MAX3510 데이터 시트보기 (PDF) - Maxim Integrated

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MAX3510

Upstream CATV Amplifier

Maxim Integrated 

Upstream CATV Amplifier

VOUT-

SHDN

VCC1

VIN+

PGA BIAS

CELL

MAX3510

CEXT2

VOUT+

VIN-

Figure 1. Functional Diagram

DAC

SERIAL DATA INTERFACE

CS SDA SCLK

RAMP

GENERATOR

POWER

AMP

BIAS

VCC2

TXEN

CEXT1

Detailed Description

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).

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.

Power Amplifier

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.

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).

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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