THS7347_15 Datasheet, PDF (14/33 Pages) Texas Instruments – 3-Channel RGBHV Video Buffer with I2C Control, 2:1 Input Mux, Monitor Pass-Through, and Selectable Input Bias Modes

THS7347
SLOS531B – MAY 2007 – REVISED OCTOBER 2011
www.ti.com
INPUT OVERVOLTAGE PROTECTION
The THS7347 is built using a very high-speed complementary bipolar and CMOS process. The internal junction
breakdown voltages are relatively low for these very small geometry devices. These breakdowns are reflected in
the Absolute Maximum Ratings table. All input and output device pins are protected with internal ESD protection
diodes to the power supplies, as shown in Figure 3.
V_S+
External
Input/
Output
Pin
Internal
Circuitry
Figure 3. 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
A typical application circuit usng the THS7347 as an ac-coupled input video buffer is shown in Figure 4. It shows
the THS7347 driving a video ADC (such as the TVP7000) with 0-dB gain and also driving an output line with
6-dB gain. The Horizontal and Vertical Sync signals are also driven to the ADC and the Monitor Output
separately. Although the computer resolution R’G’B’HV signals are shown, these channels can easily be the
high-definition video (HD), enhanced-definition (ED), or standard-definition (SD) Y’P’_BP’_R (sometimes labeled
Y’U’V’ or incorrectly labeled Y’C’_BC’_R) channels. These channels could also be S-Video Y’/C’ channels and the
composite video baseband signal (CVBS). Note that the R’G’B’ channels could be professional/broadcast G’B’R’
signals or other R’G’B’ variations based on the placement of the sync signals that are commonly called R’G’sB’
(sync on Green) or R’sG’sB’s (sync on all signals).
The second set of inputs (B-Channels) shown are connected to another set of inputs. Again, these inputs can be
either HD, ED, SD, or R'G'B'/G'B'R' video signals. The THS7347 flexibility allows for virtually any input signal to
be driven into the THS7347 regardless of the other set of inputs. Simple control of the I²C configures the
THS7347 for any conceivable combination. For example, the THS7347 can be configured to have Channel 1
Input connected to input A while Channel 2 and Channel 3 are connected to input B. See the multiple application
notes sections explaining the I²C interface later in this document for details on configuring these options.
Note that the Y' term is used for the luma channels throughout this document, rather than the more common
luminance (Y) term. The reason for this usage is to account for the true definition of luminance as stipulated by
the CIE (International Commission on Illumination). Video departs from true luminance because a nonlinear term,
gamma, is added to the true RGB signals to form R'G'B' signals. These R'G'B' signals are then utilized to
mathematically create luma (Y'). Therefore, true luminance (Y) is not maintained, and thus the difference in
terminology arises.
This rationale is also utilized for the chroma (C') term. Chroma is derived from the nonlinear R'G'B' terms and
therefore it is also nonlinear. True chominance (C) is derived from linear RGB, and thus the difference between
chroma (C') and chrominance (C) exists. 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 labeled RGB) is also called G'B'R' (again commonly labeled 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 approach 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
14
Copyright © 2007–2011, Texas Instruments Incorporated