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參數(shù)資料
| 型號： | AD8320-EB |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Serial Digital Controlled Variable Gain Line Driver |
| 中文描述： | 串行數(shù)字控制可變增益線路驅(qū)動器 |
| 文件頁數(shù)： | 10/20頁 |
| 文件大小： | 583K |
| 代理商： | AD8320-EB |
AD8320
–10–
REV. 0
T
ES
VALID DATA WORD G1
MSB. . . .LSB
GAIN TRANSFER (G1)
T
DS
T
EH
8 CLOCK CYCLES
GAIN TRANSFER (G2)
T
OFF
T
GS
ANALOG
OUTPUT
SIGNAL AMPLITUDE (p-p)
PD
PEDESTAL
CLK
SDATA
DATEN
T
ON
T
C
T
WH
VALID DATA WORD G2
Figure 43. Serial Interface Timing
what is referred to as sleep and standby modes, and VCC supply
switching via PFET S or equivalent, as described in the applica-
tions section, would be required.
APPLIC AT IONS
T he AD8320 is primarily intended to be used as the return path
(also called upstream path) line driver in cable modem and
cable telephony applications. Data to be transmitted is modu-
lated in either QPSK or QAM format. T his is done either in
DSP or by a dedicated QPSK /QAM modulator such as the
AD9853.
T he amplifier receives its input signal either from the dedicated
QPSK /QAM modulator or from a DAC. In both cases, the
signal must be low-pass filtered before being applied to the line
driving amplifier.
DIPLEXER
75
V
TO MODEM
RECEIVE
CIRCUITRY
CENTRAL
OFFICE
SUBSCRIBER
7TH ORDER
ELLIPTIC
LOW PASS
FILTER
AD8320
AD9853
Figure 44. Block Diagram of Cable Modem’s Upstream
Driver Section
T he amplifier drives the line through a diplexer. T he insertion
loss of a diplexer is typically –3 dB. As a result, the line driver
must deliver a power level roughly 3 dB greater than required by
the applicable cable modem standard so that diplexer losses are
canceled out.
Because the distance to the central office varies from subscriber
to subscriber, signals from different subscribers will be attenu-
ated by differing amounts. As a result, the line driver is required
to vary its gain so that all signals arriving at the central office
have the same amplitude.
T he power amplifier has two basic modes of operation; forward
or power-up mode and reverse or power-down mode. In the
power-up mode (
PD
= 1), the power amplifier stage is enabled
and the differential output core signal is amplified by 20 dB.
With a core attenuation range of 0 dB to –36 dB and 6 dB of
input gain, the overall AD8320 gain range is 26 dB to –10 dB.
In this mode, the single-ended output signal maintains a dc
level of VCC/2. T his dc output level provides for optimum large
signal linearity and allows for dc coupling the output if neces-
sary. T he output stage is unique in that it maintains a dynamic
output impedance of 75
. T his allows for a direct 75
cable
connection and results in 6 dB of added load power versus using
a series 75
back-termination resistor as required with tradi-
tional low output impedance amplifiers. T he power amplifier
will also drive lower or higher output loads, although the device’s
gain (not gain range) will change accordingly (see Applications
section).
In the power-down mode (
PD
= 0), the power amplifier is turned
off and a “reverse” amplifier (the inner triangle in Figure 42) is
enabled. During this 1 to 0 transition, the output power amplifier’s
input stage is also disabled, resulting in no forward output signal
(S21 is 0), although the attenuator core and input amplifier/
buffer signals are not affected (S11
≈
0). T he function of the
reverse amplifier is to maintain 75
and VCC/2 at the output
port (VOUT ) during power-down. T his is required to minimize
line reflections (S22
≈
0) and ensures proper filter operation for
any forward mode device sharing the same bus (i.e., in a multi-
plexed configuration). (See Applications section.) In the time
domain, as
PD
switches states, a transitional glitch and pedestal
offset results. (See Figures 31 and 43.) T he powered down
supply current drops to 32 mA versus 97 mA (V
CC
= 12 V) in
power-up mode.
Generally, using the power-down low input (
PD
) for switching
allows for multiple devices to be multiplexed via splitters (N-1
off, 1 on) and reduces overall total power consumption as re-
quired for cable data applications. For cable telephony, the
power-down current generally needs to be much lower during
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