THS7303 Datasheet, PDF (31/43 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

THS7303
www.ti.com
SLOS479 – OCTOBER 2005
APPLICATION INFORMATION (continued)
LOW PASS FILTER AND BYPASS MODES
Each channel of the THS7303 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 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 resulting in significant 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 well beyond the standard 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
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 THS7303 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
THS7303 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 THS7303 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 THS7303. 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 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 flat
frequency response in the video spectrum. The 16-MHz filter can also be used for SD signals to ensure there is
no amplitude aberration, and to have an exceptional low group delay within the SD video frequency range.
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 within the ED frequency range.
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