Datasheet

LOW-PASS FILTER
DAC/Encoder
+2.6Vto+5V Enable
75 W
Out1
Channel1
R
75 W
LPF
3-Pole
20-MHz
6dB
x1
Level
Shift
Out2
Channel2
R
75 W
75 W
LPF
3-Pole
20-MHz
6dBx1
Level
Shift
Channel3
R
Out3
75 W
75 W
LPF
3-Pole
20-MHz
6dBx1
Level
Shift
330 Fm
330 Fm
330 Fm
THS7319
SBOS468A – JUNE 2009 – REVISED JULY 2009 .............................................................................................................................................................. www.ti.com
Figure 52 shows a typical configuration where the change in phase (radians/second) divided by a
input is dc-coupled and the output is also ac-coupled. change in frequency. An increase in group delay
AC-coupled inputs are generally required when corresponds to a time domain pulse response that
current-sink DACs are used or the input is connected has overshoot and some possible ringing associated
to an unknown source, such as when the THS7319 is with the overshoot.
used as an input device.
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
Each channel of the THS7319 incorporates a
significant overshoot and ringing. While these filters
third-order low-pass filter. These video reconstruction
may help meet the video standard specifications with
filters minimize DAC images from being passed onto
respect to amplitude attenuation, the group delay is
the video receiver. Depending on the receiver design,
well beyond the standard specifications. Considering
failure to eliminate these DAC images can cause
this group delay with the fact that video can go from a
picture quality problems as a result of aliasing of the
white pixel to a black pixel over and over again, it is
ADC in the receiver. Another benefit of the filter is to
easy to see that ringing can occur. Ringing typically
smooth out aberrations in the signal that DACs
causes a display to have ghosting or fuzziness
typically have associated with the digital stepping of
appear on the edges of a sharp transition. On the
the signal. This benefit helps with picture quality and
other hand, a Bessel filter has ideal group delay
ensures that the signal meets video bandwidth
response, but the rate of attenuation is typically too
requirements.
low for acceptable image rejection. Thus, the
Each filter has an associated Butterworth
Butterworth filter is a respectable compromise for
characteristic. The benefit of the Butterworth
both attenuation and group delay.
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
Figure 52. Typical DC Input System Driving AC-Coupled Video Lines
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