LM7131BCM5X National Semiconductor, LM7131BCM5X Datasheet - Page 18

• Manufacturer Part Number: LM7131BCM5X
• Description: Tiny High Speed Single Supply Operational Amplifier
• Manufacturer: National Semiconductor

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Using the LM7131
CCD Amplifiers
The LM7131 has enough gain bandwidth to amplify low level
signals from a CCD or similar image sensor and drive a flash
analog-to-digital converter with one amplifier stage.
Signals from CCDs, which are used in scanners, copiers,
and digital cameras, often have an output signal in the 100
mV–300 mV range. See Figure 14 for a conceptual diagram.
With a gain of 6 the output to the flash analog-to- digital con-
verter is 1.8V, matching 90% of the converter’s 2V input
range. With a −3db bandwidth of 70 MHz for a gain of +1, the
bandwidth at a gain of 6 will be 11.6 MHz. This 11.6 MHz
bandwidth will result in a time constant of about 13.6 ns. This
will allow the output to settle to 7 bits of accuracy within 4.9
time constants, or about 66 ns. Slewing time for a 1.8V step
will be about 12 ns. The total slewing and settling time will be
about 78 ns of the 150 ns pixel valid time. This will leave
about 72 ns total for the flash converter signal acquisition
time and tolerance for timing signals.
For scanners and copiers with moving scan bars, the
SOT23-5 package is small enough to be placed next to the
light sensor. The LM7131 can drive a cable to the main elec-
tronics section from the scan bar. This can reduce noise
pickup by amplifying the signal before sending on the cable.
A/D Reference Drivers
The LM7131’s output and drive capability make it a good
choice for driving analog-to-digital references which have
suddenly changing loads. The small size of the SOT23-5
package allow the LM7131 to be placed very close to the A/D
reference pin, maximizing response. The small size avoids
the penalty of increased board space. Often the SOT23-5
package is small enough that it can fit in space used by the
(Continued)
FIGURE 13.
18
large capacitors previously attached to the A/D reference. By
acting as a buffer for a reference voltage, noise pickup can
be reduced and the accuracy may be increased.
For additional space savings, the LM4040 precision voltage
reference and LM385 low current voltage reference are
available in a tiny SOT23-3 package.
Video Gain of +2
The design of the LM7131 has been optimized for gain of +2
video applications. Typical values for differential gain and
phase are 0.25% differential gain and 0.75 degree differen-
tial phase. See Figure 12 .
Improving Video Performance
Differential gain and phase performance can be improved by
keeping the active video portion of the signal above 300 mV.
The sync signal can go below 300 mV without affecting the
video quality. If it is possible to AC couple the signal and shift
the output voltage slightly higher, much better video perfor-
mance is possible. For a +5V single supply, an output range
between 2.0V and 3.0V can have a differential gain of 0.07%
and differential phase of 0.3 degree when driving a 150
load. For a +3V single supply, the output should be between
1.0V and 2.0V.
Cable Driving with +5V Supplies
The LM7131 can easily drive a back-terminated 75
cable (150
ures 2, 3, 4 . This makes it a good choice for video output for
portable equipment, personal digital devices, and desktop
video applications.
load) when powered by a +5V supply. See Fig-
DS012313-21
video