AD9119BBCZ datasheet(52/66 Pages) AD

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AD9119/AD9129

Data Sheet

Rev. B | Page 52 of 66

Peak DAC Output Power Capability

The maximum peak power capability of a differential current

output DAC is dependent on its peak differential ac current,

IPEAK, and the equivalent load resistance it sees. In the case of a

1:1 balun with 50 Ω source termination, the equivalent load that

is seen by the DAC ac current source is 25 Ω. If the AD9119/

AD9129 is programmed for an IOUTFS = 20 mA, its peak ac current

is 9.375 mA and its peak power, delivered to the equivalent

load, is 2.2 mW (that is, P = I2R). Because the source and load

resistance seen by the 1:1 balun are equal, this power is shared

equally. Hence, the output load receives 1.1 mW, or 0.4 dBm

peak power.

To calculate the rms power delivered to the load, consider the

following:

•

Peak-to-rms of digital waveform

•

Any digital backoff from digital full scale

•

DAC sinc response and nonideal losses in the external

network

For example, a reconstructed sine wave with no digital backoff

ideally measures −2.6 dBm because it has a peak-to-rms ratio of

3 dB. If a typical balun loss of 0.4 dBm is included, the user would

expect to measure −3 dBm of actual power in the region where

the sinc response of the DAC has negligible influence. Increasing

the output power is best accomplished by increasing IOUTFS.

Output Stage Configuration

The AD9119/AD9129 are intended to serve high dynamic range

applications that require wide signal reconstruction bandwidth

(that is, a DOCSIS cable modem termination system (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 a stable, low noise 1.8 V nominal analog supply

(VDDA). The ADP150 LDO can be used to generate a clean 1.8

V supply.

The output network used to interface to the DAC should provide

a near 0 Ω dc bias path to VDDA. Any imbalance in the output

impedance over frequency between the IOUTP and IOUTN pins

degrades the distortion performance (mostly even order) and

noise performance. Component selection and layout are critical

in realizing the performance potential of the AD9119/AD9129.

Most applications that require balanced-to-unbalanced conversion

from 10 MHz to 1 GHz can take advantage of the Mini-Circuits

JTX series of transformers that offer impedance ratios of both

2:1 and 1:1.

Figure 152 shows the AD9119/AD9129 interfacing to the JTX-2-

10T transformer. This transformer provides excellent amplitude/

phase balance (that is, <1 dB/1°) up to 1 GHz while providing

a 0 Ω D dc bias path to VDDA. If filtering of the DAC images

and clock components is required, applying an analog LC filter

on the single-ended side has the advantage of preserving the

balance of the transformer.

JTX-2-10T+

MINI-CIRCUITS

50Ω

IOUTP

IOUTN

VDDA

2:1

Figure 152. Recommended Transformer for Wideband Applications with

Upper Bandwidths of up to 2.2 GHz

Figure 153 shows an interface that can be considered when

interfacing the DAC output to a self-biased differential gain block.

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

dc bias path to AGND. Its value, along with the dc blocking

capacitors, determines the lower cut-off frequency of the

composite pass-band response. (The dc blocking capacitors

form a high-pass response with the input resistance of the RF

differential gain stage.)

100Ω

IOUTP

IOUTN

RF DIFF_

AMP

VDDA

Figure 153. Interfacing the DAC Output to Self-Biased,

Differential Gain Stage

Many RF differential amplifiers consist of two single-ended

amplifiers with matched gain, thus providing no common-mode

rejection while possibly degrading the balance, due to poor

matching characteristics. Also, depending on the component

tolerances, differential LC filters can further degrade the balance

in a differential signal path. In both cases, the use of a balun could

be advantageous in rejecting the common-mode distortion and

noise components from the RF DAC prior to filtering or further

amplification.

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