Datasheet

THS7373

SBOS506A –DECEMBER 2009–REVISED AUGUST 2012

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Another benefit of the THS7373 over a passive RLC impedance at 6.75 MHz for the 9.5-MHz filter and

filter is the input and output impedance. The input approximately 1.4 Ω of output impedance at 30 MHz

impedance presented to the DAC varies significantly, for the 36-MHz filters. Thus, the system is matched

from 35 Ω to over 1.5 kΩ with a passive network, and significantly better with a THS7373 compared to a

may cause voltage variations over frequency. The passive filter.

THS7373 input impedance is 800 kΩ, and only the 2-

One final benefit of the THS7373 over a passive filter

pF input capacitance plus the PCB trace capacitance

is power dissipation. A DAC driving a video line must

impact the input impedance. As such, the voltage

be able to drive a 37.5-Ω load: the receiver 75-Ω

variation appearing at the DAC output is better

resistor and the 75-Ω impedance matching resistor

controlled with a fixed termination resistor and the

next to the DAC to maintain the source impedance

high input impedance buffer of the THS7373.

requirement. This requirement forces the DAC to

On the output side of the filter, a passive filter again drive at least 1.25 V

(100% saturation CVBS)/37.5 Ω

has a large impedance variation over frequency. = 33.3 mA. A DAC is a current-steering element, and

EIA770 specifications require the return loss to be at this amount of current flows internally to the DAC

least 25 dB over the video frequency range of usage. even if the output is 0 V. Thus, power dissipation in

For a video system, this requirement implies that the the DAC may be very high, especially when six

source impedance (which includes the source, series channels are being driven. Using the THS7373 with a

resistor, and the filter) must be better than 75 Ω, high input impedance and the capability to drive up to

+9/–8 Ω. The THS7373 is an operational amplifier two video lines per channel can reduce DAC power

that approximates an ideal voltage source, which is dissipation significantly. This outcome is possible

desirable because the output impedance is very low because the resistance that the DAC drives can be

and can source and sink current. To properly match substantially increased. It is common to set this

the transmission line characteristic impedance of a resistance in a DAC by a current-setting resistor on

video line, a 75-Ω series resistor is placed on the the DAC itself. Thus, the resistance can be 300 Ω or

output. To minimize reflections and to maintain a more, substantially reducing the current drive

good return loss meeting EIA specifications, this demands from the DAC and saving significant

output impedance must maintain a 75-Ω impedance. amounts of power. For example, a 3.3-V, four-

A passive filter impedance variation cannot ensure channel DAC dissipates 440 mW alone for the

this level of performance. On the other hand, the steering current capability (four channels × 33.3 mA ×

THS7373 has approximately 0.8 Ω of output 3.3 V) if it must drive a 37.5-Ω load. With a 300-Ω

load, the DAC power dissipation as a result of current

steering current would only be 55 mW (four channels

× 4.16 mA × 3.3 V).