MT1530 UPSTREAM AMPLIFIER
CATV APPLICATIONS
ADVANCE DATA SHEET
DS-00010
Rev 1.2 — December 2000
Page 7 of 14
8.1.3 SHUTDOWN MODE
Shutdown mode minimizes current consumption while maintaining the programmed
gain state stored in the latch of the serial interface. All analog functions are disabled
in this mode and current consumption is reduced to 131 µA (typical). Shutdown
mode is enabled when either D7 = 0 or SHDN is set low. This mode overrides the
TXEN input pin, but TXEN should be held low to minimize glitching when exiting this
mode.
8.2 IC ARCHITECTURE
The MT1530’s signal path consists of an input amplifier, a variable attenuator, and a
power amplifier as shown in Figure 12. The input amplifier is a fixed-gain, low-noise,
differential amplifier with a nominal input impedance of 1.85 kΩ. The variable
attenuator is capable of a 51-dB attenuation range in 1-dB steps. The output power
amplifier has 12 dB of gain range in 6 dB steps.
8.2.1 INPUT AMPLIFIER
The input amplifier (IA) is a fully differential, low-noise amplifier with no gain
adjustment. Its bias may be disabled either by the external SHDN pin, or D7 of the
serial control register.
For best second-order performance, the IA should be driven differentially. However,
the device may be driven in a single-ended mode, with the other unused input
bypassed to ground with a 10 nF capacitor. The IA’s inputs are self-biasing and
must be AC coupled.
8.2.2 VARIABLE ATTENUATOR
The variable attenuator (VA) provides up to 51 dB attenuation of the input signal in
nominal 1-dB increments. The architecture is fully differential to minimize glitching
and even-order harmonics. This attenuator acts together with the output power
amplifier to achieve a nominal 63 dB of gain range, as described in section 8.2.3.
8.2.3 OUTPUT POWER AMPLIFIER
The output power amplifier (PA) is a fully differential amplifier capable of driving +64
dBmV into a 75Ω load. An off-chip transformer must be used to convert from a
differential to a single-ended output. The PA’s output impedance is nominally 300Ω,
which is transformed to 75Ω by the 4:1 impedance ratio transformer. This impedance
is maintained when the amplifier is powered down by setting TXEN = low. The
amplifier’s outputs are powered by +5V connected to the center tap of the
transformer’s secondary.
The differential nature of the PA provides superior second-order harmonic
performance. Additionally, any single-ended transients on the amplifier’s outputs that
are generated during transmit enable/disable switching are cancelled by this
architecture.
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