THS7315 Datasheet, PDF (15/26 Pages) Texas Instruments – 3-Channel SDTV Video Amplifier with 5th-Order Filters and 5.2-V/V Gain

THS7315
www.ti.com
SLOS532 – JUNE 2007
APPLICATION INFORMATION (continued)
Lastly, because of the edge rates and frequencies of operation, it is recommended (but not required) to place a
0.1-μF to 0.01-μF capacitor in parallel with the large 220-μF to 1000-μF capacitor. These large-value capacitors
are generally aluminum electrolytic. It is well-known that these types of capacitors have significantly large
equivalent series resistance, or ESR, and the respective impedances at high frequencies is rather large as a
result of the associated inductances involved with the leads and construction. The small 0.1-μF to 0.01-μF
capacitors help to pass these high-frequency ( greater than 1 MHz) signals with much lower impedance than the
large capacitors.
Although it is common to use the same capacitor values for all the video lines, the frequency bandwidth of the
chroma signal in an S-Video system is not required to go to as low (or as high) a frequency as the luma
channels. Therefore, the capacitor values of the chroma line(s) can be smaller, such as 0.1 μF.
LOW-PASS FILTER
Each channel of the THS7315 incorporates a 5th-order low-pass filter. These video reconstruction filters
minimize DAC images from passing on to the video receiver. Depending on the receiver design, failure to
eliminate these DAC images can cause picture quality problems that result from ADC aliasing. Another benefit of
the filter is that it smooths out aberrations in the signal that some DACs can demonstrate if the internal filtering is
not good. These benefits help with picture quality and ensure that the signal meets video bandwidth
requirements.
Each filter has an associated Butterworth characteristic. The benefit of the Butterworth response is that the
frequency response is flat, with a relatively steep initial attenuation at the corner frequency. The problem with the
Butterworth filter, however, is that the group delay also 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 possible ringing associated with the
overshoot).
Other filter types (such as elliptic or chebyshev) are not recommended for video applications because of the very
large group delay variations that occur near the corner frequency, also resulting in significant overshoot and
ringing. While elliptic or chebyshev filters may help meet the video standard specifications with respect to
amplitude attenuation, the group delay is well beyond the standard specifications. Combined with the fact that
video can switch 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 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. Consequently, the Butterworth filter is a respectable compromise for both attenuation and group delay.
The THS7315 filters have a nominal corner (–3 dB) frequency at 8.5 MHz and a –1 dB passband, typically at 7
MHz. This 8.5-MHz filter is ideal for SDTV, 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' signals, and computer R'G'B' video
signals. The 8.5-MHz, –3-dB 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 2nd and 3rd
Nyquist zones that is found in many video systems. This feature is important because any signal appearing
around this frequency can appear in the baseband because of aliasing effects of an ADC 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, and so forth. 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 THS7315 has over 70-dB
attenuation at 54 MHz and 81 MHz, along with over 65-dB attenuation at 108 MHz. Attenuation above 108 MHz
is at least 55 dB, ensuring that images do not affect the desired video baseband signal.
The 8.5-MHz filter frequency was chosen to account for process variations in the THS7315. To ensure the
required video frequencies are effectively passed, the filter corner frequency must be high enough to allow
component variations. The other filter design consideration is the attenuation. It must be large enough to ensure
that anti-aliasing/reconstruction filtering is enough to meet the system demands. Thus, the filter frequency
selection was not arbitrary; it is a good compromise that should meet the demands of most systems.
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