THS7375IPWR datasheet(25/36 Pages) TI

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Reducing System Gain

75 W

VideoOut

DAC

53.6 W

21.5 W

75 W

5.6V/V

THS7375

INPUT

0.44V

DAC

0.615V

100%SaturatedCVBS

2.46V

1.23V

THS7375

www.ti.com......................................................................................................................................................................................... SBOS449 – SEPTEMBER 2008

coefficient capacitors found in the BiCom3X process.

One final advantage of the THS7375 over a passive

These filters have been specified by design to

filter is power dissipation. A DAC driving a video line

account for process and temperature variations to

must be able to drive a 37.5-

Ω load—the receiver

maintain proper filter characteristics. This architecture

75-

Ω resistor and the 75-Ω source impedance

maintains a low channel-to-channel time delay, which

matching resistor next to the DAC to maintain the

is required for proper video signal performance.

source impedance requirement. This approach forces

the DAC to drive at least 1.25 VP (100% saturation

Another benefit of the THS7375 over a passive RLC

CVBS)/37.5

Ω = 33.3 mA. A DAC is a current

filter is the input and output impedance. The input

steering element and this amount of current flows

impedance

presented

the

DAC

may

vary

internally to the DAC even if the output is 0 V. Thus,

significantly with a passive network and may cause

power dissipation in the DAC may be very high,

voltage variations over frequency. The THS7375

especially when four channels are being driven. With

input impedance is 800 k

Ω, and only the 2-pF input

a high input impedance and the capability to drive up

capacitance plus the PCB trace capacitance impacts

to two video lines, utilizing the THS7375 can reduce

the input impedance. As such, the voltage variation

the

DAC

power

dissipation

significantly.

This

appearing at the DAC output is better controlled with

reduction occurs because the resistance the DAC is

a fixed termination resistor and the high input

driving can be substantially increased. It is common

impedance buffer of the THS7375.

to set this driving resistance in a DAC by a

current-setting resistor on the device. Thus, the

On the output side of the filter, a passive filter also

resistance can be 300

Ω or more, substantially

has a large impedance variation over frequency. The

reducing the current drive demands from the DAC

EIA770 specifications requires the return loss be at

and saving a substantial amount of power. For

least 25 dB over the video frequency range of usage.

example, a 3.3-V four-channel DAC dissipates 440

For a video system, this condition implies the source

mW for the steering current capability alone (four

impedance—which includes the source and the

channels × 33.3 mA × 3.3 V) if it must drive a 37.5-

Ω

series resistor and the filter—must be better than 75

load. With a 300-

Ω load, the DAC power dissipation

+9/–8

Ω. The THS7375 is an operational amplifier

as a result of current steering current would only be

that approximates an ideal voltage source. A voltage

55 mW (four channels × 4.16 mA × 3.3 V).

source is desirable because the output impedance is

very low and can source and sink current. To properly

match the transmission line characteristic impedance

of a video line, a 75-

Ω series resistor is placed on the

The THS7375 has a built-in gain of 5.6 V/V, or 14.95

output. To minimize reflections and to maintain a

dB. While this gain matches the needs of many of

good return loss, this output resistance must maintain

Texas Instruments' video processors, including the

a 75-

Ω impedance. A passive filter impedance

DaVinci family of products, the gain can be easily

variation cannot ensure this consistent performance

reduced to meet other needs. The easiest and most

while the THS7375 has about 1.4-

Ω of output

effective method of adjusting the gain lower is to

impedance at 5-MHz. Thus, the system is much

simply use a resistor divider on the input to the

better matched with a THS7375 as compared to a

THS7375, as shown in Figure 52. This solution uses

passive filter.

resistors R1 and R2 to accomplish two requirements:

1. Terminate

the

video

DAC

with

the

proper

resistance, and

2. Form a resistor divider in before the THS7375

input.

Figure 52. Configuring the THS7375 with 4-V/V (12dB) Gain

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