AD9709 Analog Devices, AD9709 Datasheet - Page 23

• 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

I and Q digital data can be fed into the AD9709 in two ways. In
dual port mode, the digital I information drives one input port,
and the digital Q information drives the other input port. If no
interpolation filter precedes the DAC, the symbol rate is the rate
at which the system clock drives the CLK and WRT pins on the
AD9709. In interleaved mode, the digital input stream at Port 1
contains the I and the Q information in alternating digital words.
Using IQSEL and IQRESET, the AD9709 can be synchronized
to the I and Q data streams. The internal timing of the AD9709
routes the selected I and Q data to the correct DAC output. In
interleaved mode, if no interpolation filter precedes the AD9709,
the symbol rate is half that of the system clock driving the digital
data stream and the IQWRT and IQCLK pins on the AD9709.
CDMA
Code division multiple access (CDMA) is an air transmit/receive
scheme where the signal in the transmit path is modulated with a
pseudorandom digital code (sometimes referred to as the spreading
code). The effect of this is to spread the transmitted signal across
a wide spectrum. Similar to a discrete multitone (DMT) wave-
form, a CDMA waveform containing multiple subscribers can
be characterized as having a high peak to average ratio (that is,
crest factor), thus demanding highly linear components in the
transmit signal path. The bandwidth of the spectrum is defined
by the CDMA standard being used, and in operation it is
implemented by using a spreading code with particular
characteristics.
Rev. B | Page 23 of 32
Distortion in the transmit path can lead to power being transmitted
out of the defined band. The ratio of power transmitted in-band to
out-of-band is often referred to as adjacent channel power (ACP).
This is a regulatory issue due to the possibility of interference
with other signals being transmitted by air. Regulatory bodies
define a spectral mask outside of the transmit band, and the ACP
must fall under this mask. If distortion in the transmit path causes
the ACP to be above the spectral mask, filtering or different
component selection is needed to meet the mask requirements.
Figure 45 displays the results of using the application circuit shown
in Figure 44 to reconstruct a wideband CDMA (W-CDMA) test
vector using a bandwidth of 8 MHz that is centered at 2.4 GHz
and sampled at 65 MHz. The IF frequency at the DAC output is
15.625 MHz. The adjacent channel power ratio (ACPR) for the
given test vector is measured at greater than 54 dB.
–100
–110
–120
–130
–30
–40
–50
–60
–70
–80
–90
Figure 45. CDMA Signal, 8 MHz Chip Rate Sampled at 65 MSPS,
CENTER 2.4GHz
Recreated at 2.4 GHz, Adjacent Channel Power > 54 dB
==
c11
c11
C0
FREQUENCY
3MHz
cu1
C0
SPAN 30MHz
AD9709
cu1