AD8325-EVAL Analog Devices, AD8325-EVAL Datasheet - Page 8

• Manufacturer Part Number: AD8325-EVAL
• Description: 5 V CATV Line Driver Fine Step Output Power Control
• Manufacturer: Analog Devices

AD8325
Output Bias, Impedance, and Termination
The differential output pins V
dc level of approximately V
ac-coupled before being applied to the load. This is accomplished
with a 1:1 transformer as seen in the typical applications circuit
of Figure 6. The transformer also converts the output signal
from differential to single-ended, while maintaining a proper
impedance match to the line. The differential output impedance
of the AD8325 is internally maintained at 75 Ω, regardless of
whether the amplifier is in transmit enable mode (TXEN = 1)
or transmit disable mode (TXEN = 0). If the output signal is
being evaluated on standard 50 Ω test equipment, a 75 Ω to 50 Ω
pad must be used to provide the test circuit with the correct
impedance match.
Power Supply Decoupling, Grounding, and Layout
Considerations
Careful attention to printed circuit board layout details will
prevent problems due to associated board parasitics. Proper RF
design techniques are mandatory. The 5 V supply power should be
delivered to each of the V
to ensure that each pin is at the same potential. The power bus
should be decoupled to ground with a 10 µF tantalum capacitor
located in close proximity to the AD8325. In addition to the
10 µF capacitor, each V
ground with a 0.1 µF ceramic chip capacitor located as close to
the pin as possible. The pin labeled BYP (Pin 21) should also be
decoupled with a 0.1 µF capacitor. The PCB should have a low-
impedance ground plane covering all unused portions of the
component side of the board, except in the area of the input and
output traces (see Figure 10). It is important that all of the
AD8325’s ground pins are connected to the ground plane to
ensure proper grounding of all internal nodes. The differential
SLEEP
TXEN
5V
DATEN
SDATA
CLK
CC
CC
pin should be individually decoupled to
CC
pins via a low impedance power bus
OUT+
/2. Therefore, the outputs should be
10 F
25V
and V
0.1 F
0.1 F
0.1 F
OUT–
are also biased to a
DATEN
SDATA
CLK
GND1
V
TXEN
SLEEP
GND2
V
V
GND3
GND4
GND5
OUT–
CC
CC 1
CC2
AD8325 TSSOP
TOKO 617DB-A0070
GND11
GND10
GND9
GND8
GND7
GND6
OUT+
V
V
V
V
BYP
V
V
CC6
CC5
CC4
CC3
TO DIPLEXER Z
IN–
IN+
input and output traces should be kept as short and symmetrical
as possible. In addition, the input and output traces should be
kept far apart in order to minimize coupling (crosstalk) through
the board. Following these guidelines will improve the overall
performance of the AD8325 in all applications.
Initial Power-Up
When the 5 V supply is first applied to the V
AD8325, the gain setting of the amplifier is indeterminate.
Therefore, as power is first applied to the amplifier, the TXEN
pin should be held low (Logic 0) thus preventing forward signal
transmission. After power has been applied to the amplifier, the
gain can be set to the desired level by following the procedure in
the SPI Programming and Gain Adjustment section. The TXEN
pin can then be brought from Logic 0 to 1, enabling forward
signal transmission at the desired gain level.
Between Burst Operation
The asynchronous TXEN pin is used to place the AD8325 into
“Between Burst” mode while maintaining a differential output
impedance of 75 Ω. Applying a Logic 0 to the TXEN pin acti-
vates the on-chip reverse amplifier, providing a 74% reduction
in consumed power. The supply current is reduced from approxi-
mately 133 mA to approximately 35 mA. In this mode of
operation, between burst noise is minimized and the amplifier
can no longer transmit in the upstream direction. In addition to
the TXEN pin, the AD8325 also incorporates an asynchronous
SLEEP pin, which may be used to place the amplifier in a high
output impedance state and further reduce the supply current to
approximately 4 mA. Applying a Logic 0 to the SLEEP pin
places the amplifier into SLEEP mode. Transitioning into or
out of SLEEP mode will result in a transient voltage at the output
of the amplifier. Therefore, use only the TXEN pin for DOCSIS
compliant “Between Burst” operation.
0.1 F
IN
0.1 F
0.1 F
0.1 F
0.1 F
= 75
0.1 F
0.1 F
165
Z
IN
V
V
CC
= 150
IN–
IN+
pins of the