THS7376 Datasheet, PDF (48/61 Pages) Texas Instruments – 4-Channel Video Amplifier with One SD and Three HD 8th-Order Filters with 6-dB Gain

THS7376
SBOS692 – JUNE 2013
www.ti.com
LOW-PASS FILTER (LPF)
Each channel of the THS7376 incorporates a sixth-order or eighth-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 because of aliasing of
the ADC in the receiver. If the DAC sampling freqeuncy is different than the ADC sampling frequency, then the
images will not fold properly back into the base-band and picture quality may suffer. Another benefit of the filter is
to smooth out aberrations in the signal that some DACs can have if the internal filtering is not very good. 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 problem with this
characteristic is that the group delay rises near the corner frequency. Group delay is defined as the change in
phase (radians per 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
because of the very large group delay variations near the corner frequency resulting in significant overshoot and
ringing. Although these filters may help meet the video standard specifications with respect to amplitude
attenuation, the group delay is well beyond standard specifications. Considering this delay 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 an 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 THS7376 SD filter has a nominal corner (–3-dB) frequency at 10 MHz and a –1-dB passband typically at 9
MHz. This 10-MHz filter is ideal for SD NTSC, PAL, and SECAM composite video (CVBS) signals. The 10-MHz,
–3-dB corner frequency is designed to achieve 46 dB of attenuation at 27 MHz—a common sampling frequency
between the DAC and 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.
The device HD filters have a nominal corner (–3-dB) frequency at 42 MHz and a –1-dB passband typically at 39
MHz. This 42-MHz filter is ideal for 720p, 1080i, 1080p24, or 1080p30 component video. This filter is also ideal
for oversampling systems where the video DAC upsamples the video signal (such as 480i or 480p upsampled to
>74MHz). The benefit is an extremely flat passband response along with almost no group delay within the HD
video passband.
Keep in mind that images do not stop at the DAC sampling frequency, f_S (for example, 27 MHz for traditional SD
DACs); they continue around the sampling frequencies of 2x f_S, 3x f_S, 4x f_S, and so on (that is, 54 MHz, 81 MHz,
108 MHz, and so forth). An ADC will fold down these images into the baseband signal which meanis that the
low-pass filter must eliminate these higher-order images. The device filters are Butterworth filters and, as such,
do not bounce at higher frequencies, thus maintaining good attenuation performance.
The filter frequencies are chosen to account for process variations in the device. 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 antialiasing and
reconstruction filtering is sufficient to meet system demands. Thus, selection of the filter frequencies is not
arbitrarily selected and is a good compromise that should meet the demands of most systems.
BENEFITS OVER PASSIVE FILTERING
Two key benefits of using an integrated filter system, such as the THS7376, over a passive system are PCB area
and filter variations. The small TSSOP-14 package for four video channels is much smaller over a passive RLC
network, especially a seventh-order passive network. Additionally, consider that inductors have ±5% up to ±20%
tolerances and capacitors typically have ±5% up to ±10% tolerances. Using a Monte Carlo analysis shows that
the filter corner frequency (–3 dB), flatness (–1 dB), Q factor (or peaking), and channel-to-channel delay have
wide variations. These variances can lead to potential performance and quality issues in mass-production
environments. The THS7376 solves most of these problems with the corner frequency being essentially the only
variable.
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