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+

-

Input

Pin

800kW

Input

Cin

Internal

Circuitry

3.3V

3.3V

3.01MW

140mVLevel

Shifter

V

800k

(1)

OUTPUT MODE OF OPERATION – DC COUPLED

THS7316

SLOS521 – MARCH 2007

APPLICATION INFORMATION (continued)

Figure 30. AC-Bias Input Mode Circuit Configuration

The dc voltage appearing at the input pin is approximately equal to:

The THS7316 allowable input range is approximately (+Vs – 1.5V) which allows for a wide input voltage range.

As such, the input dc-bias point is flexible with the output dc-bias point being the primary factor. For example, if

the output dc-bias point is desired to be 1.65-V on a 3.3-V supply, then the input dc-bias point should be (1.65 V

– 280 mV) /2 = 0.685 V. Thus, the pull-up resistor calculates to about 3.01-M

Ω resulting in 0.693 V. If the input

dc-bias point is desired to be 0.685 V with a 5-V power supply, then the pull-up resistor calculates to about

5.1-M

Ω.

The internal 800-k

Ω resistor has approximately a ±20% variance. As such, the calculations should take this into

account. For the 0.693 V example above using an ideal 3.01-M

Ω resistor, the input dc-bias voltage is about

0.693 V

±0.11 V.

One other issue that must be taken into account is that the dc-bias point is a function of the power supply. As

such, there may be an impact on power supply rejection (PSRR) on the system. To help reduce the impact, the

input capacitor combined with the pull-up resistance functions as a low-pass filter. Additionally, the time to

charge the capacitor to the final dc-bias point is also a function of the pull-up resistor and the input capacitor.

Lastly, the input capacitor forms a high-pass filter with the parallel impedance of the pull-up resistor and the

800-k

Ω resistor. It is good to have this high pass filter at about 3-Hz to minimize any potential droop on a P'

high-pass corner frequency.

signals.

The THS7316 incorporates a rail-to-rail output stage that can be used to drive the line directly without the need

for large ac-coupling capacitors as shown in Figure 31. This offers the best line tilt and field tilt (or droop)

performance since there is no ac-coupling occurring. Remember that if the input is ac-coupled, then the resulting

tilt due to the input ac-coupling is seen on the output regardless of the output coupling. The 80-mA output

current drive capability of the THS7316 was designed to drive two video lines per channel simultaneously –

essentially a 75-

Ω load – while keeping the output dynamic range as wide as possible.

16

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