AD9763AST Analog Devices Inc, AD9763AST Datasheet - Page 32

• Manufacturer Part Number: AD9763AST
• Description: IC DAC 10BIT DUAL 125MSPS 48LQFP
• Manufacturer: Analog Devices Inc
• Series: TxDAC+®r

Specifications of AD9763AST

• Rohs Status: RoHS non-compliant
• Settling Time: 35ns
• Number Of Bits: 10
• Data Interface: Parallel
• Number Of Converters: 2
• Voltage Supply Source: Analog and Digital
• Power Dissipation (max): 450mW
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 48-LQFP
• For Use With: AD9763-EBZ - BOARD EVAL FOR AD9763

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 between the two
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. F | 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
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
to compensate for any mismatch in gain between the two
channels. The mismatch can 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
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
OR
RA
SET1
RA
10
10
0.1µF
Figure 83. Typical Analog QAM Architecture
and R
DAC
DAC
BBQP
0 TO I
BBIP
BBIN
BBQN
SET2
AD976x
FREQUENCY
OUTFS
, the effective load resistance of each
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
RA
FSEA30B
Σ
CAL
AD8346
can be used
V
MOD
TO
MIXER