AD9709 Analog Devices, AD9709 Datasheet - Page 17

• Manufacturer Part Number: AD9709
• Description: 8-Bit, 125 MSPS Dual TxDAC+ Digital-to-Analog Converter
• Manufacturer: Analog Devices

Specifications of AD9709

• Resolution (bits): 8bit
• Dac Update Rate: 125MSPS
• Dac Settling Time: 35ns
• Max Pos Supply (v): +5.5V
• Single-supply: Yes
• Dac Type: Current Out
• Dac Input Format: Par

Digital signal paths should be kept short, and run lengths should be
matched to avoid propagation delay mismatch. The insertion of
a low value (that is, 20 Ω to 100 Ω) resistor network between
the AD9709 digital inputs and driver outputs may be helpful in
reducing any overshooting and ringing at the digital inputs that
contribute to digital feedthrough. For longer board traces and
high data update rates, stripline techniques with proper
impedance and termination resistors should be considered to
maintain “clean” digital inputs.
The external clock driver circuitry provides the AD9709 with a
low-jitter clock input meeting the minimum and maximum logic
levels while providing fast edges. Fast clock edges help minimize
jitter manifesting itself as phase noise on a reconstructed waveform.
Therefore, the clock input should be driven by the fastest logic
family suitable for the application.
Note that the clock input can also be driven via a sine wave, which
is centered around the digital threshold (that is, DVDDx/2) and
meets the minimum and maximum logic threshold. This typically
results in a slight degradation in the phase noise, which becomes
more noticeable at higher sampling rates and output frequencies.
In addition, at higher sampling rates, the 20% tolerance of the
digital logic threshold should be considered because it affects
the effective clock duty cycle and, subsequently, cut into the
required data setup and hold times.
Rev. B | Page 17 of 32
Input Clock and Data Timing Relationship
SNR in a DAC is dependent on the relationship between the
position of the clock edges and the point in time at which the
input data changes. The AD9709 is rising-edge triggered and
therefore exhibits SNR sensitivity when the data transition is
close to this edge. In general, the goal when applying the AD9709 is
to make the data transition close to the falling clock edge. This
becomes more important as the sample rate increases. Figure 32
shows the relationship of SNR to clock/data placement.
Figure 32. SNR vs. Clock Placement @ f
60
50
40
30
20
10
0
–4
–3
TIME OF DATA CHANGE RELATIVE TO
–2
RISING CLOCK EDGE (ns)
–1
0
OUT
= 20 MHz and f
1
2
CLK
3
AD9709
= 125 MSPS
4