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参数资料
型号： AD9878
厂商： Analog Devices, Inc.
元件分类： 开关
英文描述： Mixed-Signal Front End for Broadband Applications
中文描述： 混合信号的宽带应用前端
文件页数： 23/36页
文件大小： 749K
代理商： AD9878
AD9878
Tx THROUGHPUT AND LATENCY
Data inputs affect the output fairly quickly, but remain effective
due to the AD9878 filter characteristics. Data transmit latency
through the AD9878 is easiest to describe in terms of f
SYSCLK
clock
cycles (4 × f
MCLK
). The numbers provided indicate the number of
f
SYSCLK
cycles before the AD9878 output responds to a change in
the input.
Rev. A | Page 23 of 36
Latency of I/Q data from the time it enters the data assembler
(AD9878 input) to the time of DAC output is 119 f
SYSCLK
clock
cycles (29.75 f
MCLK
cycles). DC values applied to the data assembler
input take up to 176 f
SYSCLK
clock cycles (44 f
MCLK
cycles) to
propagate and settle at the DAC output.
Frequency hopping is accomplished via changing the PROFILE
input pin. The time required to switch from one frequency to
another is less than 232 f
SYSCLK
cycles (58.5 f
MCLK
cycles).
DAC
A 12-bit digital-to-analog converter (DAC) is used to convert the
digitally processed waveform into an analog signal. The worst-
case spurious signals due to the DAC are the harmonics of the
fundamental signal and their aliases (see the Analog Devices
www.analog.com/dds
). The conversion process
(
3
=
±
×
n
f
f
n
CARRIER
SYSCLK
with an external RLC filter at the DAC output. It is important
for this analog filter to have a sufficiently flat gain and linear
phase response across the bandwidth of interest to avoid
modulation impairments. A relatively inexpensive seventh-
order, elliptical, low-pass filter is sufficient to suppress the
aliased components for HFC network applications.
)
.
These are typically filtered
The AD9878 provides true and complement current outputs. The
full-scale output current is set by the R
SET
resistor at Pin 49 and
the DAC gain register. Assuming maximum DAC gain, the value
of R
SET
for a full-scale I
OUT
is determined using the equation:
OUT
OUT
DACRSET
SET
I
I
V
R
4
39
32
=
=
For example, if a full-scale output current of 20 mA is desired,
then R
SET
= (39.4/0.02), or approximately 2 k.
The following equation calculates the full-scale output current,
including the programmable DAC gain control:
(
)
20
5
5
10
4
39
GAIN
N
SET
OUT
R
I
+
×
=
where
N
GAIN
is the value of DAC fine gain control [3:0].
The full-scale output current range of the AD9878 is 4 to
20 mA. Full-scale output currents outside this range degrade
SFDR performance. SFDR is also slightly affected by output
matching—that is, the two outputs should be terminated equally
for best SFDR performance. The output load should be located
as close as possible to the AD9878 package to minimize stray
capacitance and inductance. The load can be a simple resistor to
ground, an op amp current-to-voltage converter, or a transformer-
coupled circuit. It is best not to directly drive a highly reactive
load, such as an LC filter. Driving an LC filter without a
transformer requires that the filter be doubly terminated for
best performance—that is, both the filter input and output should
be resistively terminated with the appropriate values. The parallel
combination of the two terminations determines the load that
the AD9878 sees for signals within the filter pass band. For
example, a 50 terminated input/output low-pass filter looks
like a 25 load to the AD9878. The output compliance voltage
of the AD9878 is 0.5 V to +1.5 V. Any signal developed at the
DAC output should not exceed 1.5 V; otherwise, signal distortion
results. Furthermore, the signal can extend below ground as much
as 0.5 V without damage or signal distortion. The AD9878 true
and complement outputs can be differentially combined for
common-mode rejection using a broadband 1:1 transformer.
Using a grounded center tap results in signals at the AD9878 DAC
output pins that are symmetrical about ground. As previously
mentioned, by differentially combining the two signals, the user
can provide some degree of common-mode signal rejection.
A differential combiner can consist of a transformer or an
op amp. The object is to combine or amplify the difference
between only two signals and to reject any common—usually
undesirable—characteristics, such as 60 Hz hum or clock
feedthrough, that is equally present on both signals.
AD832x
AD9878
VARIABLE GAIN
CABLE DRIVER
AMPLIFIER
3
Tx
CA
DAC
75
LOW-PASS
FILTER
CA_EN
CA_DATA
CA_CLK
0
Figure 27. Cable Amplifier Connection
Connecting the AD9878 true and complement outputs to the
differential inputs of the programmable gain cable drivers
AD8321/AD8323 or AD8322/AD8327 (see Figure 27)
provides an optimized solution for the standard compliant
cable modem upstream channel. The cable driver’s gain
can be programmed through a direct 3-wire interface
using the AD9878 profile registers.
PROGRAMMING THE AD8321/AD8323 OR
AD8322/AD8327/AD8238 CABLE-DRIVER
AMPLIFIERS
Users can program the gain of the AD832x family of cable-driver
amplifiers via the AD9878 cable amplifier control interface. Two
(one per profile) 8-bit registers within the AD9878 store the gain
value to be written to the serial 3-wire port. Typically, either the
AD8321/AD8323 or AD8322/AD8327 variable gain cable
amplifiers are connected to the chip’s 3-wire cable amplifier
相关PDF资料
PDF描述
AD9878-EB Mixed-Signal Front End for Broadband Applications
AD9878BST Mixed-Signal Front End for Broadband Applications
AD9878BSTZ Mixed-Signal Front End for Broadband Applications
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相关代理商/技术参数
参数描述
AD9878BST 制造商:Analog Devices 功能描述:Mixed Signal Front End 100-Pin LQFP
AD9878BSTRL 制造商:Analog Devices 功能描述:Mixed Signal Front End 100-Pin LQFP T/R 制造商:Analog Devices 功能描述:MIXED SGNL FRONT END 100LQFP - Tape and Reel
AD9878BSTZ 功能描述:IC FRONT-END MIXED-SGNL 100-LQFP RoHS:是 类别:集成电路 (IC) >> 数据采集 - 模拟前端 (AFE) 系列:- 产品培训模块:Lead (SnPb) Finish for COTS Obsolescence Mitigation Program 标准包装:2,500 系列:- 位数:- 通道数:2 功率（瓦特）:- 电压 - 电源，模拟:3 V ~ 3.6 V 电压 - 电源，数字:3 V ~ 3.6 V 封装/外壳:32-VFQFN 裸露焊盘 供应商设备封装:32-QFN（5x5） 包装:带卷 (TR)
AD9878-EB 制造商:Analog Devices 功能描述:EVAL KIT FOR MIXED-SGNL FRONT END FOR BROADBAND APPLICATIONS - Bulk
AD9879 制造商:AD 制造商全称:Analog Devices 功能描述:Mixed-Signal Front End Set-Top Box, Cable Modem
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