LMH6521 datasheet(32/47 Pages) TI2

32 / 47 page

½ LMH2832

INPx

INMx

OUTPx

OUTMx

64.9

SPI

VS+ = 5V

GND

0.1 µF

1:2

OUT_AMP

½ ADS54J40

5.5 pF

24 nH

82 nH

24 nH

100

VOCM

5.1

ZS = 75-

LMH2832

SBOS709A – JULY 2016 – REVISED JULY 2016

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Application Information (continued)

10.1.1.4 ADC Input Common-Mode Voltage Considerations (DC-Coupled Input)

The LMH2832 is designed to primarily be used in ac-coupled applications only. However, the LMH2832 can be

dc-coupled if certain strict conditions are met. The LMH2832 has an internal common-mode bias equal to the

mid-supply voltage, so any dc coupling on the input or output must have a common-mode voltage that is also set

to mid-supply. To dc couple to an ADC input, the mid-supply voltage of the LMH2832 must be centered around

the ADC input common-mode. This common-mode matching can be accomplished by shifting the supplies to

center the mid-supply voltage around the ADC input common-mode voltage. However, shifting the supplies also

changes the ground reference for the digital inputs, which then likely requires a voltage-shifted interface as well.

The LMH2832 is not recommended to be operated as dc-coupled unless absolutely necessary.

10.2 Typical Applications

10.2.1 DOCSIS 3.X Driver

The LMH2832 is designed to perform best when driving differential input ADCs in high-speed applications.

Figure 61 shows an example diagram of the LMH2832 driving an ADC with a fifth-order, low-pass filter for a 75-Ω

impedance, data over cable service interface specification (DOCSIS) 3.X upstream receiver return path

application. The primary interface between the amplifier and the ADC is usually an antialiasing filter to suppress

high-frequency harmonics that otherwise alias back into the ADC FFT spectrum. Filters range from single-order

real RC poles to higher-order, resistor-inductor-capacitor (RLC) filters, depending on the application

requirements. Series output resistors (RO) help isolate the amplifier from any capacitive load presented by the

filter, and can also be used to create a matched impedance to drive transmission lines.

Figure 61. DOCSIS 3.X Driver with the ADS54J40 and a 300-MHz, 4th-Order, Butterworth, Low-Pass Filter

10.2.1.1 Design Requirements

Table 12 shows example design requirements for the LMH2832 in an upstream receiver application.

Table 12. Example Design Requirements

SPECIFICATION

DESIGN REQUIREMENTS

Supply voltage

4.75 to 5.25 V

Usable input frequency range

300 MHz

System voltage gain and range

33-dB voltage gain with 30-dB range

Source impedance

75 Ω, single-ended

Signal path SNR at 175 MHz (measured at ADC)

> 50 dBFS

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