Circuit Note

CN-0205

Circuits from the Lab™ reference circuits are engineered and

testedfor quickandeasysystemintegrationtohelpsolvetoday’s

analog, mixed-signal, and RF design challenges. For more

informationand/orsupport, visitwww.analog.com/CN0205.

Devices Connected/Referenced

AD9122

Dual Channel, 1.2 GSPS, 16-Bit, TxDAC®

Digital-to Analog Converter

ADL5375

Broadband Quadrature Modulator

Interfacing the ADL5375 I/Q Modulator to the

AD9122 Dual Channel, 1.2 GSPS High Speed DAC

Rev. 0

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EVALUATION AND DESIGN SUPPORT

Circuit Evaluation Boards

AD9122/ADL5375 Evaluation Board (AD9122-M5375-EBZ)

Design and Integration Files

Schematics, Layout Files, Bill of Materials

CIRCUIT FUNCTION AND BENEFITS

This circuit provides a simple and flexible interface between

the AD9122 dual high speed TxDAC digital-to-analog

converter and the ADL5375-05 broadband I/Q modulator.

Because the DAC outputs and ADL5375-05 I/Q modulator

inputs share a common bias level of 0.5 V, there is no need

for any active or passive level shifting circuitry. The dc coupled

interface facilitates I/Q modulator local oscillator (LO) leakage

compensation by the DAC.

The 1.2 GSPS AD9122 DAC sampling rate and the wide

bandwidth of the ADL5375-05 modulator I and Q inputs

ensure that both zero-IF (ZIF) or complex-IF (CIF)

architectures can be supported. In addition to filtering Nyquist

images, the baseband filter provides excellent rejection of both

differential-mode and common-mode DAC spurs.

CIRCUIT DESCRIPTION

The circuit and board shown in Figure 1 and Figure2 utilize the

AD9122 TxDAC and the ADL5375-05 wideband transmit

modulator. Signal biasing and scaling in the interface circuit is

controlled by the four ground-referenced resistors (RBIP, RBIN,

RBQP, RBQN) and the two shunt resistors (RSLI,RSLQ),

respectively.

RBIP

50Ω

RBIN

50Ω

67

66

IBBN

IBBP

AD9122

ADL5375-05

RBQN

50Ω

RBQP

50Ω

59

58

21

22

9

10

RSLI

100Ω

RSLQ

100Ω

IOUT1N

IOUT1P

IOUT2P

IOUT2N

QBBP

QBBN

LOW-PASS

FILTER

LOW-PASS

FILTER

Figure 1. Interface Between the AD9122 and ADL5375-05 with 50 Ω Resistors to Ground to Establish the

500 mV DC Bias for the ADL5375-05 Baseband Inputs (Simplified Schematic)