AD9747 Analog Devices, AD9747 Datasheet - Page 25

• Manufacturer Part Number: AD9747
• Description: Dual 16-Bit 250 MSPS Digital-to-Analog Converters
• Manufacturer: Analog Devices

Specifications of AD9747

• Resolution (bits): 16bit
• Dac Update Rate: 250MSPS
• Dac Settling Time: n/a
• Max Pos Supply (v): +3.47V
• Single-supply: No
• Dac Type: Current Out
• Dac Input Format: Par

AUXILIARY DACS
Two auxiliary DACs are provided on the AD9741/AD9743/
AD9745/AD9746/AD9747. A functional diagram is shown
in Figure 37. The auxiliary DACs are current output devices
with two output pins, AUXP and AUXN. The active pin can
be programmed to either source or sink current. When either
sinking or sourcing, the full-scale current magnitude is 2 mA.
The available compliance range at the auxiliary DAC outputs
depends on whether the output is configured to a sink or source
current. When sourcing current, the compliance voltage is 0 V
to 1.6 V, but when sinking current, the output compliance
voltage reduces to 0.8 V to 1.6 V. Either output can be used, but
only one output of the auxiliary DAC (P or N) is active at any
time. The inactive pin is always in a high impedance state
(>100 kΩ).
In a single side band transmitter application, the combination of
the input referred dc offset voltage of the quadrature modulator
and the DAC output offset voltage can result in local oscillator
(LO) feedthrough at the modulator output, which degrades
system performance. The auxiliary DACs can be used to remove
the dc offset and the resulting LO feedthrough. The circuit
configuration for using the auxiliary DACs for performing
dc offset correction depends on the details of the DAC and
modulator interface. An example of a dc-coupled configuration
with low-pass filtering is outlined in the Power Dissipation
section.
Figure 36. Transfer Function for Each Analog Operating Mode
–10
–20
–30
–40
0
V
BIAS
0mA
2mA
0mA
2mA
TO
TO
Figure 37. Auxiliary DAC Functional Diagram
0.5
NORMAL
SOURCE
SINK
OR
RZ
F
S
NEGATIVE
POSITIVE
OR
MIX
1.5
AUXP
AUXN
2
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AD9741/AD9743/AD9745/AD9746/AD9747
POWER DISSIPATION
Figure 39 shows the power dissipation and current draw of the
AD9741/AD9743/AD9745/AD9746/AD9747. It shows that the
devices have a quiescent power dissipation of about 190 mW.
Most of this comes from the AVDD33 supply. Total power
dissipation increases about 50% as the clock rate is increased
to the maximum clock rate of 250 MHz.
Figure 38. DAC DC Coupled to Quadrature Modulator with Passive DC Shift
25Ω TO 50Ω
350
310
270
230
190
150
15
12
AD9747
DAC1 OR
9
6
3
0
DAC2
0
0
25
25
Figure 39. AD9747 Power Dissipation vs. f
Figure 40. DVDD33 Current vs. f
50
50
75
75
AD9747
DAC1 OR
FILTERING
OPTIONAL
DAC2
PASSIVE
AUX
100
100
f
f
DAC
DAC
AD9747
f
125
OUT
125
MODULATOR V+
QUADRATURE
(MHz)
(MHz)
= NYQUIST
150
150
AD9741
f
OUT
175
175
25Ω TO 50Ω
DAC
= DC
QUAD MOD
I OR Q INPUTS
200
200
DAC
225
225
250
250