300MHz High-Speed Op Amp

_______________________________________________________________________________________

9

Then:

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

factor:

√Hz, and assum-

ing a signal bandwidth of 300MHz (471MHz noise

bandwidth), total output noise in this bandwidth is:

Note that for both DC and noise calculations, errors are

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 75

Ω 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.

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 the 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.

capacitive load. Figure 6 shows the Bode plots that

result when a 20

Ω 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

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.

R

e

KT x

nV

Hz at

C

e

nV

nV

pA x

nV Hz

EQ

R

T

=

+

(

=

=

°

=

75

500

500

325

4

325

2 3

25

5

2 3

2

325

5 5

2

2

2

Ω

Ω

Ω

Ω

.

/

.

.

||

e

= 5.5nV x

OUT

1

500

500

471

239

+













=

x

MHz

µ

e

= e

1+

R

R

OUT

T

f

g

















MAX477

Figure 3. Output Offset Voltage

15

10

5

0

1M

10M

100M

1G

-20

-5

-10

-15

FREQUENCY (Hz)