LMH6601_09 Datasheet, PDF (22/28 Pages) National Semiconductor (TI) – 250 MHz, 2.4V CMOS Operational Amplifier with Shutdown

FIGURE 11. Headroom Considerations with AC Coupled Amplifiers
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If a stage has a 3 V_PP unclipped swing capability available at
a given node, as shown in Figure 11, the maximum allowable
amplitude for an arbitrary waveform is ½ of 3V or 1.5 V_PP. This
is due to the shift in the average value of the waveform as the
duty cycle varies. Figure 11 shows what would happen if a
2 V_PP signal were applied. A low duty cycle waveform, such
as the one in Figure 11B, would have high positive excur-
sions. At low enough duty cycles, the waveform could get
clipped on the top, as shown, or a more subtle loss of linearity
could occur prior to full-blown clipping. The converse of this
occurs with high duty cycle waveforms and negative clipping,
as depicted in Figure 11C.
HOW TO PICK THE RIGHT VIDEO AMPLIFIER
Apart from output current drive and voltage swing, the op amp
used for a video amplifier/cable driver should also possess
the minimum requirement for speed and slew rate. For video
type loads, it is best to consider Large Signal Bandwidth (or
LSBW in the National Semiconductor data sheet tables) as
video signals could be as large as 2 V_PP when applied to the
commonly used gain of +2 configuration. Because of this rel-
atively large swing, the op amp Slew Rate (SR) limitation
should also be considered. Table 2 shows these require-
ments for various video line rates calculated using a rudimen-
tary technique and intended as a first order estimate only.
TABLE 2. Rise Time, −3 dB BW, and Slew Rate Requirements for Various Video Line Rates
Video
Standard
TV_NTSC
VGA
SVGA
XGA
SXGA
UXGA
Line Rate
(HxV)
451x483
640x480
800x600
1024x768
1280x1024
1600x1200
Refresh Rate
(Hz)
30
75
75
75
75
75
Horizontal
Active (KH%)
84
80
76
77
75
74
Vertical
Active (KV%)
92
95
96
95
96
96
Pixel Time
(ns)
118.3
33.0
20.3
12.4
7.3
4.9
Rise Time
(ns)
39.4
11.0
6.8
4.1
2.4
1.6
LSBW
(MHz)
9
32
52
85
143
213
SR
(V/μs)
41
146
237
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655
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