400MHz, Ultra-Low-Distortion Op Amps

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9

_______________Detailed Description

Choosing Resistor Values

Unity-Gain Configuration

The MAX4108 is internally compensated for unity gain.

When configured for unity gain, the device requires a

small resistor in series with the feedback path. This

resistor improves the AC response by reducing the Q

of the tank circuit, which is formed by parasitic feed-

back inductance and capacitance.

Inverting and Noninverting Configurations

The values of the gain-setting feedback and input resis-

tors are important design considerations. Large resistor

values will increase voltage noise, and will interact with

the amplifier’s input and PC board capacitance to gen-

erate undesirable poles and zeros, which can decrease

bandwidth or cause oscillations. For example, a nonin-

verting gain of +2, using 1k

Ω resistors combined with

2pF of input capacitance and 0.5pF of board capaci-

tance, will cause a feedback pole at 128MHz. If this

pole is within the anticipated amplifier bandwidth, it will

jeopardize stability. Reducing these 1k

Ω resistors to

100

Ω will extend the pole frequency to 1.28GHz, but

could limit output swing by adding 200

Ω in parallel with

the amplifier’s load. Clearly, the selection of resistor val-

ues must be tailored to the specific application.

The MAX4108/MAX4109/MAX4308/MAX4309 are ultra-

low-distortion, high-bandwidth op amps. The output

distortion will be degraded as the total load resistance

seen by the amplifier decreases. To minimize distortion

products, keep the input and gain-setting resistors rela-

tively large. A 500

Ω feedback resistor combined with

an appropriate input resistor to set the gain will provide

excellent AC performance without significantly increas-

ing distortion.

_____________________Pin Description

PIN

NAME

FUNCTION

1

N.C.

No Connection. Not internally connected.

2

IN-

Inverting Input

3

IN+

Noninverting Input

4, 5

6

OUT

Amplifier Output

7, 8

PART

MAX4108

MAX4109

MAX4308

MAX4309

24

500

500

500

∞

500

125

56

GAIN (V/V)

1

2

5

10

Figure 1a. Using an Isolation Resistor for High Capacitive

Loads

0

20

25

15

10

5

190

10

40

70

100

130

160

220

CAPACITANCE (pF)

MAX4109/MAX4309

MAX4108

MAX4308