THS7313 Datasheet, PDF (21/48 Pages) Texas Instruments – 3-Channel Low Power SDTV Video Amplifier with I2C Control, 6-dB Gain, SAG Correction, 2:1 Input MUX, and Selectable Input Bias Modes

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APPLICATION INFORMATION (continued)
Input
V_S+
Input
Pin
Internal
Circuitry
Level
Shifter
THS7313
SLOS483 – NOVEMBER 2005
Figure 49. Equivalent DC + 135 mV Input Mode Circuit
INPUT MODES OF OPERATION – AC BIAS
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 V_S+÷ 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-kΩ.
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.
Input
V_S+
Rpu
(See Note A)
Input Pin
C_I
Rpd
(See Note A)
V_S+ V_S+
75 kW
25 kW
Internal
Circuitry
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
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