AD9789BBCZ datasheet(56/76 Pages) AD

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AD9789

Rev. A | Page 56 of 76

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.

CURRENT

SCALING

FSC[9:0]

AD9789

DAC

FULL-SCALE

CURRENT

10kΩ

1nF

VREF

I120

AVSS

I120

VBG

1.2V

–

Figure 108. Voltage Reference Circuit

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, IFS, in milliamperes.

IFS = 0.023 × FSC[9:0] + 8.58

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.

200

400

600

800

DAC GAIN CODE

1000

Figure 109. Full-Scale Current vs. DAC Gain Code

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.

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

Recommended IFS (mA)

FSC[9:0]

512

720

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.

90Ω

70Ω

IOUTP

IOUTN

90Ω

JTX-2-10T

Figure 110. Recommended Transformer Output Stage

for Single-Tone/Multitone Measurements

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.

90Ω

70Ω

IOUTP

IOUTN

MABACT0039

90Ω

Figure 111. Recommended Transformer Output Stage for Mix Mode

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