CN-0016

Circuit Note

Rev. A | Page 2 of 2

It is generally necessary to low-pass filter the DAC outputs to

remove image frequencies when driving a modulator. The

above interface lends itself well to the introduction of such a

filter. The filter can be inserted between the dc bias setting

resistors and the ac swing-limiting resistor. Doing so establishes

the input and output impedances for the filter.

A simulated filter example is shown in Figure 3 with a third-order

elliptical filter with a 3 dB frequency of 3 MHz. Matching input

and output impedances makes the filter design easier, so the

shunt resistor chosen is 100 Ω, producing an ac swing of 1 V p-p

differential for a 0 mA to 20 mA DAC full-scale output current.

In a practical application, the use of standard value components,

along with the input impedance of the I/Q modulator (2900 kΩ

in parallel with a few picofarads of input capacitance), will slightly

change the frequency response of this circuit.

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

LPI

2.7nH

LNI

2.7nH

1.1nF

C1I

1.1nF

C2I

LNQ

2.7nH

LPQ

2.7nH

1.1nF

C1Q

1.1nF

C2Q

Figure 3. DAC Modulator Interface with 3 MHz Third-Order, Low-Pass Filter

(Calculated Component Values)

All the power supply pins of the ADL5370 must be connected to

the same 5 V source. Adjacent pins of the same name can be

tied together and decoupled to a large area ground plane with a

0.1 µF capacitor. These capacitors should be located as close as

possible to the device. The power supply can range between

4.75 V and 5.25 V.

The COM1 pin, COM2 pin, COM3 pin, and COM4 pin should

be tied to the same ground plane through low impedance paths.

The exposed paddle on the underside of the package should

also be soldered to a low thermal and electrical impedance

ground plane. If the ground plane spans multiple layers on the

circuit board, they should be stitched together with nine vias

under the exposed paddle. The AN-772 application note

discusses the thermal and electrical grounding of the

LFCSP_VQ in greater detail.

COMMON VARIATIONS

The interface described here can be used to interface

any TxDAC converter with ground referenced 0 mA to 20 mA

output currents to any I/Q modulator with a 0.5 V input bias

level. For zero-IF applications, the AD9783 dual DAC provides

an LVDS interface, while the CMOS-driven AD9788 dual DAC

can generate a fine resolution complex IF input to the I/Q

modulator. The ADL5370/ADL5371/ADL5372/ADL5373/

ADL5374 family of I/Q modulators provides narrow-band

operation with high output 1 dB compression point and OIP3,

whereas the ADL5375 provides broadband high performance

operation from 400 MHz to 6 GHz. The ADL5385 I/Q modula-

tor uses a 2 × LO and operates from 50 MHz to 2.2 GHz.

LEARN MORE

AN-772 Application Note, A Design and Manufacturing Guide

for the Lead Frame Chip Scale Package (LFCSP). Analog

Devices.

MT-016 Tutorial, Basic DAC Architectures III: Segmented DACs.

Analog Devices.

MT-017 Tutorial, Oversampling Interpolating DACs. Analog

Devices.

MT-031 Tutorial, Grounding Data Converters and Solving the

Mystery of 'AGND' and 'DGND'. Analog Devices.

MT-080 Tutorial, Mixers and Modulators. Analog Devices.

MT-101 Tutorial, Decoupling Techniques. Analog Devices.

Zumbahlen, Hank. 2006. Basic Linear Design. Analog Devices.

ISBN 0915550281. Chapters 4 and 11. Also available as

Linear Circuit Design Handbook. Elsevier-Newnes, 2008,

ISBN 0750687037, Chapters 4 and 11.

Data Sheets

AD9779A Data Sheet.

ADL5370 Data Sheet.

REVISION HISTORY

5/09—Rev. 0 to Rev. A

Updated Format..................................................................Universal

10/08—Revision 0: Initial Version

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