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參數(shù)資料
| 型號(hào)： | AD9877-EB |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Mixed-Signal Front End Set-Top Box, Cable Modem |
| 中文描述： | 混合信號(hào)前端機(jī)頂盒，電纜調(diào)制解調(diào)器 |
| 文件頁(yè)數(shù)： | 28/36頁(yè) |
| 文件大?。?/td> | 1094K |
| 代理商： | AD9877-EB |
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AD9877
The AD9877 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 particular full-scale I
OUT
is determined using
the following equation:
Rev. B | Page 28 of 36
R
SET
= 32
V
DACRSET
/
I
OUT
= 39.4/
I
OUT
For example, if a full-scale output current of 20 mA is desired,
then R
SET
= (39.4/0.02) Ω or approximately 2 kΩ.
The following equation calculates the full-scale output current,
including the programmable DAC gain control.
I
OUT
= [39.4/
R
SET
] × 10
(7.5 + 0.5
NGAIN
)/20
where
N
GAIN
is the value of DAC fine gain control [3:0].
The full-scale output current range of the AD9877 is 4 to
20 mA. Full-scale output currents outside of this range degrade
SFDR performance. SFDR is also slightly affected by output
matching; the two outputs should be terminated equally for best
SFDR performance. The output load should be located as close
as possible to the AD9877 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 attempt to directly
drive highly reactive loads (such as an LC filter). Driving an LC
filter without a transformer requires that the filter be doubly
terminated for best performance.
The filter input and output should both be resistively
terminated with the appropriate values. The parallel
combination of the two terminations will determine the load
that the AD9877 will see for signals within the filter pass band.
For example, a 50 Ω terminated input/output low-pass filter will
look like a 25 Ω load to the AD9877. The output compliance
voltage of the AD9877 is 0.5 V to +1.5 V. To avoid signal
distortion, any signal developed at the DAC output should not
exceed 1.5 V. Furthermore, the signal may extend below ground
as much as 0.5 V without damage or signal distortion.
The AD9877 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 AD9877 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 might consist of
a transformer or an operational amplifier. The object is to
combine or amplify only the difference between two signals and
to reject any common, usually undesirable characteristic, such
as 60 Hz hum or clock feedthrough that is equally present on
both individual signals.
Connecting the AD9877 true and complement outputs to the
differential inputs of the gain programmable cable drivers
AD8321/AD8323 or AD8322/AD8327 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 profile registers of the AD9877.
3
LOW-PASS
FILTER
Tx
AD832x
AD9877
CA
75
Ω
VARIABLE GAIN
CABLE DRIVER
AMPLIFIER
CA_EN
CA_DATA
CA_CLK
DAC
0
Figure 37. Cable Amplifier Connection
CA_EN
CA_CLK
CA_DATA
MSB
8
t
MCLK
8
t
MCLK
8
t
MCLK
4
t
MCLK
4
t
MCLK
LSB
0
Figure 38. Cable Amplifier Interface Timing
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