Circuit Note

CN-0019

Circuit Designs Using Analog Devices Products

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Devices Connected/Referenced

AD9779A

Dual 16-Bit, 1 GSPS DAC

ADL5373

2300 MHz to 3000 MHz I/Q Modulator

Interfacing the ADL5373 I/Q Modulator to the AD9779A

Dual-Channel, 1 GSPS High Speed DAC

Rev. A

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CIRCUIT FUNCTION AND BENEFITS

This circuit provides a simple, elegant interface between the

ADL5373 I/Q modulator and the AD9779A high speed DAC.

The ADL5373 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 ADL5373 is designed to interface with minimal

components to members of Analog Devices family of

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

ADL5373 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

19

20

IBBN

IBBP

AD9779A

ADL5373

RBQN

50

Ω

RBQP

50

Ω

84

83

23

24

RSLI

100

Ω

RSLQ

100

Ω

OUT1_N

OUT1_P

OUT2_P

OUT2_N

QBBP

QBBN

Figure 1. Interface Between the AD9779A and ADL5373 with 50 Ω Resistors to Ground to Establish the

500 mV DC Bias for the ADL5373 Baseband Inputs (Simplified Schematic)