Circuit Note

CN-0016

Circuit Designs Using Analog Devices Products

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Devices Connected/Referenced

AD9779A

Dual 16-Bit, 1 GSPS DAC

ADL5370

300 MHz to 1000 MHz I/Q Modulator

Interfacing the ADL5370 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

ADL5370 I/Q modulator and the AD9779A high speed DAC.

The ADL5370 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 ADL5370 is designed to interface with minimal

components to members of Analog Devices family of

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

ADL5370 require a dc common-mode bias voltage of 500 mV.

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

RBIP

50

Ω

RBIN

50

Ω

93

92

19

20

IBBN

IBBP

AD9779A

ADL5370

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 ADL5370 with 50 Ω Resistors

to Ground to Establish the 500 mV DC Bias for the ADL5370 Baseband Inputs

(Simplified Schematic)

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

the differential pair, as shown in Figure 1. It has the effect of

reducing the ac swing without changing the dc bias already

established by the 50 Ω resistors and the DAC output current.

The value of this ac swing-limiting resistor is chosen based on

the desired ac voltage swing. Figure 2 shows the relationship

between the swing-limiting resistor and the peak-to-peak ac

swing that it produces when 50 Ω bias-setting resistors are used.

Note that all Analog Devices I/Q modulators present a relatively

high input impedance on their baseband inputs (typically >1 kΩ).

As a result, the input impedance of the I/Q modulator will have

no effect on the scaling of the DAC output signal.

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

10

100

1k

10k

Figure 2. Relationship Between the AC Swing-Limiting Resistor and the

Peak-to-Peak Voltage Swing with 50 Ω Bias-Setting Resistors