AD9739A datasheet(39/44 Pages) AD

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AD9739A

Rev. 0 | Page 39 of 44

Output Stage Configuration

The AD9739A is intended to serve high dynamic range

applications that require wide signal reconstruction bandwidth

(that is, DOCSIS CMTS) and/or high IF/RF signal generation.

Optimum ac performance can only be realized if the DAC

output is configured for differential (that is, balanced) operation

with its output common-mode voltage biased to analog ground.

The output network used to interface to the DAC should

provide a near 0 Ω dc bias path to analog ground. Any

imbalance in the output impedance between the IOUTP and

IOUTN pins results in asymmetrical signal swings that degrade

the distortion performance (mostly even order) and noise

performance. Component selection and layout are critical in

realizing the AD9739A’s performance potential.

MINI-CIRCUITS®

TC1-33-75G+

90Ω

IOUTP

IOUTN

70Ω

Figure 97. Recommended Balun for Wideband Applications with Upper

Bandwidths of up to 2.2 GHz

Most applications requiring balanced-to-unbalanced conversion

can take advantage of the Ruthroff 1:1 balun configuration

shown in Figure 97. This configuration provides excellent

amplitude/phase balance over a wide frequency range while

providing a 0 Ω dc bias path to each DAC output. Also, its

design provides exceptional bandwidth and can be considered

for applications requiring signal reconstruction of up to 2.2 GHz.

The characterization plots shown in this data sheet are based on

the AD9739A evaluation board, which uses this configuration.

Figure 98 compares the measured frequency response for

normal and mix mode using the AD9739A evaluation board vs.

the ideal frequency response.

–36

–33

–30

–27

–24

–21

–18

–15

–12

–9

–6

–3

500

1000

1500

2000

2500

3000

3500

FREQUENCY (MHz)

IDEAL BASEBAND MODE

MIX MODE

TC1-33-75G

BASEBAND

TC1-33-75G

IDEAL MIX MODE

Figure 98. Measured vs. Ideal Frequency Response for Normal (Baseband)

and Mix-Mode Operation Using a TC1-33-75G Transformer on

the AD9739A EVB

Figure 99 shows an interface that can be considered when

interfacing the DAC output to a self-biased differential gain

block. The inductors shown serve as RF chokes (L) that provide

the dc bias path to analog ground. The value of the inductor,

along with the dc blocking capacitors (C), determines the lower

cutoff frequency of the composite pass-band response. An RF

balun should also be considered before the RF differential gain

stage and any filtering to ensure symmetrical common-mode

impedance seen by the DAC output while suppressing any

common mode noise, harmonics, and clock spurs prior to

amplification.

90Ω

IOUTP

IOUTN

70Ω

RF DIFF

AMP

OPTIONAL BALUN AND FILTER

90Ω

LPF

Figure 99. Interfacing the DAC Output to the Self-Biased Differential

Gain Stage

For applications operating the AD9739A in mix mode with output

frequencies extending beyond 2.2 GHz, the circuits shown in

Figure 100 should be considered. The circuit in Figure 100 uses

a wideband balun with a configuration similar to the one shown

in Figure 99 to provide a dc bias path for the DAC outputs. The

circuit in Figure 101 takes advantage of ceramic chip baluns to

provide a dc bias path for the DAC outputs while providing

excellent amplitude/phase balance over a narrower RF band.

These low cost, low insertion loss baluns are available for

different popular RF bands and provide excellent amplitude/

phase balance over their specified frequency range.

MINI-CIRCUITS

TC1-1-462M

90Ω

IOUTP

IOUTN

70Ω

90Ω

Figure 100. Recommended Mix-Mode Configuration Offering Extended RF

Bandwidth Using a TC1-1-43A+ Balun

MURATA

JOHANSON TECHNOLOGY

CHIP BALUNS

180Ω

IOUTP

IOUTN

70Ω

Figure 101. Lowest Cost and Size Configuration for Narrow RF Band

Operation

NONIDEAL SPECTRAL ARTIFACTS

The AD9739A output spectrum contains spectral artifacts that

are not part of the original digital input waveform. These non-

ideal artifacts included harmonics (including alias harmonics),

images, and clock spurs. Figure 102 shows a spectral plot of the

AD9739A within the first Nyquist zone (that is, dc to fDAC/2)

reconstructing a 650 MHz, 0 dBFS sine wave at 2.4 GSPS. Besides

the desired fundamental tone at the −7.8 dBm level, the spectrum

also reveals these nonideal artifacts that also appear as spurs

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