MAX3507 Datasheet, PDF (10/12 Pages) Maxim Integrated Products – Upstream CATV Amplifier with On-Chip Anti-Alias Filter

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On-Chip Anti-Alias Filter
drop. In general, this will not be a problem, as modern
RF transformers have adequate bandwidth.
Input Circuit
To achieve rated performance, the inputs of the
MAX3507 must be driven differentially with an appropri-
ate input level. The differential input impedance is
approximately 4kΩ. The MAX3507 has sufficient gain to
produce an output level of +64dBmV (QPSK through a
2:1 transformer) when driven with a +34dBmV input sig-
nal. Rated performance is achieved with this input level.
When a lower input level is present, the maximum out-
put level will be reduced proportionally and output lin-
earity will increase. If an input level greater than
+34dBmV is used, the 3rd-order distortion performance
will degrade.
If a single-ended source drives the MAX3507, one of
the input terminals must be capacitively coupled to
ground (IN+ or IN-). The value of this capacitor must be
large enough to look like a short circuit at the lowest fre-
quency of interest. For operation at 5MHz with a 75Ω
source impedance, a value of 0.001µF will suffice.
Layout Issues
A well-designed PC board is an essential part of an RF
circuit. For best performance pay attention to power-
supply layout issues as well as the output circuit layout.
Power-Supply Layout
For minimal coupling between different sections of the
MAX3507, the ideal power-supply layout is a star con-
figuration. This configuration has a large-value decou-
TOP VIEW
Pin Configuration
pling capacitor at the central power-supply node. The
power-supply traces branch out from this node, each
going to a separate power-supply node in the circuit. At
the end of each of these traces is a decoupling capaci-
tor that provides very low impedance at the frequency
of interest. This arrangement provides local power-sup-
ply decoupling at each power-supply pin. The power-
supply traces must be made as thick as practical.
Ground inductance degrades distortion performance.
Therefore, ground plane connections to pins 4, 8, 12, 23,
and 26 should be made with multiple vias if necessary.
Output Circuit Layout
The differential implementation of the MAX3507’s output
has the benefit of significantly reducing even-order dis-
tortion, the most significant of which is 2nd-harmonic
distortion. The degree of distortion cancellation
depends on the amplitude and phase balance of the
overall circuit. It is important to keep the trace lengths
from the output pins equal.
Chip Information
TRANSISTOR COUNT: 1457
BIASF 1
DC+ 2
DC- 3
GND 4
V_CC 5
IN+ 6
IN- 7
MAX3507
21 TXEN
20 N.C.
19 OUT+
18 OUT-
17 N.C.
16 CEXT
15 N.C.
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