LMH6718 datasheet(14/16 Pages) NSC

14 / 16 page

Application Circuits

SINGLE SUPPLY CABLE DRIVER

Figure 14 below shows the LMH6718 driving 10m of 75

Ω

coaxial cable. The LMH6718 is set for a gain of +2V/V to

compensate for the divide-by-two voltage drop at V

O. The

response after 10m of cable is illustrated in Figure 15

DIFFERENTIAL LINE DRIVER WITH LOAD IMPEDANCE

CONVERSION

The circuit shown in Figure 16, operates as a differential line

driver. The transformer converts the load impedance to a

value that best matches the LMH6718’s output capabilities.

The single-ended input signal is converted to a differential

signal by the LMH6718. The line’s characteristic impedance

is matched at both the input and the output. The schematic

shows Unshielded Twisted Pair for the transmission line;

other types of lines can also be driven.

Set up the LMH6718 as a difference amplifier:

•

Set the Channel 1 amplifier to a gain of +1V/V

•

Set the Channel 2 amplifier to a gain of −1V/V

Make the best use of the LMH6718’s output drive capability

as follows:

where R

eq is the transformed value of the load impedance,

max is the output Voltage Range, and Imax is the maximum

Output Current.

Match the line’s characteristic impedance:

Select the transformer so that it loads the line with a value

very near Z

O over frequency range. The output impedance

of the LMH6718 also affects the match. With an ideal trans-

former we obtain:

where Z

O(6718)(j

ω) is the output impedance of the

LMH6718 and |Z

O(6718)(j

ω)|<<R

The load voltage and current will fall in the ranges:

The LMH6718’s high output drive current and low distortion

make it a good choice for this application.

20040153

FIGURE 14. Single Supply Cable Driver

20040154

FIGURE 15. Response After 10m of Cable

20040155

FIGURE 16. Differential Line Driver with Load

Impedance Conversion

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