THS7314 Datasheet, PDF (19/23 Pages) Texas Instruments – 3-Channel SDTV Video Amplifier With 5th-Order Filters and 6-dB Gain

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THS7314
SLOS513 – DECEMBER 2006
APPLICATION INFORMATION (continued)
LOW PASS FILTER
Each channel of the THS7314 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. 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 can easily occur. 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 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 THS7314 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 quite as
fast as an ideal Butterworth response, but it is an acceptable initial rate of attenuation considering the pulse and
group delay characteristic benefits. The THS7314 still achieves 47-dB of attenuation at 27-MHz, which typically
exceeds most SD video requirements.
The THS7314 filters have a nominal corner (-3dB) frequency at 8.5-MHz and a –1 dB passband typically at
7-MHz. This 8.5-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, Y'U'V', broadcast G’B’R’ (R’G’B’)
signals, and computer video signals. The 8.5-MHz -3dB corner frequency was designed to allow a maximally flat
video signal while achieving 47-dB of attenuation at 27-MHz – a common sampling frequency between the
DAC/ADC 2^nd and 3^rd Nyquist zones found in many video systems. 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.
Keep in mind that images do not stop at 27-MHz, they continue around the sampling frequencies of 54-MHz,
81-MHz, 108-MHz, etc. Because of these multiple images that an ADC can fold down into the baseband signal,
the low pass filter must also eliminate these higher order images. The THS7314 has over 70-dB attenuation at
54-MHz, 68-dB attenuation at 81-MHz, and over 60-dB attenuation at 108-MHz. Attenuation above 108-MHz is
at least 55-dB which makes sure that images do not effect the desired video baseband signal.
The 8.5-MHz filter frequency was chosen to account for process variations in the THS7314. To ensure the
required video frequencies are effectively passed, the filter corner frequency must be high enough to allow
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 arbitrarily selected and is a good compromise that should meet the demands of most
systems.
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