Circuit Note

CN-0021

Circuit Designs Using Analog Devices Products

Apply these product pairings quickly and with confidence.

For more information and/or support call 1-800-AnalogD

(1-800-262-5643) or visit www.analog.com/circuit.

Devices Connected/Referenced

AD9779A

Dual 16-Bit, 1 GSPS DAC

ADL5375-05

400 MHz to 6000 MHz I/Q Modulator

Interfacing the ADL5375 I/Q Modulator to the AD9779A

Dual-Channel, 1 GSPS High Speed DAC

Rev. A

“Circuits from the Lab” from Analog Devices have been designed and built by Analog Devices

engineers. Standard engineering practices have been employed in the design and construction of

eachcircuit,andtheirfunctionandperformance havebeentestedandverifiedinalabenvironment

atroomtemperature.However,youaresolelyresponsible fortestingthe circuit anddetermining its

suitability and applicability for your use and application. Accordingly, in no event shall Analog

Devices be liable for direct, indirect, special, incidental, consequential or punitive damages due to

anycausewhatsoeverconnectedtotheuseofany“CircuitfromtheLab”. (Continuedonlastpage)

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.

Tel: 781.329.4700

www.analog.com

Fax: 781.461.3113

©2008–2009 Analog Devices, Inc. All rights reserved.

CIRCUIT FUNCTION AND BENEFITS

This circuit provides a simple, elegant interface between the

ADL5375 I/Q modulator and the AD9779A high speed DAC.

The ADL5375 and the AD9779A are well-matched devices

because they have the same bias levels and similarly high

signal-to-noise ratios (SNR). The matched bias levels of 500 mV

allow for a “glueless” interface—there is no requirement for a

level shifting network that would add noise and insertion loss

along with extra components. The addition of the swing-

limiting resistors (RSLI, RSLQ) allows the DAC swing to be

scaled appropriately without loss of resolution or of the 0.5 V

bias level. The high SNR of each device preserves a high SNR

through the circuit.

CIRCUIT DESCRIPTION

The ADL5375 is designed to interface with minimal

components to members of Analog Devices family of

TxDAC® converters (AD97xx). The baseband inputs of the

ADL5375 require a dc common-mode bias voltage of 500 mV.

With each AD9779A output swinging from 0 mA to 20 mA, a

single 50 Ω resistor to ground from each of the DAC outputs

provides the desired 500 mV dc bias. With just the four 50 Ω

resistors in place, the voltage swing on each pin is 1 V p-p.

This results in a differential voltage swing of 2 V p-p on each

input pair.

By adding resistors RSLI and RSLQ to the interface, the output

swing of the DAC can be reduced without any loss of DAC

resolution. The resistor is placed as a shunt between each side of

RBIP

50

Ω

RBIN

50

Ω

93

92

OUT1_N

OUT1_P

IBBN

IBBP

AD9779A

ADL5375-05

RBQN

50

Ω

RBQP

50

Ω

84

83

OUT2_P

OUT2_N

QBBP

QBBN

21

22

9

10

RSLI

100

Ω

RSLQ

100

Ω

Figure 1. Interface Between the AD9779A and ADL5375 with 50 Ω Resistors to Ground to Establish the

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