AD9879-EB datasheet(25/32 Pages) AD

25 / 32 page

AD9879

Rev. A | Page 25 of 32

DIGITAL-TO-ANALOG CONVERTER

A 12-bit digital-to-analog converter (DAC) is used to convert

the digitally processed waveform into an analog signal. The

worst-case spurious signals due to the DAC are the harmonics

of the fundamental signal and their aliases. The conversion

process produces aliased components of the fundamental signal

at n × fSYSCLK ± fCARRIER (n = 1, 2, 3). These are typically filtered

with an external RLC filter at the DAC output. It is important

for this analog filter to have a sufficiently flat gain and linear

phase response across the bandwidth of interest to avoid

modulation impairments. A relatively inexpensive seventh-

order elliptical low-pass filter is sufficient to suppress the

aliased components for HFC network applications.

The AD9879 provides true and complement current outputs.

The full-scale output current is set by the RSET resistor at Pin 49

and the DAC gain register. Assuming maximum DAC gain, the

value of RSET for a particular full-scale IOUT is determined using

the following equation:

RSET = 32 VDACRSET/IOUT = 39.4/IOUT

(22)

For example, if a full-scale output current of 20 mA is desired,

then RSET = (39.4/0.02) Ω, or approximately 2 kΩ.

The following equation calculates the full-scale output current

including the programmable DAC gain control.

IOUT = [39.4/RSET] × 10(−7.5 + 0.5 NGAIN)/20

(23)

where NGAIN is the value of DAC fine gain control [3:0].

The full-scale output current range of the AD9879 is 4 mA to

20 mA. Full-scale output currents outside of this range degrade

SFDR performance. SFDR is also slightly affected by output

matching; the two outputs should be terminated equally for best

SFDR performance. The output load should be located as close

as possible to the AD9879 package to minimize stray capaci-

tance and inductance.

The load can be a simple resistor to ground, an op amp current-

to-voltage converter, or a transformer-coupled circuit. It is best

not to attempt to directly drive highly reactive loads (such as an

LC filter).

Driving an LC filter without a transformer requires the filter to

be doubly terminated for best performance. The filter input and

output should both be resistively terminated with the appro-

priate values. The parallel combination of the two terminations

determines the load the AD9879 sees for signals within the

filter pass band. For example, a 50 Ω terminated input/output

low-pass filter looks like a 25 Ω load to the AD9879. The

output compliance voltage of the AD9879 is −0.5 V to +1.5 V.

To avoid signal distortion, any signal developed at the DAC

output should not exceed +1.5 V. Furthermore, the signal may

extend below ground as much as 0.5 V without damage or

signal distortion.

The AD9879 true and complement outputs can be differentially

combined for common-mode rejection using a broadband 1:1

transformer. Using a grounded center tap results in signals at

the AD9879 DAC output pins that are symmetrical about

ground. As previously mentioned, by differentially combining

the two signals, the user can provide some degree of common-

mode signal rejection. A differential combiner might consist of

a transformer or an operational amplifier. The object is to

combine or amplify only the difference between two signals and

to reject any common, usually undesirable, characteristic, such

as 60 Hz hum or clock feedthrough that is equally present on

both individual signals.

Connecting the AD9879 true and complement outputs to the

differential inputs of the gain programmable cable drivers

AD8321/AD8323 or AD8322/AD8327 provides an optimized

solution for the standard compliant cable modem upstream

channel. The cable driver’s gain can be programmed through a

direct 3-wire interface using the profile registers of the AD9879.

LOW-PASS

FILTER

AD832x

AD9879

Ω

VARIABLE GAIN

CABLE DRIVER

AMPLIFIER

CA_EN

CA_DATA

CA_CLK

DAC

Figure 19. Cable Amplifier Connection

CA_EN

CA_CLK

CA_DATA

MSB

tMCLK

LSB

Figure 20. Cable Amplifier Interface Timing

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