LMH6601_14 Datasheet, PDF (25/37 Pages) National Semiconductor (TI) – LMH6601/LMH6601Q 250 MHz, 2.4V CMOS Operational Amplifier with Shutdown

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(A)
50% DUTY CYCLE 2V_p-p
NO CLIPPING
LMH6601, LMH6601-Q1
SNOSAK9E – JUNE 2006 – REVISED MARCH 2013
4.0V (+) CLIPPING
2.5V
1.0V (-) CLIPPING
(B)
2V_p-p
LOW DUTY CYCLE
CLIPPED POSITIVE
4.0V (+) CLIPPING
2.5V
1.0V (-) CLIPPING
4.0V (+) CLIPPING
(C)
HIGH DUTY CYCLE _2Vp-p
CLIPPED NEGATIVE
2.5V
1.0V (-) CLIPPING
Figure 58. Headroom Considerations with AC Coupled Amplifiers
If a stage has a 3 V_PP unclipped swing capability available at a given node, as shown in Figure 58, 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 58 shows what would happen if a 2 V_PP signal were applied. A
low duty cycle waveform, such as the one in Figure 58B, would have high positive excursions. 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 58C.
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 Texas Instruments 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 relatively large swing, the op
amp Slew Rate (SR) limitation should also be considered. Table 2 shows these requirements for various video
line rates calculated using a rudimentary 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
387
655
973
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