THS7353 Datasheet, PDF (31/51 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
www.ti.com
SLOS484 – NOVEMBER 2005
APPLICATION INFORMATION (continued)
LOW PASS FILTER AND BYPASS MODES
Each channel of the THS7353 incorporates a 5^th-Order Low Pass Filter. These video anti-aliasing /
reconstruction filters minimize the images from being passed onto the video decoder / ADC or to the line.
Depending on the system design, failure to eliminate these images can cause picture quality problems due to
aliasing of the data converter. 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. The benefit of the Butterworth response is the amplitude 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 amplitude. An increase in group delay corresponds to a time domain pulse response that has
overshoot and some ringing associated with the overshoot. Thus, the use of other type of filters such as elliptic or
chebyshev are not recommended for video applications due to their very large group delay variations near the
corner frequency. This results in significant overshoot or ringing on fast edge rates such as the sync signals or
when a luma or color-difference signal changes from 0% to 100% or visa-versa. Ringing typically causes a
display to have ghosting or fuzziness appear on the edges of a sharp transition. On the other hand, a Bessel
filter has ideally flat 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 THS7353 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
THS7353 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. Because resistor-to-resistor and capacitor-to-capacitor
matching is very tight, the filter Q sensitivities are essentially eliminated. This maintains a low channel-to-channel
time delay which is required for proper video signal performance.
The THS7353 filters have a nominal corner (-3 dB) frequency selectable at 9 MHz, 16 MHz, and 35 MHz along
with a bypass mode. The 9-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 , G'B'R', and Y’U’V’ video
signals. The -3-dB corner frequency was designed to be 9 MHz to allow a maximally flat video signal while
achieving over 40-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 9-MHz filter frequency was chosen to account for process variations in the THS7353. To ensure the required
video frequencies are not affected very much, 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 is enough to meet the system demands. Thus, the selection of the filter
frequencies was not chosen arbitrarily.
The 16-MHz filter was designed to pass 480p and 576p Y’P’_BP’_R and G'B'R' video signals – sometimes referred
as enhanced definition (ED). Additionally, this filter can be used to pass computer VGA signals with very flat
frequency response in the video spectrum. The use the 16-MHz filter for SD signals ensures there is no
amplitude aberrations, and to have an exceptional low group delay.
The 35-MHz filter is designed to pass high definition (HD) 720p and 1080i Y’P’_BP’_R video signals along with
G’B’R’ (R’G’B’) SVGA and XGA signals. If a 4:2:2 system is used, the P’_BP’_R channels do not require the full
bandwidth as required by the Y’ channel. But, it is still recommended to use the same filter frequency of the Y’
channel to match the group delay and timing of all 3 signals. Otherwise, extra delay compensation is required to
minimize timing variations. This filter is also useful for passing 480p/576p signals with little amplitude or group
delay variations.
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