TA0310 Datasheet, PDF (11/14 Pages) STMicroelectronics – High Speed Operational Amplifiers for 75Ω Video Lines

Notes on video line driving
TA0310
Reconstruction Filtering:
The output stage of an MPEG decoder is a DAC (digital analog converter). This DAC makes an analog
video signal from the digital one used for video treatments on previous stages. During this conversion the
sampling frequency will be unfortunately in the video spectrum as a parasitic frequency which causes
distortion and noise in the video band. A low-pass filter (called a reconstruction filter) is used to remove
this parasitic frequency. The cut-off frequency of this filter fits with the useful video bandwidth (6MHz,
12MHz or 30MHz). The order is directly linked to the value of the sampling frequency compared to the
video band. An increasing of the sampling frequency allows a lower order of the filter. Some of the newer
designs of DAC target a higher frequency sampling in order to remove the filtering on the future solutions.
Figure 13: Filtering of the analog output
Video Chip
+Vcc
DAC
Fs
LPF
+
T_- SH
Rg ^Rfb
-Vcc
Digital-to-Analog Filtering
Buffering
Conversion
(Reconstruction)
video line
75Ω
75Ω
Matching and Line
Examples:
Figure 14: STi5518 (set-top-box chip). Fs=27MHz for SD outputs. 6MHz LPF to limit the 27MHz
Amplitude
1Vp-p
Filter
Parasitic frequency due
to sampling of the DAC
6MHz
27MHz
(STi5518)
21MHz
Frequency
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