THS7313 datasheet(21/48 Pages) TI

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Input

Internal

Circuitry

Level

Shifter

VS+

INPUT MODES OF OPERATION – AC BIAS

Input Pin

Input

Rpd

(See Note A)

Rpu

(See Note A)

75 kW

25 kW

Internal

Circuitry

VS+

THS7313

SLOS483 – NOVEMBER 2005

APPLICATION INFORMATION (continued)

Figure 49. Equivalent DC + 135 mV Input Mode Circuit

Other applications require an ac-coupled input. The ac coupling ensures that a source dc-input level does not

alter, or clip, the resulting output video signal. The first ac coupling mode is the AC Bias mode where a simple

internal dc bias voltage is applied to the input signal on the THS7313 side of the external 1-

µF coupling

capacitor.

The applied dc bias voltage is set internally by a resistor divider circuit as shown in Figure 50. The dc bias

voltage is set to VS+÷ 4. With a 3.3-V power supply, the input bias voltage is nominally 0.825 V, and with 5-V

supply, the input bias voltage is nominally 1.25 V. The input impedance with this mode is approximately 19-k

Ω.

With a 1-

µF input capacitor, it sets a high-pass corner frequency of about 9-Hz. If a lower frequency is desired,

increasing the capacitor decreases the corner frequency proportionally. For example, using a 4.7-

µF capacitor

results in a 1.8-Hz high pass corner frequency, and results in lower droop (tilt). Using any capacitor value is

acceptable for this mode of operation.

It is sometimes desirable to adjust the bias voltage to another level other than the one dictated by the internal

resistors. There are two ways this is accomplished:

1. The first is to add an external resistor between the input pin and either the +Vs or GND. This creates a new

bias voltage equal to +Vs

× [25 k / {25 k + (75 k || Rpu)}] for raising the bias voltage, or +Vs × [(25 k || Rpd) /

{(25 k || Rpd) + 75 k}] for reducing the bias voltage.

2. The second method to set the AC-Bias voltage is to use the Rpu and Rpd external resistors, but place the

THS7313 in dc input bias mode. Since the dc mode is very high impedance, the resulting bias voltage is

equal to +Vs

× (Rpd / {Rpd + Rpu}).

This mode of operation is recommended for use with chroma (C’), P’B, P’R, U’, V’, and nonsync B’ and R’ signals.

NOTE: Use external pull-up and/or pull-down resistors if changing the ac-bias input voltage is desired.

Figure 50. Equivalent AC Bias Input Mode Circuit

THS7313 Datasheet(PDF) 21 Page - Texas Instruments

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