ADA4303-2
APPLICATIONS
The ADA4303-2 active splitter is primarily intended for use
in the downstream path of television set-top boxes (STBs) that
contain multiple tuners. It is typically located directly after the
diplexer in a CATV customer premise unit. The ADA4303-2
provides a single-ended input and two single-ended outputs
that allow the delivery of the RF signal to two different signal
paths. These paths can include, but are not limited to, a main
picture tuner, a picture-in-picture (PIP) tuner, an out-of-band
(OOB) tuner, a digital video recorder (DVR), and a cable
modem (CM).
EVALUATION BOARD
The ADA4303-2 evaluation board allows designers to assess the
performance of the part in their particular applications. The board
includes 75 Ω coaxial connectors and 75 Ω controlled-impedance
signal traces that carry the input and output signals. Power (5 V)
is applied to the red VCC loop connector, and ground is connected
to the black GND loop connector.
The board has two 249 Ω resistors between each output and
ground that set the gain of the overall circuit to 3 dB and
improve output-to-output isolation. A schematic of the
ADA4303-2 evaluation board is shown in Figure 17.
The ADA4303-2 exhibits composite second-order (CSO) and
composite triple beat (CTB) products that are −62 dBc and
−72 dBc, respectively. The use of the SiGe process also allows
the ADA4303-2 to achieve a noise figure (NF) of less than 4.5 dB.
RF LAYOUT CONSIDERATIONS
Appropriate impedance matching techniques are mandatory
when designing a circuit board for the ADA4303-2. Improper
characteristic impedances on traces can cause reflections that
can lead to poor linearity. The characteristic impedance of the
signal trace from each output should be 75 Ω.
CIRCUIT DESCRIPTION
The ADA4303-2 consists of a low noise buffer amplifier followed
by a resistive power divider. This arrangement provides 3 dB of
gain relative to the RF signal present at the input of the device.
The input and each output must be properly matched to a 75 Ω
environment for distortion and noise performance to match the
data sheet specifications. In addition, to achieve the specified gain,
a 1% 249 Ω resistor should be installed to ground on each output.
AC coupling capacitors of 0.01 μF are recommended for the
input and outputs.
POWER SUPPLY
The 5 V supply should be applied to each of the VCC pins and
RF choke via a low impedance power bus. The power bus should
be decoupled to ground using a 10 μF tantalum capacitor and
a 0.1 μF ceramic chip capacitor located close to the ADA4303-2.
In addition, the VCC pins should be decoupled to ground with
a 0.1 μF ceramic chip capacitor located as close to each of the
pins as possible.
A 1 μH RF choke (Coilcraft chip inductor 0805LS-102X) is
required to correctly bias internal nodes of the ADA4303-2.
It should be connected between the 5 V supply and IL (Pin 11).
VCC
GND
C1
0.1µF
+
C5
10µF
L1
1.0μH
C6
0.1µF
C3
0.01μF
12
11
10
J2
J3
249ꢀ
249ꢀ
1
2
3
9
8
VCC
C2
0.01μF
VO1
VO2
J1
ADA4303-2
VIN
C4
0.01μF
7
GND
GND
4
5
6
NC = NO CONNECT
Figure 17. ADA4303-2 Evaluation Board Schematic
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