AD9709 Analog Devices, AD9709 Datasheet - Page 22

• Manufacturer Part Number: AD9709
• Description: 8-Bit, 125 MSPS Dual TxDAC+ Digital-to-Analog Converter
• Manufacturer: Analog Devices

Specifications of AD9709

• Resolution (bits): 8bit
• Dac Update Rate: 125MSPS
• Dac Settling Time: 35ns
• Max Pos Supply (v): +5.5V
• Single-supply: Yes
• Dac Type: Current Out
• Dac Input Format: Par

AD9709
APPLICATIONS INFORMATION
QUADRATURE AMPLITUDE MODULATION (QAM)
USING THE AD9709
QAM is one of the most widely used digital modulation
schemes in digital communications systems. This modulation
technique can be found in FDM as well as spread spectrum
(that is, CDMA) based systems. A QAM signal is a carrier
frequency that is modulated in both amplitude (that is, AM
modulation) and phase (that is, PM modulation). It can be
generated by independently modulating two carriers of identical
frequency but with a 90° phase difference. This results in an
in-phase (I) carrier component and a quadrature (Q) carrier
component at a 90° phase shift with respect to the I component.
The I and Q components are then summed to provide a QAM
signal at the specified carrier frequency.
A common and traditional implementation of a QAM
modulator is shown in Figure 43. The modulation is performed
in the analog domain in which two DACs are used to generate
the baseband I and Q components. Each component is then
typically applied to a Nyquist filter before being applied to a
quadrature mixer. The matching Nyquist filters shape and limit
each component’s spectral envelope while minimizing intersymbol
interference. The DAC is typically updated at the QAM symbol
rate, or at a multiple of the QAM symbol rate if an interpolating
filter precedes the DAC. The use of an interpolating filter typically
eases the implementation and complexity of the analog filter,
which can be a significant contributor to mismatches in gain
and phase between the two baseband channels. A quadrature
mixer modulates the I and Q components with the in-phase and
quadrature carrier frequencies and then sums the two outputs
to provide the QAM signal.
TEKTRONIX
AWG2021
OPTION 4
NOTES
1. DAC FULL-SCALE OUTPUT CURRENT = I
2. RA, RB, AND RL ARE THIN FILM RESISTOR NETWORKS
WITH
WITH 0.1% MATCHING, 1% ACCURACY AVAILABLE
FROM OHMTEK ORNXXXXD SERIES OR EQUIVALENT.
WRT1/IQWRT
WRT2/IQSEL
CLK1/IQCLK
SLEEP
DCOM1/
DCOM2
22nF
256Ω
MODE
DVDD1/
DVDD2
2kΩ
20kΩ
LATCH
LATCH
Q DAC
I DAC
FSADJ1
Figure 44. Baseband QAM Implementation Using an AD9709 and AD8346
AD9709
OUTFS
22nF
256Ω
ACOM
.
FSADJ2
AVDD
DAC
DAC
Q
I
2kΩ
20kΩ
REFIO
I
I
I
I
OUT
OUT
OUT
OUT
A
B
A
B
0.1µF
Rev. B | Page 22 of 32
CA
CA
RL
RL
RL
RL
DIFFERENTIAL
RLC FILTER
RL = 200Ω
RA = 2500Ω
RB = 500Ω
RP = 200Ω
CA = 280pF
CB = 45pF
LA = 10µH
I
AVDD = 5.0V
VCM = 1.2V
OUTFS
LA
LA
LA
LA
= 11mA
RL
RL
RL
CB
CB
RL
RB
RB
RB
RB
In this implementation, it is much more difficult to maintain
proper gain and phase matching between the I and Q channels.
The circuit implementation shown in Figure 44 helps improve
the matching between the I and Q channels, and it shows a path
for upconversion using the AD8346 quadrature modulator. The
AD9709 provides both I and Q DACs with a common reference
that will improve the gain matching and stability. R
used to compensate for any mismatch in gain between the two
channels. The mismatch may be attributed to the mismatch
between R
channel, and/or the voltage offset of the control amplifier in each
DAC. The differential voltage outputs of both DACs in the
AD9709 are fed into the respective differential inputs of the
AD8346 via matching networks.
RA
VDIFF = 1.82V p-p
C
FILTER
AVDD
RA
ASIC
DSP
OR
RA
SET1
RA
8
8
0.1µF
Figure 43. Typical Analog QAM Architecture
and R
DAC
DAC
BBQP
0 TO I
BBIP
BBIN
BBQN
AD976x
SET2
FREQUENCY
OUTFS
, the effective load resistance of each
CARRIER
VPBF
NYQUIST
SPLITTER
FILTERS
AD8346
PHASE
V
DAC
+
RL
VOUT
LOIP
LOIN
RB
QUADRATURE
MODULATOR
SPECTRUM ANALYZER
0°
SIGNAL GENERATOR
ROHDE & SCHWARZ
AVDD
ROHDE & SCHWARZ
90°
OR EQUIVALENT
RA
FSEA30B
Σ
AD8346
V
CAL
MOD
TO
MIXER
can be