AD9752 datasheet(17/23 Pages) AD

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REV. 0

AD9752

–17–

AD9752

(“I DAC”)

AD9752

(“Q DAC”)

IOUTA

IOUTB

QOUTA

QOUTB

DCOM

FSADJ

REFIO

SLEEP

RSET2

1.9k

0.1 F

CLK

Q DATA

INPUT

I DATA

INPUT

DVDD

AVDD

100W

500

100

CFILTER

100

CFILTER

100

500

634

0.1 F

+5V

VPBF

BBIP

BBIN

BBQP

BBQN

AD8346

PHASE

SPLITTER

LOIP

LOIN

VOUT

500mV p-p WITH

VCM=1.2V

NOTE: 500

RESISTOR NETWORK - OHMTEK ORN5000D

100

RESISTOR NETWORK - TOMC1603-100D

REFLO

ACOM

REFLO

AVDD

REFIO

FSADJ

RSET1

RCAL

220

AVDD

1.82V

LATCHES

500

DAC

LATCHES

Figure 37. Baseband QAM Implementation Using Two AD9752s

AD9752

CARRIER

FREQUENCY

MIXER

DSP

ASIC

NYQUIST

FILTERS

QUADRATURE

MODULATOR

Figure 36. Typical Analog QAM Architecture

In this implementation, it is much more difficult to maintain

proper gain and phase matching between the I and Q channels.

The circuit implementation shown in Figure 37 helps improve

upon the matching and temperature stability characteristics

between the I and Q channels, as well as showing a path for up-

conversion using the AD8346 quadrature modulator. Using a

single voltage reference derived from U1 to set the gain for both

the I and Q channels will improve the gain matching and stabil-

ity. RCAL can be used to compensate for any mismatch in gain

between the two channels. This mismatch may be attributed to

the mismatch between RSET1 and RSET2, effective load resistance

of each channel, and/or the voltage offset of the control ampli-

fier in each DAC. The differential voltage outputs of U1 and U2

are fed into the respective differential inputs of the AD8346 via

matching networks.

Using the same matching techniques described above, Figure 38

shows an example of the AD9752 used in a W-CDMA transmit-

ter application using the AD6122 CDMA 3 V transmitter IF

subsystem. The AD6122 has functions, such as external gain

control and low distortion characteristics, needed for the supe-

rior Adjacent Channel Power (ACP) requirements of W-CDMA.

CDMA

Carrier Division Multiple Access, or CDMA, is an air transmit/

receive scheme where the signal in the transmit path is modu-

lated with a pseudorandom digital code (sometimes referred to

as the spreading code). The effect of this is to spread the trans-

mitted signal across a wide spectrum. Similar to a DMT wave-

form, a CDMA waveform containing multiple subscribers can

be characterized as having a high peak to average ratio (i.e.,

crest factor), thus demanding highly linear components in the

transmit signal path. The bandwidth of the spectrum is defined

by the CDMA standard being used, and in operation is imple-

mented by using a spreading code with particular characteristics.

Distortion in the transmit path can lead to power being trans-

mitted out of the defined band. The ratio of power transmitted

in-band to out-of-band is often referred to as Adjacent Channel

Power (ACP). This is a regulatory issue due to the possibility of

interference with other signals being transmitted by air. Regula-

tory bodies define a spectral mask outside of the transmit band,

and the ACP must fall under this mask. If distortion in the

transmit path cause the ACP to be above the spectral mask,

then filtering, or different component selection is needed to

meet the mask requirements.

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