THS7313 Datasheet, PDF (28/48 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

THS7313
SLOS483 – NOVEMBER 2005
www.ti.com
APPLICATION INFORMATION (continued)
LOW PASS FILTER MODE
Each channel of the THS7313 incorporates a 5^th-Order Low Pass Filter. These video reconstruction filters
minimize DAC images from being passed onto the video receiver. Depending on the receiver design, failure to
eliminate these DAC images can cause picture quality problems due to aliasing of the ADC. Another benefit of
the filter is to smooth out aberrations in the signal which some DACs can have if their own internal filtering is not
good. This helps with picture quality and helps insure the signal meets video bandwidth requirements.
Each filter has a Butterworth characteristic associated with it. They have been modified with a slightly lower Q
than the traditional Q associated with the Butterworth response. The benefit of the Butterworth response is the
frequency response is flat with a relatively steep initial attenuation at the corner frequency. The problem is that
the group delay rises near the corner frequency. Group delay is defined as the change in phase (radians/second)
divided by a change in frequency. An increase in group delay corresponds to a time domain pulse response that
has overshoot and some possible ringing associated with the overshoot.
The use of other types of filters such as elliptic or chebyshev is not recommended for video applications due to
their large group delay variations near the corner frequency which results in overshoot and ringing. While these
elliptic or chebyshev filters may help meet the video standard specifications with respect to amplitude
attenuation, their group delay is beyond the standards specifications. Couple this with the fact that video can go
from a white pixel to a black pixel over and over again, ringing occurs. Ringing typically causes a display to have
ghosting or fuzziness appear on the edges of a sharp transition. However, a Bessel filter has an ideal group
delay response, but the rate of attenuation is typically too low for acceptable image rejection. Thus, the
Butterworth filter is a respectable compromise for both attenuation and group delay.
The THS7313 filter has a slightly lower group delay variation near the corner frequency compared to an ideal
Butterworth filter. This results in a time domain pulse response which still has some overshoot, but not as much
as a true Butterworth filter. Additionally, the initial rate of attenuation in the frequency response is not as fast as
an ideal Butterworth response, but it is an acceptable initial rate of attenuation considering the pulse and group
delay characteristic benefits.
One concern about an active filter in an integrated circuit is the variation of the filter characteristics when the
ambient temperature and the subsequent die temperature changes. To minimize temperature effects, the
THS7313 uses thin-film metal resistors and high quality - low temperature coefficient capacitors found in the
BiCom-3 process. The filters have been specified by design to account for process variations and temperature
variations to maintain proper filter characteristics. This maintains a low channel-to-channel time delay which is
required for proper video signal performance.
The THS7313 filters have a nominal corner (-3 dB) frequency selectable at 8 MHz. The 8-MHz filter is ideal for
standard definition (SD) NTSC, PAL, and SECAM composite video (CVBS) signals. It is also useful for S-Video
signals (Y’/C’), 480i / 576i Y’P’_BP’_R , and Y’U’V’ video signals. The -3-dB corner frequency was designed to be 8
MHz to allow a maximally flat video signal while achieving 48-dB of attenuation at 27 MHz – a common
frequency between the ADC 2^nd and 3^rd Nyquist zones found in many video receivers. This is important because
any signal appearing around this frequency can appear in the baseband due to aliasing effects of an analog to
digital converter found in a receiver.
The 8-MHz filter frequency was chosen to account for process variations in the THS7313. To ensure the required
video frequencies are the least affected, the filter corner frequency must be high enough to allow for component
variations. The other consideration is the attenuation must be large enough to ensure the anti-aliasing /
reconstruction filtering meets the system demands. Thus, the selection of the filter frequencies was not chosen
arbitrarily.
The I²C interface of the THS7313 allows each channel to be configured totally independent of the other
channels. One of the benefits is that a multiple output encoder (or DAC) can be routed through one THS7313
with the proper input configuration regardless of the signal. This is useful for a portable system or in a low cost
system where only one set (or 2 sets in parallel) is desired on the output of the system. An update of the I²C
commands changes the THS7313 channels. An example is shown in Figure 56 where the input MUX allows for
one set of component video signals to be put into the THS7313, and then through an I²C update, a
CVBS/S-Video set of signals is sent through the THS7313 with the proper input mode.
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