CN-0018 Datasheet, PDF (2/2 Pages) Analog Devices – Interfacing the ADL5372 I/Q Modulator to the AD9779A Dual-Channel, 1 GSPS High Speed DAC

CN-0018
Circuit Note
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.
AD9779A
OUT1_P
OUT1_N
93
RBIP
50Ω
RBIN
92 50Ω
1.1nF
C1I
LPI
2.7nH
1.1nF
C2I
LNI
2.7nH
RSLI
100Ω
ADL5372
19
IBBP
20
IBBN
OUT2_N
OUT2_P
84
RBQN
50Ω
RBQP
83 ^50Ω
1.1nF
C1Q
LNQ
2.7nH
1.1nF
C2Q
RSLQ
100Ω
LPQ
2.7nH
23
QBBN
24
QBBP
Figure 3. DAC Modulator Interface with 3 MHz Third-Order, Low-Pass Filter
(Calculated Component Values)
All the power supply pins of the ADL5372 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.
ADL5372 Data Sheet.
REVISION HISTORY
5/09—Rev. 0 to Rev. A
Updated Format..................................................................Universal
10/08—Revision 0: Initial Version
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