300MHz High-Speed Op Amp

10

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Flash ADC Preamp

The MAX477’s high output-drive capability and ability

to drive capacitive loads make it well suited for buffer-

ing the low-impedance input of a high-speed flash

ADC. With its low output impedance, the MAX477 can

drive the inputs of the ADC while maintaining accuracy.

Figure 7 shows a preamp for digitizing video, using the

250Msps MAX100 and the 500Msps MAX101 flash

ADCs. Both of these ADCs have a 50

Ω input resistance

and a 1.2GHz input bandwidth.

High-Performance Video

Distribution Amplifier

In a gain of +2 configuration, the MAX477 makes an

excellent driver for back-terminated 75

Ω video coaxial

cables (Figure 8). The high output-current drive allows

the attachment of up to six ±2Vp-p, 150

Ω loads to the

MAX477 at +25°C. With the output limited to ±1Vp-p,

the number of loads may double. The MAX4278 is a

similar amplifier configured for a gain of +2 without the

need for external gain-setting resistors. For multiple

gain-of-2 video line drivers in a single package, see the

MAX496/MAX497 data sheet.

Wide-Bandwidth Bessel Filter

Two high-impedance inputs allow the MAX477 to be

used in all standard active filter topologies. The filter

design is straightforward because the component val-

ues can be chosen independently of op amp bias.

Figure 9 shows a wide-bandwidth, second-order Bessel

filter using a multiple feedback topology. The compo-

nent values are chosen for a gain of +2, a -3dB band-

width of 10MHz, and a 28ns delay. Figure 10a shows a

square-wave pulse response, and Figure 10b shows the

filter’s frequency response and delay. Notice the flat

delay in the passband, which is characteristic of the

Bessel filter.

MAX477

Figure 5. Capacitive-Load Driving Circuit

MAX477

VIDEO IN

75

Ω

500

Ω

500

Ω

75

Ω

75

Ω

75

Ω

75

Ω

75

Ω

75

Ω

75

Ω

75

Ω

OUT1

OUT2

OUTN

Figure 8. High-Performance Video Distribution Amplifier

MAX477

VIDEO IN

500

Ω

500

Ω

FLASH ADC

(MAX100/MAX101)

Figure 7. Preamp for Video Digitizer

Isolation Resistor

1

0

-1

-2

1M

10M

100M

1G

-6

-3

-4

-5

FREQUENCY (Hz)