OPA360 Datasheet, PDF (10/24 Pages) Burr-Brown (TI) – 3V Video Amplifier with 6dB Gain and Filter in SC70

OPA360
SB0S294E − DECEMBER 2003 − REVISED SEPTEMBER 2006
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 9a) creates a single low frequency
pole (−3dB frequency) at 5Hz. If this capacitor is made
much smaller, excessive phase shift in the critical 50 to
100Hz range produces field tilt which can interfere with
proper recovery of synchronization signals in the television
receiver.
The OPA360 sag correction circuit (Figure 9b, see also
Figure 14) 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.
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220µF _75Ω
75Ω
a) Traditional Video Circuit
47µF
75Ω
22µF
75Ω
Figure 10. Standard Video Circuit with 220mF
Capacitor (top trace) vs OPA360 with 22mF and
47mF Capacitors
A field tilt equivalent to that achieved using the standard
220µF coupling capacitor can be achieved with a
22µF/67µF combination − see Figure 11. These capacitor
values are optimized—sag correction capacitors larger
than 22µF do not provide significant improvement. Smaller
sag correction capacitors will lead to higher tilt.
b) OPA360 with Sag Correction
Figure 9. Traditional Video Circuit vs OPA360
with Sag Correction
To achieve good performance, a 22µF sag correction and
47µF coupling capacitor can be used. Figure 10 and
Figure 11 show comparisons for a standard video circuit
with a 220µF coupling capacitor and the OPA360 with sag
correction.
Figure 10 shows that the 22µF/47µF combination leads to
only a slightly greater tilt in the 50Hz, 1/2 black − 1/2 white
video signal. No degradation in video quality is observed.
Figure 11. 220mF Standard Video Circuit (top
trace) vs OPA360 with 22mF and 67mF Capacitors
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