LMH6601MG NSC [National Semiconductor], LMH6601MG Datasheet - Page 23

• Manufacturer Part Number: LMH6601MG
• Description: 250 MHz, 2.4V CMOS Op Amp with Shutdown
• Manufacturer: NSC [National Semiconductor]

Application Information
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
For any video line rate (HxV corresponding to the number of
Active horizontal and vertical lines), the speed requirements
can be estimated if the Horizontal Active (KH%) and Vertical
Active (KV%) numbers are known. These percentages cor-
respond to the percentages of the active number of lines
(horizontal or vertical) to the total number of lines as set by
VESA standards. Here are the general expressions and the
specific calculations for the SVGA line rate shown in Table 2.
Requiring that an “On” pixel is illuminated to at least 90% of
its final value before changing state will result in the rise/fall
time equal to, at most,
Assuming a single pole frequency response roll-off charac-
teristic for the closed loop amplifier used, we have:
Rise/Fall times are 10%-90% transition times, which for a 2
V
1.6V (80% of 2V). So, the Slew Rate requirement can be
calculated as follows:
PP
TV_NTSC
Standard
SVGA
SXGA
UXGA
Video
VGA
XGA
video step would correspond to a total voltage shift of
TABLE 2. Rise Time, −3 dB BW, and Slew Rate Requirements for Various Video Line Rates
1280x1024
1600x1200
Line Rate
1024x768
451x483
640x480
800x600
(HxV)
1
⁄
3
the pixel time as shown below:
Refresh Rate
(Hz)
30
75
75
75
75
75
(Continued)
Active (KH%)
Horizontal
84
80
76
77
75
74
(5)
(6)
(7)
(8)
23
Active (KV%)
Vertical
sheet tables) as video signals could be as large as 2 V
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.
The LMH6601 specifications show that it would be a suitable
choice for video amplifiers up to and including the SVGA line
rate as demonstrated above.
For more information about this topic and others relating to
video amplifiers, please see Application Note 1013:
http://www.national.com/an/AN/AN-1013.pdf#page=1
CURRENT TO VOLTAGE CONVERSION
(TRANSIMPEDANCE AMPLIFIER (TIA))
Being capable of high speed and having ultra low input bias
current makes the LMH6601 a natural choice for Current to
Voltage applications such as photodiode I-V conversion. In
these type of applications, as shown in Figure 13 below, the
photodiode is tied to the inverting input of the amplifier with
R
F
92
95
96
95
96
96
set to the proper gain (gain is measured in Ohms).
FIGURE 13. Typical Connection of a Photodiode
Pixel Time
118.3
(ns)
33.0
20.3
12.4
Detector to an op amp
7.3
4.9
Rise Time
39.4
(ns)
11.0
6.8
4.1
2.4
1.6
LSBW
(MHz)
143
213
32
52
85
9
www.national.com
(V/µs)
20136458
146
237
387
655
973
SR
41
PP