Upstream CATV Amplifier
MAX3510
VOUT-
MAX3510
CEXT2
VOUT+
SHDN
PGA BIAS
CELL
V 1
CC
VIN+
VIN-
V 2
CC
POWER
AMP
BIAS
TXEN
D/A CONVERTER
RAMP
GENERATOR
SERIAL DATA INTERFACE
CS SDA SCLK
CEXT1
Figure 1. Functional Diagram
Power Amplifier
Detailed Description
The power amplifier is a Class A differential amplifier
capable of driving +64dBmV differentially. This archi-
tecture provides superior even-order distortion perfor-
mance but requires that a transformer be used to
convert to a single-ended output. In transmit-disable
mode, the power amplifier is shut off. An internal resis-
tor is placed across the output, so that the output
impedance remains matched when the amplifier is in
transmit-disable mode. Disabling the output devices
also allows the lowest standby noise.
The following sections describe the blocks shown in the
functional diagram (Figure 1).
Programmable-Gain Amplifier
The programmable-gain amplifier (PGA) consists of the
variable-gain amplifier (VGA) and the digital-to-analog
converter (DAC), which provide better than 52dB of
output level control in 1dB steps.
The PGA is implemented as a programmable Gilbert-
cell attenuator. It uses a differential architecture to
achieve maximum linearity. The gain of the PGA is
determined by a 6-bit word (D5–D0) programmed
through the serial data interface (Tables 2 and 3).
To achieve the proper load line, the output impedance
of the power amplifier is 300Ω differential. To match this
output impedance to a 75Ω load, the transformer must
have a turns ratio (voltage ratio) of 2:1 (4:1 impedance
ratio).
Specified performance is achieved when the input is
driven differentially. The device may be driven single-
ended; however, a slight increase in even-order distor-
tion may result at low output levels. To drive the device
in this manner, one of the input pins must be capaci-
tively coupled to ground. Use a capacitor value large
enough to allow for a low-impedance path to ground at
the lowest frequency of operation. For operation down
to 5MHz, a 0.001µF capacitor is suggested.
The differential amplifier is biased directly from the +5V
supply using the center tap of the output transformer.
This provides a significant benefit when switching
between transmit mode and transmit-disable mode.
Stored energy due to bias currents will cancel within
the transformer and prevent switching transients from
reaching the load.
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