CN-0020 [ADI]

Interfacing the ADL5374 I/Q Modulator to the AD9779A Dual-Channel, 1 GSPS High Speed DAC; 接口的ADL5374 I / Q调制器的AD9779A双通道， 1 GSPS高速DAC


型号：	CN-0020
厂家：	ADI
描述：	Interfacing the ADL5374 I/Q Modulator to the AD9779A Dual-Channel, 1 GSPS High Speed DAC 接口的ADL5374 I / Q调制器的AD9779A双通道， 1 GSPS高速DAC
文件：	总3页 (文件大小：134K)
中文：	中文翻译
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Circuit Note

CN-0020

Devices Connected/Referenced

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.

AD9779A

ADL5374

Dual 16-Bit, 1 GSPS DAC

3000 MHz to 4000 MHz I/Q Modulator

Interfacing the ADL5374 I/Q Modulator to the AD9779A

Dual-Channel, 1 GSPS High Speed DAC

CIRCUIT DESCRIPTION

CIRCUIT FUNCTION AND BENEFITS

The ADL5374 is designed to interface with minimal

components to members of Analog Devices family of

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

ADL5374 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.

This circuit provides a simple, elegant interface between the

ADL5374 I/Q modulator and the AD9779A high speed DAC.

The ADL5374 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.

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

AD9779A

ADL5374

OUT1_P

IBBP

RBIP

50Ω

RSLI

100Ω

RBIN

50Ω

OUT1_N

IBBN

OUT2_N

OUT2_P

QBBN

RBQN

50Ω

RSLQ

100Ω

RBQP

50Ω

QBBP

Figure 1. Interface Between the AD9779A and ADL5374 with 50 Ω Resistors to Ground to Establish the

500 mV DC Bias for the ADL5374 Baseband Inputs (Simplified Schematic)

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

each circuit, and their function and performance have been tested and verified in a lab environment

at room temperature. However, you are solely responsible for testing the circuit and determining 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

anycause whatsoever connectedto the use ofany“Circuit fromthe Lab”. (Continued on last page)

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Tel: 781.329.4700 www.analog.com

CN-0020

Circuit Note

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

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

order elliptical filter with a 3 dB frequency of 10 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. The simulated frequency response of this

filter is shown in Figure 4. 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.

All the power supply pins of the ADL5374 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.

100

10k

(Ω)

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

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

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.

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.

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.

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/

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.

–10

–20

–30

–40

–50

–60

LPI

AD9779A

ADL5374

MAGNITUDE

771.1nH

OUT1_P

IBBP

RBIP

50Ω

RSLI

53.62nF

C1I

350.1pF

C2I

100Ω

RBIN

50Ω

GROUP DELAY

OUT1_N

OUT2_N

OUT2_P

IBBN

LNI

771.1nH

LNQ

771.1nH

QBBN

QBBP

RBQN

50Ω

53.62nF

C1Q

350.1pF

C2Q

RSLQ

100Ω

RBQP

50Ω

100

LPQ

771.1nH

FREQUENCY (MHz)

Figure 4. Simulated Frequency Response for DAC Modulator Interface with

10 MHz Third-Order Bessel Filter

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

(Calculated Component Values)

Rev. A | Page 2 of 3

Circuit Note

CN-0020

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.

ADL5374 Data Sheet.

REVISION HISTORY

5/09—Rev. 0 to Rev. A

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

10/08—Revision 0: Initial Version

(Continued from first page) "Circuits from the Lab" are intended only for use with Analog Devices products and are the intellectual property of Analog Devices or its licensors. While you may

use the "Circuits from the Lab" in the design of your product, no other license is granted by implication or otherwise under any patents or other intellectual property by application or use of

the "Circuits from the Lab". Information furnished by Analog Devices is believed to be accurate and reliable. However, "Circuits from the Lab" are supplied "as is" and without warranties of any

kind, express, implied, or statutory including, but not limited to, any implied warranty of merchantability, noninfringement or fitness for a particular purpose and no responsibility is assumed

by Analog Devices for their use, nor for any infringements of patents or other rights of third parties that may result from their use. Analog Devices reserves the right to change any "Circuits

from the Lab" at any time without notice, but is under no obligation to do so. Trademarks and registered trademarks are the property of their respective owners.

registered trademarks are the property of their respective owners.

CN08223-0-5/09(A)

Rev. A | Page 3 of 3

CN-0020 [ADI]

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