OPA358 Datasheet, PDF (9/14 Pages) Burr-Brown (TI) – 3V Single-Supply 80MHz High-Speed Op Amp in SC70

OPA358
www.ti.com
Chrominance-to-luminence gain inequality (or relative
chrominance level) is a change in the gain ratio of the
chrominance and luminence components of a video
signal, which are at different frequencies. A common test
pattern is the pulse in test pattern CCIR 17, shown in
Figure 5. As in Figure 3 and Figure 4, the top waveform
shows the full test pattern. The middle and bottom
waveform are a more detailed view of the critical portion of
the full waveform, with the middle waveform representing
the input signal from the video generator and the bottom
waveform being the OPA358 output to the line.
SB0S296C − MARCH 2004 − REVISED FEBRUARY 2005
100Hz range produces field tilt which can interfere with
proper recovery of synchronization signals in the television
receiver.
600mV
0V
Figure 5. CCIR 17 Test Pattern (PAL)
Gain errors most commonly appear as attenuation or
peaking of the chrominance information. This shows up in
the picture as incorrect color saturation. Delay distortion
will cause color smearing or bleeding, particularly at the
edges of objects in the picture. It may also cause poor
reproduction of sharp luminence transitions.
Figure 3 through Figure 5 show that the OPA358 causes
no visible distortion or change in gain throughout the entire
video frequency range.
OUTPUT SWING TO GND (SYNC PULSE)
Figure 6 shows the output swing capability of the OPA358
by driving the input with a sync level of 0V. The output of
the OPA358 swings very close to 0V, typically to within less
than 5mV with an 150Ω load connected to ground.
SAG CORRECTION
Sag correction provides excellent video performance with
two small output coupling capacitors. It eliminates the
traditional, large 220µF output capacitor. The traditional
220µF circuit (Figure 7a) creates a single low frequency
pole (−3dB frequency) at 5Hz. If this capacitor is made
much smaller, excessive phase shift in the critical 50Hz to
Figure 6. OPA358 Output Swing with Input Sync
Level at 0V
The OPA358 with sag correction (Figure 7b) creates an
amplitude response peak in the 20Hz region. This small
amount of peaking (a few tenths of a dB) provides
compensation of the phase response in the critical 50Hz
to 100Hz range, greatly reducing field tilt. Note that two
significantly smaller and lower-cost capacitors are
required.
220µF _75Ω
G=2
75Ω
a) Traditional Video Circuit
47µF
75Ω
1.3kΩ _22µF
75Ω
499Ω
1kΩ
825Ω
DC Gain = 2.8
AC Gain = 2
b) OPA358 with Sag Correction
Figure 7. Traditional Video Circuit vs OPA358
with Sag Correction
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