THS7320 Datasheet, PDF (25/35 Pages) Texas Instruments – 3-Channel ED Filter Video Amplifier with 4-V/V Gain

THS7320
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SBOS565B – JULY 2011 – REVISED SEPTEMBER 2012
LOW-PASS FILTER
Each channel of the THS7320 video section incorporates a third-order, low-pass filter. These video
reconstruction filters minimize DAC images from being passed on to the video receiver. Depending on the
receiver design, failure to eliminate these DAC images can cause picture quality problems as a result of aliasing
of the ADC in the receiver. Another benefit of the filter is to smooth out aberrations in the signal that DACs
typically have associated with the digital stepping of the signal. This benefit helps with picture quality and
ensures 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 concern with the
Butterworth characteristic 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, is not recommended for video applications
because of the very large group delay variations near the corner frequency resulting in significant overshoot and
ringing. While these filters may help meet the video standard specifications with respect to amplitude attenuation,
the group delay is well beyond the standard specifications. Considering this group delay with the fact that video
can go from a white pixel to a black pixel over and over again, it is easy to see that ringing can 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 an acceptable compromise for both attenuation and group delay.
The THS7320 filters have a nominal corner (–3-dB) frequency at 20 MHz and a –1-dB passband typically at
17 MHz. This 20-MHz filter is ideal for enhanced definition (ED) NTSC or PAL 480p/576p Y’P’_BP’_R or
G'B'R'/R'G'B' signals. For oversampled systems, the THS7320 works well for passing standard definition (SD)
NTSC, PAL, or SECAM composite video (CVBS), S-Video signals (Y’C’), 480i/576i Y’P’_BP’_R, Y’U’V’, broadcast
G’B’R’ signals, and R'G'B' video signals. The 20-MHz, –3-dB corner frequency was designed to achieve 27-dB of
attenuation at 54 MHz—a common sampling frequency between the DAC/ADC second and third Nyquist zones
found in many video systems. This consideration is important because any signal that appears around this
frequency can also appear in the baseband as a result of aliasing effects of an ADC found in a receiver. Another
specification ensured for the THS7320 is attenuation at 43 MHz. This frequency is derived from the fact that the
ED Y' signal has an 11-MHz bandwidth. Following standard sampling theory, this means that the second Nyquist
zone image starts at 54 MHz – 11 MHz = 43 MHz.
Keep in mind that images do not stop at the DAC sampling frequency, f_S (for example, 54 MHz for traditional ED
DACs); they continue around the sampling frequency harmonics of 2 × f_S, 3 × f_S, 4 × f_S, and so on (that is,
108 MHz, 162 MHz, 216 MHz, etc.). Because of these multiple images, an ADC can fold down into the baseband
signal, meaning that the low-pass filter must also eliminate these higher-order images. The THS7320 filters are
designed to attenuate all of these higher frequencies without bounce effect that some filters can allow.
The filter frequencies were chosen to account for process variations in the THS7320. 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 that the attenuation must be large enough to ensure the anti-
aliasing/reconstruction filtering is sufficient to meet the system demands. Thus, the recommendations for the filter
frequencies was not arbitrarily selected and is a good compromise that should meet the demands of most
systems.
Copyright © 2011–2012, Texas Instruments Incorporated
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