AD9851
W CLK #1
Differential DAC output connection (Figure 9) for reduction of
common-mode signals and to allow highly reactive filters to be
driven without a filter input termination resistor (see above
single-ended example, Figure 8). A 6 dB power advantage is
obtained at the filter output as compared with the single-ended
example, since the filter need not be doubly terminated.
W CLK IOUT
AD9851
#1
FQ UD
RESET
W CLK #1
FQ UD
FQ UD
90
PHASE
MICROPROCESSOR
OR
DIFFERENCE
8-BIT DATA BUS
RESET
REF
CLOCK
MICROCONTROLLER
RESET
DIFFERENTIAL
TRANSFORMER-COUPLED
RESET
FQ UD
IOUT
W CLK #2
OUTPUT
21
FILTER
REFERENCE
CLOCK
AD9851
50⍀
#2
W CLK
AD9851
W CLK #2
DDS
20
Figure 7. Application Showing Synchronization of Two
AD9851 DDSs to Form a Quadrature Oscillator
50⍀
1:1 TRANSFORMER
i.e., MINI-CIRCUITS T1–1T
After a common RESET command is issued, separate W_CLKs
allow independent programming of each AD9851 40-bit input
register via the 8-bit data bus or serial input pin. A common
FQ_UD pulse is issued after programming is completed to
simultaneously engage both oscillators at their specified fre-
quency and phase.
Figure 9. Differential DAC Output Connection for Reduc-
tion of Common-Mode Signals
The AD9851 RSET input being driven by an external DAC
(Figure 10) to provide amplitude modulation or fixed, digital
amplitude control of the DAC output current. Full description
of this application is found as a “Technical Note” on the AD9851
web page (site address is www.analog.com) under “Related
Information.” An Analog Devices application note for the
AD9850, AN-423, describes another method of amplitude
control using an enhancement-mode MOSFET that is equally
applicable to the AD9851.
BANDPASS
AMPLIFIER
FILTER
240MHz
IOUT
AD9851
50⍀
50⍀
30MHz
CLOCK
AD9851
SPECTRUM
FINAL OUTPUT
SPECTRUM
NOTE: If the 6× REFCLK Multiplier of the AD9851 is en-
gaged, the 125 MHz clocking source shown in Figure 10 can be
reduced by a factor of six.
FUNDAMENTAL
F
+ F
O
C
F
– F
O
F
+ F
O
C
C
IMAGE
IMAGE
IMAGE
F
BANDPASS
FILTER
CLK
60 120
180 240
240
FREQUENCY – MHz
FREQUENCY – MHz
Figure 8. Deriving a High Frequency Output Signal from
the AD9851 by Using an “Alias” or Image Signal
+5V
+5V
330⍀
+5V
DIFFERENTIAL
TRANSFORMER-COUPLED
OUTPUT
20mA
MAX
DATA
4k⍀
12
9
21
20
10-BIT DAC
AD9731
GENERATOR
e.g., DG-2020
IOUT
10 BITS
R
SET
50⍀
200⍀
AD9851
DDS
–5V
IOUT
125MHz
50⍀
1:1 TRANSFORMER
CONTROL
DATA
COMPUTER
Figure 10. The AD9851 RSET Input Being Driven by an External DAC
REV. C
–7–