THS7368IPWR datasheet(30/45 Pages) TI2

30 / 45 page

THS7368

SBOS497 – DECEMBER 2009

www.ti.com

source impedance (which includes the source, series

drive at least 1.25 VP (100% saturation CVBS)/37.5 Ω

resistor, and the filter) must be better than 75

Ω,

= 33.3 mA. A DAC is a current-steering element, and

+9/–8

Ω. The THS7368 is an operational amplifier

this amount of current flows internally to the DAC

that approximates an ideal voltage source, which is

even if the output is 0 V. Thus, power dissipation in

desirable because the output impedance is very low

the DAC may be very high, especially when six

and can source and sink current. To properly match

channels are being driven. Using the THS7368 with a

the transmission line characteristic impedance of a

high input impedance and the capability to drive up to

video line, a 75-

Ω series resistor is placed on the

two video lines per channel can reduce DAC power

output. To minimize reflections and to maintain a

dissipation significantly. This outcome is possible

good return loss meeting EIA specifications, this

because the resistance that the DAC drives can be

output impedance must maintain a 75-

Ω impedance.

substantially increased. It is common to set this

A wide impedance variation of a passive filter cannot

resistance in a DAC by a current-setting resistor on

ensure this level of performance. On the other hand,

the DAC itself. Thus, the resistance can be 300

Ω or

the THS7368 has approximately 0.7

Ω of output

more,

substantially

reducing

the

current

drive

impedance, or a return loss of 46 dB, at 6.75 MHz for

demands from the DAC and saving significant

the SD filters and approximately 1.7

Ω of output

amounts of power. For example, a 3.3-V, six-channel

impedance, or a return loss of 39 dB, at 30 MHz for

DAC dissipates 660 mW alone for the steering

the SF-HD filters. Thus, the system is matched

current capability (six channels × 33.3 mA × 3.3 V) if

significantly better with a THS7368 compared to a

it must drive a 37.5-

Ω load. With a 300-Ω load, the

passive filter.

DAC power dissipation as a result of current steering

current would only be 82 mW (six channels × 4.16

One final benefit of the THS7368 over a passive filter

mA × 3.3 V).

is power dissipation. A DAC driving a video line must

be able to drive a 37.5-

Ω load: the receiver 75-Ω

resistor and the 75-

Ω impedance matching resistor

next to the DAC to maintain the source impedance

requirement. This requirement forces the DAC to

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