THS7319_14 Datasheet, PDF (20/34 Pages) Texas Instruments – 3-Channel, Very Low Power Video Amplifiers with EDTV Filters and 6-dB Gain

THS7319
SBOS468A – JUNE 2009 – REVISED JULY 2009.............................................................................................................................................................. www.ti.com
Figure 52 shows a typical configuration where the
input is dc-coupled and the output is also ac-coupled.
AC-coupled inputs are generally required when
current-sink DACs are used or the input is connected
to an unknown source, such as when the THS7319 is
used as an input device.
LOW-PASS FILTER
Each channel of the THS7319 incorporates a
third-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 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, are 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 a respectable compromise for
both attenuation and group delay.
Channel 1
Channel 2
Channel 3
x1
R
x1
R
x1
R
Level
Shift
Level
Shift
Level
Shift
3-Pole
20-MHz
LPF
3-Pole
20-MHz
LPF
3-Pole
20-MHz
LPF
6 dB
6 dB
6 dB
75 W ^{330 mF Out 1}
75 W ^{330 mF Out 2}
75 W ^{330 mF Out 3}
+2.6 V to +5 V Enable
Figure 52. Typical DC Input System Driving AC-Coupled Video Lines
75 W
75 W
75 W
20
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