THS7303 Datasheet, PDF (21/43 Pages) Texas Instruments – 3-Channel Low Power Video Amplifier with I2C Control, Selectable Filters, 6-dB Gain, SAG Correction, 2:1 Input MUX, and Selectable Input Bias Modes

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APPLICATION INFORMATION (continued)
V_S+
External
Input/
Output
Pin
Internal
Circuitry
THS7303
SLOS479 – OCTOBER 2005
Figure 58. Internal ESD Protection
These diodes provide moderate protection to input overdrive voltages above and below the supplies. The
protection diodes can typically support 30-mA of continuous current when overdriven.
TYPICAL CONFIGURATION and VIDEO TERMINOLOGY
A typical application circuit using the THS7303 as a video buffer is shown in Figure 59. It shows a DAC (or
encoder such as the THS8200) driving the three input channels of the THS7303. Although the high-definition
video (HD) or enhanced-definition (ED) Y’P’_BP’_R (sometimes Y’U’V’ is used or it is incorrectly labeled Y’C’_BC’_R)
channels are shown, these channels can easily be S-Video Y’C’ channels and the composite video baseband
signal (CVBS) of a standard definition video (SD) system. These signals can also be G’B’R’ (R'G'B') signals or
other variations. Note that for computer signals the sync should be embedded within the signal for a system with
only 3-outputs. This is sometimes labeled as R’G’sB’ (sync on green) or R’sG’sB’s (sync on all signals).
The second set of inputs (B-Channels) shown are being driven from an external input typically used as a
pass-through function. These are either HD, ED, or SD video signals. The flexibility of the THS7303 allows for
almost any input signal to be driven into the THS7303 regardless of the other set of inputs. Control of the I²C
configures each channel of the THS7303 independently of the other channels. For example, the THS7303 can
be configured to have Channel 1 Input connected to input A with 35-MHz LPF while Channels 2 and 3 are
connected to input B with 16-MHz LPF. See the various sections explaining the I²C interface later in this data
sheet for more information.
Note that the Y’ term is used for the luma channels throughout this document rather than the more common
luminance (Y) term. The reason is to account for the definition of luminance as stipulated by the CIE -
International Commission on Illumination. Video departs from true luminance since a nonlinear term, gamma, is
added to the true RGB signals to form R’G’B’ signals. These R’G’B’ signals are then used to mathematically
create luma (Y’). Thus luminance (Y) is not maintained providing a difference in terminology.
This rationale is also used for the chroma (C’) term. Chroma is derived from the non-linear R’G’B’ terms and thus
it is nonlinear. Chominance (C) is derived from linear RGB giving the difference between chroma (C’) and
chrominance (C). The color difference signals (P’_B / P’_R / U’ / V’) are also referenced this way to denote the
nonlinear (gamma corrected) signals.
R’G’B’ (commonly mislabeled RGB) is also called G’B’R’ (again commonly mislabeled as GBR) in professional
video systems. The SMPTE component standard stipulates that the luma information is placed on the first
channel, the blue color difference is placed on the second channel, and the red color difference signal is placed
on the third channel. This is consistent with the Y'P'_BP'_R nomenclature. Because the luma channel (Y') carries the
sync information and the green channel (G') also carries the sync information, it makes logical sense that G' be
placed first in the system. Since the blue color difference channel (P'_B) is next and the red color difference
channel (P'_R) is last, then it also makes logical sense to place the B' signal on the second channel and the R'
signal on the third channel respectfully. Thus hardware compatibility is better achieved when using G'B'R' rather
than R'G'B'. Note that for many G'B'R' systems sync is embeded on all three channels, but may not always be
the case in all systems.
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