MAX477EPA 查看數據表（PDF） - Maxim Integrated

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MAX477EPA

300MHz High-Speed Op Amp

Maxim Integrated 

300MHz High-Speed Op Amp

As an example, consider RS = 75Ω, Rf = Rg = 500Ω.

Then:

( ) REQ = 75Ω + 500Ω ||500Ω = 325Ω

eR = 4KT x 325 = 2.3nV / Hz at + 25°C

eT = (5nV)2 + (2.3nV)2 + (2pA x 325)2 = 5.5nV Hz

3) The MAX477’s output-referred noise is simply total

input-referred noise, eT, multiplied by the gain

factor:

e OUT

=

eT



1+

Rf









Rg





In the above example, with eT = 5.5nV√Hz, and assum-

ing a signal bandwidth of 300MHz (471MHz noise

bandwidth), total output noise in this bandwidth is:

e OUT

=

5.5nV x 1 +

500 

500 

x

471MHz = 239µVRMS

Note that for both DC and noise calculations, errors are

dominated by offset voltage (VOS) and input noise volt-

age (en). For a current-mode feedback amplifier with

offset and noise errors significantly higher, the calcula-

tions are very different.

Driving Capacitive Loads

The MAX477 provides maximum AC performance with

no output load capacitance. This is the case when the

MAX477 is driving a correctly terminated transmission

line (i.e., a back-terminated 75Ω cable). However, the

MAX477 is capable of driving capacitive loads up to

100pF without oscillations, but with reduced AC perfor-

mance.

Driving large capacitive loads increases the chance of

oscillations in most amplifier circuits. This is especially

true for circuits with high loop gain, such as voltage fol-

lowers. The amplifier’s output resistance and the load

capacitor combine to add a pole and excess phase to

the loop response. If the frequency of this pole is low

enough and phase margin is degraded sufficiently,

oscillations may occur.

A second problem when driving capacitive loads

results from the amplifier’s output impedance, which

looks inductive at high frequency. This inductance

forms an L-C resonant circuit with the capacitive load,

which causes peaking in the frequency response and

degrades the amplifier’s gain margin.

Rg

Rf

IB-

VOUT

RS

MAX477

IB+

VIN

Figure 3. Output Offset Voltage

15

10

5

0

-5

-10

-15

-20

1M

CL = 100pF

CL = 41pF

CL = 22pF

CL = 0pF

10M

100M

1G

FREQUENCY (Hz)

Figure 4. Effect of CLOAD on Frequency Response (AVCL = +1V/V)

The MAX477 drives capacitive loads up to 100pF with-

out oscillation. However, some peaking (in the frequen-

cy domain) or ringing (in the time domain) may occur.

This is shown in Figure 4 and in the Small and Large-

Signal Pulse Response graphs in the Typical Operating

Characteristics.

To drive larger-capacitance loads or to reduce ringing,

add an isolation resistor between the amplifier’s output

and the load, as shown in Figure 5.

The value of RISO depends on the circuit’s gain and the

capacitive load. Figure 6 shows the Bode plots that

result when a 20Ω isolation resistor is used with a volt-

age follower driving a range of capacitive loads. At the

higher capacitor values, the bandwidth is dominated by

the RC network, formed by RISO and CL; the bandwidth

of the amplifier itself is much higher. Note that adding

an isolation resistor degrades gain accuracy. The load

and isolation resistor form a divider that decreases the

voltage delivered to the load.

_______________________________________________________________________________________ 9

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