AD9851
level as determined by the sin(x)/x roll-off of the quantized D/A
converter output. In fact, depending on the f/system clock rela-
tionship, the 1st aliased image can equal the fundamental
amplitude (when f
OUT
= 1/2 system clock). A low-pass filter is
generally placed between the output of the D/A converter and
the input of the comparator to suppress the jitter-producing
effects of non-harmonically related aliased images and other
spurious signals. Consideration must be given to the relationship
of the selected output frequency, the system clock frequency and
alias frequencies to avoid unwanted output anomalies.
Images need not be thought of as useless by-products of a DAC.
In fact, with bandpass filtering around an image and some
amount of post-filter amplification, the image can become the
primary output signal (see Figure 8). Since images are not har-
monics, they retain a 1:1
∆frequency
relationship to the funda-
mental output. That is, if the fundamental is shifted 1 kHz, then
the image is also shifted 1 kHz. This relationship accounts for
the frequency stability of an image, which is identical to that of
the fundamental. Users should recognize that the lower image of
an image pair surrounding an integer multiple of the system clock
will move in a direction opposite the fundamental. Images of an
image pair located above an integer multiple of the system clock
will move in the same direction as a fundamental movement.
The frequency band where images exist is much richer in spuri-
ous signals and therefore, more hostile in terms of SFDR. Users
of this technique should empirically determine what frequencies
are usable if their SFDR requirements are demanding.
A good “rule-of-thumb” for applying the AD9851 as a clock
generator is to limit the fundamental output frequency to 40% of
Reference Clock frequency to avoid generating aliased signals
that are too close to the output band of interest (generally dc—
highest selected output frequency) to be filtered. This practice
will ease the complexity and cost of the external filter require-
ment for the clock generator application.
The reference clock input of the AD9851 has minimum limita-
tion of 1 MHz without 6× REFCLK Multiplier engaged and
5 MHz with multiplier engaged. The device has internal cir-
cuitry that senses when the clock rate has dropped below the
minimum and automatically places itself in the power-down
mode. In this mode, the on-chip comparator is also disabled.
This is important information for those who may wish to use the
on-chip comparator for purposes other than squaring the DDS
sine wave output. When the clock frequency returns above the
minimum threshold, the device resumes normal operation after
5
µs
(typically). This shutdown mode prevents excessive current
leakage in the dynamic registers of the device.
The impact of reference clock phase noise in DDS systems is
actually reduced, since the DDS output is the result of a division
of the input frequency. The amount of apparent phase noise
reduction, expressed in dB, is found using: 20 log f
OUT
/f
CLK
.
Where f
OUT
is the fundamental DDS output frequency and f
CLK
is the system clock frequency. From this standpoint, using the
highest system clock input frequency makes good sense in reduc-
ing the effects of reference clock phase noise contribution to the
output signals’ overall phase noise. As an example, an oscilla-
tor with –100 dBc phase noise operating at 180 MHz would
appear as a –125 dB contribution to DDS overall phase noise for
a 10 MHz output. Engaging the 6× REFCLK Multiplier has
generally been found to increase overall output phase noise. This
REV. C
–9–
increase is due to the inherent 6× (15.5 dB) phase gain transfer
function of the 6× REFCLK Multiplier, as well as noise gener-
ated internally by the clock multiplier circuit. By using a low
phase noise reference clock input to the AD9851, users can be
assured of better than –100 dBc/Hz phase noise performance
for output frequencies up to 50 MHz at offsets from 1 kHz to
100 kHz.
Programming the AD9851
The AD9851 contains a 40-bit register that stores the 32-bit
frequency control word, the 5-bit phase modulation word,
6× REFCLK Multiplier enable and the power-down function.
This register can be loaded in parallel or serial mode. A logic
high engages functions; for example, to power-down the IC
(sleep mode), a logic high must be programmed in that bit
location. Those users who are familiar with the AD9850 DDS
will find only a slight change in programming the AD9851,
specifically, data[0] of W0 (parallel load) and W32 (serial load)
now contains a “6× REFCLK Multiplier Enable” bit that needs
to be set high to enable or low to disable the internal reference
clock multiplier.
Note: setting “data[1]” high in programming word W0 (paral-
lel mode) or word W33 high in serial mode is not allowed (see
Tables I and III). This bit controls a “factory test mode” that
will cause abnormal operation in the AD9851 if set high. If
erroneously entered (as evidenced by Pin 2 changing from an
input pin to an output signal), an exit is provided by asserting
RESET. Unintentional entry to the factory test mode can
occur if an FQ_UD pulse is sent after initial power-up and
RESET of the AD9851. Since RESET does not clear the 40-
bit input register, this will transfer the random power-up values
of the input register to the DDS core. The random values may
invoke the factory test mode or power-down mode. Never issue
an FQ_UD command if the 40-bit input register contents are
unknown.
In the default parallel load mode, the 40-bit input register is
loaded using an 8-bit bus. W_CLK is used to load the register
in five iterations of eight bytes. The rising edge of FQ_UD
transfers the contents of the register into the device to be acted
upon and resets the word address pointer to W0. Subsequent
W_CLK rising edges load 8-bit data, starting at W0 and then
move the word pointer to the next word. After W0 through W4
are loaded, additional W_CLK edges are ignored until either a
RESET is asserted or an FQ_UD rising edge resets the address
pointer to W0 in preparation for the next 8-bit load. See Fig-
ure 13.
In serial load mode, forty subsequent rising edges of W_CLK
will shift and load the 1-bit data on Pin 25 (D7) through the
40-bit register in “shift-register” fashion. Any further W_CLK
rising edges after the register is full will shift data out causing
data that is left in the register to be out-of-sequence and cor-
rupted. The serial mode must be entered from the default
parallel mode, see Figure 17. Data is loaded beginning with
W0 and ending with W39. One note of caution: the 8-bit
parallel
word (W0)—xxxxx011—that invokes the serial mode
should be overwritten with a valid 40-bit serial word immedi-
ately after entering the serial mode to prevent unintended
engaging of the 6× REFCLK Multiplier or entry into the fac-
tory test mode. Exit from serial mode to parallel mode is only
possible using the RESET command.
AD [ ANALOG DEVICES ]
