400MHz, Ultra-Low-Distortion Op Amps

______________________________________________________________________________________

11

before the reactive load prevents ringing and oscilla-

tion. At higher capacitive loads, AC performance will be

controlled by the interaction of the load capacitance

and isolation resistor. Figures 3a–3c show the effect of

an isolation resistor on the MAX4108/MAX4109/

MAX4308/MAX4309 closed-loop response.

Coaxial cable and other transmission lines are easily

driven when terminated at both ends with their charac-

teristic impedance. When driving back-terminated

transmission lines, the capacitance of the transmission

line is essentially eliminated.

ADC Input Buffers

Input buffer amplifiers can be a source of significant

errors in high-speed ADC applications. The input buffer

is usually required to rapidly charge and discharge the

ADC’s input, which is often capacitive (see the section

Driving Capacitive Loads). In addition, a high-speed

ADC’s input impedance often changes very rapidly

during the conversion cycle, requiring an amplifier with

very low output impedance at high frequencies to main-

tain measurement accuracy. The combination of high

speed, fast slew rate, low noise, and a low and stable

distortion over load makes the MAX4108/MAX4109/

MAX4308/MAX4309 ideally suited for use as buffer

amplifiers in high-speed ADC applications.

12

-8

0.1

1

100

10

1000

-2

FREQUENCY (MHz)

6

4

10

-6

-4

0

2

8

Figure 3a. MAX4108 Response vs. Capacitive Load with

11

1

0.1

1

100

10

1000

4

5

FREQUENCY (MHz)

8

10

2

3

6

7

9

Figure 3b. MAX4308 Response vs. Capacitive Load with

4

-6

0.1

1

100

10

1000

-2

FREQUENCY (MHz)

1

3

-5

-3

-4

-1

0

2

Figure 3c. MAX4108/MAX4309 Response vs. Capacitive Load