AD9767 Analog Devices, AD9767 Datasheet - Page 32

• Manufacturer Part Number: AD9767
• Manufacturer: Analog Devices

Specifications of AD9767

• Resolution (bits): 14bit
• 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

AD9763/AD9765/AD9767
QUADRATURE AMPLITUDE MODULATION (QAM)
EXAMPLE USING THE AD9763
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 83. 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
NOTES
1. DAC FULL-SCALE OUTPUT CURRENT = I
2. RA, RB, AND RL ARE THIN FILM RESISTOR NETWORKS
TEKTRONIX
AWG2021
OPTION 4
WITH 0.1% MATCHING, 1% ACCURACY AVAILABLE
FROM OHMTEK ORNXXXXD SERIES OR EQUIVALENT.
WITH
WRT1/IQWRT
WRT2/IQSEL
CLK1/IQCLK
SLEEP
DCOM1/
DCOM2
22nF
256Ω
MODE
DVDD1/
DVDD2
2kΩ
20kΩ
LATCH
LATCH
Q DAC
I DAC
FSADJ1
Figure 84. Baseband QAM Implementation Using an AD9763 and an AD8346
OUTFS
AD9763/
AD9765/
AD9767
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. G | Page 32 of 44
CA
CA
DIFFERENTIAL
RLC FILTER
RL = 200Ω
RA = 2500Ω
RB = 500Ω
RP = 200Ω
CA = 280pF
CB = 45pF
LA = 10µH
I
AVDD = 5.0V
VCM = 1.2V
RL
RL
RL
RL
OUTFS
LA
LA
LA
LA
= 11mA
RL
RL
RL
CB
CB
RL
between the two baseband channels. A quadrature mixer
modulates the I and Q components with the in-phase and
quadrature carrier frequency and then sums the two outputs
to provide the QAM signal.
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 84 helps improve the
matching between the I and Q channels, and it shows a path for
upconversion using the AD8346 quadrature modulator. The
AD9763 provides both I and Q DACs a common reference that
improves the gain matching and stability. R
compensate for any mismatch in gain between the two channels.
The mismatch can be attributed to the mismatch between R
and R
the voltage offset of the control amplifier in each DAC. The
differential voltage outputs of both DACs in the AD9763 are
fed into the respective differential inputs of the AD8346 via
matching networks.
RB
RB
RB
RB
RA
VDIFF = 1.82V p-p
C
FILTER
AVDD
RA
ASIC
DSP
SET2
OR
RA
, the effective load resistance of each channel, and/or
RA
10
10
0.1µF
Figure 83. Typical Analog QAM Architecture
DAC
DAC
BBQP
0 TO I
BBIP
BBIN
BBQN
AD976x
FREQUENCY
OUTFS
CARRIER
VPBF
NYQUIST
FILTERS
SPLITTER
AD8346
PHASE
V
DAC
+
RL
VOUT
LOIP
LOIN
RB
QUADRATURE
MODULATOR
0°
SPECTRUM ANALYZER
SIGNAL GENERATOR
ROHDE & SCHWARZ
AVDD
ROHDE & SCHWARZ
90°
OR EQUIVALENT
CAL
RA
FSEA30B
Data Sheet
can be used to
Σ
AD8346
V
MOD
TO
MIXER
SET1