THS7353_15 Datasheet, PDF (24/57 Pages) Texas Instruments – 3-Channel Low Power Video Buffer with I2C Control, Selectable Filters, External Gain Control, 2:1 Input MUX, and Selectable Input Modes

THS7353
SLOS484B – NOVEMBER 2005 – REVISED AUGUST 2012
Figure 61 shows the equivalent input circuit while in the dc + 250 mV shift mode of operation.
V_S+
Internal
Circuitry
Input
Input
Pin
Level
Shifter
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Figure 61. Equivalent DC + 250 mV Input Mode Circuit
INPUT MODES OF OPERATION – AC BIAS
Other applications require an ac-coupled input. The ac coupling ensures that a dc input level does not alter, or
clip, the resulting output video signal, and it also allows the THS7353 to re-establish its own dc-operating point.
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 THS7353 side of the external coupling capacitor.
The applied dc bias voltage is set internally by a simple resistor divider circuit as shown in Figure 62. The dc bias
voltage is set to V_S+ ÷ 3.3. With a 3.3-V power supply, the input bias voltage is nominally 1 V and with 5-V
supply, the input bias voltage is nominally 1.5 V. The input impedance with this mode is approximately 21-kΩ.
With a 1-μF input capacitor, it sets a high-pass corner frequency of about 7.6-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 1.6-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 +V_S or GND. This creates a new
bias voltage equal to +Vs × [30 k / {30 k + (70 k || R_PU)}] for raising the bias voltage, or +Vs × [(30 k || Rpd) /
{(30 k || Rpd) + 70 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
THS7353 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 ac-bias mode is recommended for use with chroma (C’), P’_B, P’_R, U’, V’, and other nonsync signals.
Input
V_S+
Rpu
(See Note A)
Input Pin
C_I
Rpd
(See Note A)
V_S+ V_S+
70 kW
30 kW
Internal
Circuitry
NOTE: Use external pull-up and/or pull-down resistors if changing the AC-bias input voltage is desired.
Figure 62. Equivalent AC Bias Input Mode Circuit
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