MAX4106 데이터 시트보기 (PDF) - Maxim Integrated

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MAX4106

350MHz, Ultra-Low-Noise Op Amps

Maxim Integrated 

350MHz, Ultra-Low-Noise Op Amps

_____________________Pin Description

PIN

1, 5, 8

2

3

4

6

7

NAME

N.C.

IN-

IN+

VEE

OUT

VCC

FUNCTION

No Connection, not internally connected

Inverting Input

Noninverting Input

Negative Power Supply, connect to -5V

Amplifier Output

Positive Power Supply, connect to +5V

_______________General Description

Choosing Resistor Values

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 +5 (MAX4106), using a 1kΩ feedback

resistor combined with 2pF of input capacitance and

0.5pF of board capacitance, will cause a feedback

pole at 318MHz. If this pole is within the anticipated

amplifier bandwidth, it will jeopardize stability.

Reducing the 1kΩ feedback resistor to 40Ω will extend

the pole frequency to 8GHz, but could limit output

swing by adding 50Ω in parallel with the amplifier’s

load. Clearly the selection of resistor values must be

tailored to the specific application.

The MAX4106/MAX4107 are ultra-low-noise, high-band-

width op amps. The output noise voltage can be domi-

nated by resistor thermal noise, so keep the feedback

and input resistors small. Setting the input resistor to

30Ω and choosing the feedback resistor to suit the gain

will provide excellent AC performance without signifi-

cantly degrading noise performance.

Driving Capacitive Loads

The MAX4106/MAX4107 are optimized for AC perfor-

mance. They are not designed to drive highly reactive

loads. Reactive loads will decrease phase margin and

may produce excessive ringing and oscillation. Figure

1a shows a circuit that eliminates this problem, and

Figure 1b is a graph of the optimal isolation resistor

(RS) vs. capacitive load. Figures 2a and 2b show how a

capacitive load causes excessive peaking of the ampli-

fier’s bandwidth if the capacitive load is not isolated

(RS) from the amplifier. A small isolation resistor (usual-

ly 10Ω to 20Ω) placed before the reactive load prevents

ringing and oscillation. At higher capacitive loads, AC

performance will be controlled by the interaction of the

load capacitance and isolation resistor. Figures 3a and

3b show the effect of an isolation resistor on the

MAX4106/MAX4107 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.

RG

30Ω

RF

MAX4106

MAX4107

VIN

RS

CL

RL

PART

MAX4106

MAX4107

RF (Ω)

120

270

GAIN (V/V)

+5

+10

Figure 1a. Using an Isolation Resistor for High Capacitive

Loads

30

25

MAX4107

20

15

10

MAX4106

5

0

10 40 70 100 130 160 190 220

CAPACITANCE (pF)

Figure 1b. Optimal Isolation Resistor (RS) vs. Capacitive Load

8 _______________________________________________________________________________________

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