THS7364IPWR datasheet(37/47 Pages) TI

37 / 47 page

THS7364

www.ti.com

SBOS530 – AUGUST 2010

series resistor, and the filter) must be better than 75

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

Ω, +/-9 Ω. The THS7364 is an operational amplifier

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

that approximates an ideal voltage source, which is

this amount of current flows internally to the DAC

desirable because the output impedance is very low

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

and can source and sink current. To properly match

the DAC may be very high, especially when six

the transmission line characteristic impedance of a

channels are being driven. Using the THS7364 with a

video line, a 75-

Ω series resistor is placed on the

high input impedance and the capability to drive up to

output. To minimize reflections and to maintain a

two video lines per channel can reduce DAC power

good return loss meeting EIA/CEA specifications, this

dissipation significantly. This outcome is possible

output impedance must maintain a 75-

Ω impedance.

because the resistance that the DAC drives can be

A wide impedance variation of a passive filter cannot

substantially increased. It is common to set this

ensure this level of performance. On the other hand,

resistance in a DAC by a current-setting resistor on

the THS7364 has approximately 0.9

Ω of output

the DAC itself. Thus, the resistance can be 300

Ω or

impedance, or a return loss of 44 dB, at 6.75 MHz for

more,

substantially

reducing

the

current

drive

the SD filters and approximately 9

Ω of output

demands from the DAC and saving significant

impedance, or a return loss of 25 dB, at 60 MHz for

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

the FHD filters. Thus, the system is matched

DAC dissipates 660 mW alone for the steering

significantly better with a THS7364 compared to a

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

passive filter.

it must drive a 37.5-

Ω load. With a 300-Ω load, the

DAC power dissipation as a result of current steering

One final benefit of the THS7364 over a passive filter

current would only be 82 mW (six channels ×

is power dissipation. A DAC driving a video line must

4.16 mA × 3.3 V).

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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