AD9739ABBCZ datasheet(59/64 Pages) AD

59 / 64 page

Data Sheet

AD9737A/AD9739A

Rev. | Page 59 of 64

OUTPUT STAGE CONFIGURATION

The AD9737A/AD9739A are 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 be realized only 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 performance potential of the AD9737A/AD9739A.

MINI-CIRCUITS®

TC1-33-75G+

90Ω

IOUTP

IOUTN

70Ω

Figure 181. 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 181. 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 AD9737A/AD9739A evaluation board, which uses this

configuration. Figure 182 compares the measured frequency

response for normal and mix-mode using the AD9737A/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 182. Measured vs. Ideal Frequency Response for Normal (Baseband)

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

the AD9737A/AD9739A EVB

Figure 183 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 183. Interfacing the DAC Output to the Self-Biased Differential

Gain Stage

For applications operating the AD9737A/AD9739A in mix-mode

with output frequencies extending beyond 2.2 GHz, the circuits

shown in Figure 184 should be considered. The circuit in

Figure 184 uses a wideband balun with a configuration similar

to the one shown in Figure 183 to provide a dc bias path for the

DAC outputs. The circuit in Figure 185 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 184. Recommended Mix-Mode Configuration Offering Extended RF

Bandwidth Using a TC1-1-43A+ Balun

MURATA

JOHANSON TECHNOLOGY

CHIP BALUNS

180Ω

IOUTP

IOUTN

70Ω

Figure 185. Lowest Cost and Size Configuration for Narrow RF Band

Operation

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