LM7131 datasheet(18/23 Pages) NSC

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Using the LM7131 (Continued)

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

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

Ω

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

Ω video

cable (150

Ω load) when powered by a +5V supply. See Fig-

ures 2, 3, 4. This makes it a good choice for video output for

portable equipment, personal digital devices, and desktop

video applications.

DS012313-21

FIGURE 13.

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