MAX477 Maxim, MAX477 Datasheet - Page 10

• Manufacturer Part Number: MAX477
• Description: 300MHz High-Speed Op Amp
• Manufacturer: Maxim

300MHz High-Speed Op Amp
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 50Ω input resistance
and a 1.2GHz input bandwidth.
Figure 5. Capacitive-Load Driving Circuit
Figure 6. Effect of C
Isolation Resistor
Figure 7. Preamp for Video Digitizer
10
V
______________________________________________________________________________________
IN
500
VIDEO IN
-1
-2
-3
-4
-5
-6
1
0
MAX477
1M
R
ISO
LOAD
MAX477
= 20
500
on Frequency Response With
10M
FREQUENCY (Hz)
C
C
L
L
C
= 22pF
= 100pF
L
= 47pF
Flash ADC Preamp
R
ISO
100M
C
L
C
(MAX100/MAX101)
L
= 0pF
FLASH ADC
1G
R
L
V
OUT
In a gain of +2 configuration, the MAX477 makes an
excellent driver for back-terminated 75Ω video coaxial
cables (Figure 8). The high output-current drive allows
the attachment of up to six ±2Vp-p, 150Ω 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.
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.
Figure 8. High-Performance Video Distribution Amplifier
VIDEO IN
500
MAX477
500
Wide-Bandwidth Bessel Filter
High-Performance Video
Distribution Amplifier
75
75
75
75
75
75
75
75
75
OUT1
OUT2
OUTN