a 32-bit parallel assembly register and a
32-bit serial assembly register. Each of the modulation registers
can be loaded from either assembly register under control of the
LOAD pin and the Transfer-Control (TC) pins (See Table II).
The Command register can be loaded only from the parallel as-
sembly register. In practical use, both serial and parallel inter-
faces can be used simultaneously if the application requires.
TC3–TC0 should be stable before the LOAD signal rises and
should not change until after LOAD falls (Figure 2).
The DSP/MPU asserts both
WR
and
CS
to load the parallel as-
sembly register (Figure 3). At the end of each write, the parallel
assembly register is shifted left by 8 or 16 bits (Depending on
CR0), and the new data is loaded into the low bits. Hence, two
16-bit writes or four 8-bit writes are used to load the parallel as-
sembly register. When loading parallel data, it is only necessary
to write as much data as will be used by that register. For in-
stance, the Command Register requires only one write to the
parallel assembly register.
Serial data is input to the chip on the rising edge of SCLK, most
significant bit first (Figure 4). The data in the assembly regis-
ters can be transferred to the modulation registers by means of
the transfer control pins.
Maximum Updating of the AD7008
LOAD
Figure 10. Accelerated Data Load Sequence
APPLICATIONS
Serial Configuration
Data is written to the AD7008 in serial mode using the two sig-
nal lines SDATA and SCLK. Data is accumulated in the serial
assembly register with the most significant bit loaded first. The
data bits are loaded on the rising edge of the serial clock. Once
data is loaded in the serial assembly register, it must be trans-
ferred to the appropriate register on chip. This is accomplished
by setting the TC bits according to Tables II and III. If you
want to load the serial assembly register into FREQ1 register,
the TC bits should be 1101. When the LOAD pin is raised,
data is transferred directly to the FREQ1 register. When oper-
ating in serial mode, some functions must still operate in parallel
mode such as loading the TC bits and updating the Command
register which is accessed only through the parallel assembly
register. See Figure 11 for a typical serial mode configuration.
U3
AD7008
CMD0
CMD1
CMD2
CMD3
Updating the AD7008 need not take place in a synchronous
fashion. However, in asynchronous systems, most of the exter-
nal clock pulses (LOAD and SCLK) must be high for greater
than one system clock period. This insures that at least one
CLOCK rising edge will occur successfully completing the latch
function (Figure 1).
However, if the AD7008 is run in a synchronous mode with the
controlling DSP or microcontroller, the AD7008 may be loaded
very rapidly. Optimal speed is attained when operated in the
16-bit load mode; the following discussion will assume that
mode is used. Each of the modulation registers require two 16
bit loads. This data is latched into the parallel assembly register
on the falling edge of the
WR
command. This strobe is not
qualified by the CLOCK pulse but must be held low for a mini-
mum of 20 ns and only need be high for 10 ns. The two 16-bit
words may be loaded in succession. While the second 16-bit
word is being latched into the parallel assembly register, the
Transfer and Control word may be presented to the TC3–TC0
pins. If the designation register is always the same, an external
register can be used to store the information on the inputs of
REV. B
–9–
WR
TC0
TC1
TC2
TC3
LOAD
SCLK
SDATA
+5V
U2
14 50MHz
V
CC
8
OUT
V
EE
RESET
7 K1115
19
20
21
22
23
24
25
26
8
9
10
11
12
13
14
15
16
27
32
33
34
35
36
41
42
31
30
38
37
C1
6
+5V
D0
V
REF
D1
0.1µF C2
5
+5V
D2
COMP
D3
0.1µF
D4
2 R4
D5
IOUT
49.9
D6
R3
1
D7
D8
49.9
IOUT
D9
D10
D11
R5
4
D12
FSADJUST
D13
390
D14
D15
V
AA
3
+5V
WR
V
DD
17
+5V
CS
V
DD
28
+5V
TC0
V
DD
39
+5V
TC1
TC2
TC3
44
AGND
LOAD
7
DGND
SCLK
18
DGND
SDATA
29
DGND
FSELECT
43
DGND
CLK
RESET
40
TEST
SLEEP
Figure 11. General Purpose Serial Interface
AD [ ANALOG DEVICES ]
