AD9789 Analog Devices, AD9789 Datasheet - Page 56

• Manufacturer Part Number: AD9789
• Description: 14-Bit, 2400 MSPS RF DAC with 4-Channel Signal Processing
• Manufacturer: Analog Devices

Specifications of AD9789

• Resolution (bits): 14bit
• Dac Update Rate: 2.4GSPS
• Dac Settling Time: 13ns
• Max Pos Supply (v): +3.47V
• Single-supply: No
• Dac Type: Current Out
• Dac Input Format: LVDS,Par

AD9789
VOLTAGE REFERENCE
The AD9789 output current is set by a combination of digital
control bits and the I120 reference current, as shown in
Figure 108.
The reference current is obtained by forcing the band gap
voltage across an external 10 kΩ resistor from I120 (Pin B14) to
ground. The 1.2 V nominal band gap voltage, VREF (Pin C14),
generates a 120 μA reference current in the 10 kΩ resistor. This
current is adjusted digitally by FSC[7:0] (Register 0x3C[7:0])
and FSC[9:8] (Register 0x3D[1:0]) to set the output full-scale
current, I
The full-scale output current range is approximately 8.6 mA to
32.1 mA for register values from 0x000 to 0x3FF. The default
value of 0x200 generates 20 mA full scale. The typical range is
shown in Figure 109.
Always connect a 10 kΩ resistor from the I120 pin to ground
and use the digital controls to adjust the full-scale current. The
AD9789 is not a multiplying DAC. Applying an analog signal to
I120 is not supported.
I
FS
35
30
25
20
15
10
5
0
1nF
= 0.023 × FSC[9:0] + 8.58
0
AVSS
FS
, in milliamperes.
Figure 109. Full-Scale Current vs. DAC Gain Code
10kΩ
VREF
I120
Figure 108. Voltage Reference Circuit
200
I120
1.2V
V
BG
AD9789
+
–
400
DAC GAIN CODE
CURRENT
SCALING
FSC[9:0]
600
FULL-SCALE
CURRENT
800
DAC
1000
Rev. A | Page 56 of 76
VREF (Pin C14) must be bypassed to ground with a 1 nF
capacitor. The band gap voltage is present on this pin and can
be buffered for use in external circuitry. The typical output
impedance is near 5 kΩ. If desired, an external reference can be
used to overdrive the internal reference by connecting it to the
VREF pin.
IPTAT (Pin D14) is used for factory testing and can simply be
left floating. IPTAT is an output current that is proportional to
absolute temperature. At 25°C, the output current is approx-
imately 10 μA and follows a slope of approximately 20 nA/°C.
For optimal DOCSIS 3.0 ACLR performance, the full-scale
output current settings provided in Table 75 are recommended.
Table 75. Recommended Full-Scale Current Settings vs.
Number of QAM Channels
Number of
QAM Channels
1
2
3
4
DAC OUTPUT STAGES
To properly evaluate the AD9789 in the lab, three distinct
output coupling circuits were used.
Figure 110 shows the optimal output network when measuring
traditional DAC performance specifications such as SFDR and
IMD performance with sine waves.
Figure 111 shows the optimal output network when measuring
signals in mix mode (second or third Nyquist zone). The band-
width of the center tap transformer is not sufficient to support
mix mode outputs, so the best solution is to use a wideband
balun by itself.
Figure 111. Recommended Transformer Output Stage for Mix Mode
Figure 110. Recommended Transformer Output Stage
IOUTN
IOUTP
70Ω
IOUTP
IOUTN
for Single-Tone/Multitone Measurements
70Ω
90Ω
Recommended I
20
25
25
25
90Ω
90Ω
90Ω
MABACT0039
FS
JTX-2-10T
(mA)
FSC[9:0]
512
720
720
720