AD9709AST datasheet(13/27 Pages) AD

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REV. 0

AD9709

–13–

TIME OF DATA CHANGE RELATIVE TO

RISING CLOCK EDGE – ns

–4

–2

–3

–1

Figure 29. SINAD vs. Clock Placement @ fOUT = 20 MHz

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 so

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 29 shows

the relationship of SNR to clock/data placement.

SLEEP MODE OPERATION

The AD9709 has a power down function that turns off the

output current and reduces the supply current to less than

8.5 mA over the speciﬁed supply range of 3.0 V to 5.5 V and

temperature range. This mode can be activated by applying a

logic level 1 to the SLEEP pin. The SLEEP pin logic threshold

is equal to 0.5

× AVDD. This digital input also contains an

active pull-down circuit that ensures the AD9709 remains

enabled if this input is left disconnected. The AD9709 takes

less than 50 ns to power down and approximately 5

µs to

power back up.

POWER DISSIPATION

The power dissipation, PD, of the AD9709 is dependent on

several factors that include: (1) The power supply voltages

(AVDD and DVDD), (2) the full-scale current output IOUTFS,

(3) the update rate fCLOCK, (4) and the reconstructed digital

input waveform. The power dissipation is directly proportional

to the analog supply current, IAVDD, and the digital supply cur-

rent, IDVDD. IAVDD is directly proportional to IOUTFS as shown in

Figure 30 and is insensitive to fCLOCK.

Conversely, IDVDD is dependent on both the digital input wave-

form, fCLOCK, and digital supply DVDD. Figures 31 and 32

show IDVDD as a function of full-scale sine wave output ratios

(fOUT/fCLOCK) for various update rates with DVDD = 5 V and

DVDD = 3 V, respectively. Note how IDVDD is reduced by more

than a factor of 2 when DVDD is reduced from 5 V to 3 V.

APPLYING THE AD9709

Output Conﬁgurations

The following sections illustrate some typical output conﬁgura-

tions for the AD9709. Unless otherwise noted, it is assumed

that IOUTFS is set to a nominal 20 mA. For applications requiring

the optimum dynamic performance, a differential output

conﬁguration is suggested. A differential output conﬁguration

may consist of either an RF transformer or a differential op amp

conﬁguration. The transformer conﬁguration provides the opti-

mum high-frequency performance and is recommended for any

application allowing for ac coupling. The differential op amp

conﬁguration is suitable for applications requiring dc coupling,

a bipolar output, signal gain and/or level shifting, within the

bandwidth of the chosen op amp.

IOUTFS – mA

Figure 30. IAVDD vs. IOUTFS

RATIO – fOUT/fCLK

0.1

0.2

0.3

0.4

0.5

125MSPS

100MSPS

65MSPS

25MSPS

5MSPS

Figure 31. IDVDD vs. Ratio @ DVDD = 5 V

RATIO – fOUT/fCLK

0.1

0.2

0.3

0.4

0.5

125MSPS

100MSPS

65MSPS

25MSPS

5MSPS

Figure 32. IDVDD vs. Ratio @ DVDD = 3 V

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