Choosing Resistor Values

Unity-Gain Configurations

The MAX4265 and MAX4268 are internally compensat-

ed for unity gain. When configured for unity gain, they

path (Figure 1). This resistor improves AC response by

reducing the Q of the tank circuit, which is formed by

parasitic feedback 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 interact with the

amplifier’s input and PC board capacitance to generate

undesirable poles and zeros, which can decrease

bandwidth or cause oscillations. For example, a nonin-

Ω

combined with 2pF of input capacitance and 0.5pF of

board capacitance will cause a feedback pole at

128MHz. If this pole is within the anticipated amplifier

bandwidth, it will jeopardize stability. Reducing the 1k

Ω

resistors to 100

Ω extends the pole frequency to

1.28GHz, but could limit output swing by adding 200

Ω

in parallel with the amplifier’s load. Clearly, the selec-

tion of resistor values must be tailored to the specific

application.

Distortion Considerations

The MAX4265–MAX4270 are ultra-low-distortion, high-

bandwidth op amps. Output distortion will degrade as

the total load resistance seen by the amplifier decreas-

es. To minimize distortion, keep the input and gain-set-

ting resistor values relatively large. A 500

Ω feedback

resistor combined with an appropriate input resistor to

set the gain will provide excellent AC performance with-

out significantly increasing distortion.

Noise Considerations

The amplifier’s input-referred noise-voltage density is

dominated by flicker noise at lower frequencies and by

thermal noise at higher frequencies. Because the ther-

mal noise contribution is affected by the parallel combi-

nation of the feedback resistive network, those resistor

values should be reduced in cases where the system

bandwidth is large and thermal noise is dominant. This

noise-contribution factor decreases, however, with

increasing gain settings. For example, the input noise

voltage density at the op amp input with a gain of

18nV/

√Hz. The input noise can be reduced to 8nV/√Hz

Driving Capacitive Loads

The MAX4265–MAX4270 are not designed to drive

highly reactive loads; stability is maintained with loads

up to 15pF with less than 2dB peaking in the frequency

response. To drive higher capacitive loads, place a

small isolation resistor in series between the amplifier’s

output and the capacitive load (Figure 1). This resistor

improves the amplifier’s phase margin by isolating the

capacitor from the op amp’s output.

To ensure a load capacitance that limits peaking to less

than 2dB, select a resistance value from Figure 2. For

example, if the capacitive load is 100pF, the corre-

sponding isolation resistor is 6

Ω (MAX4269). Figures 3

and 4 show the peaking that occurs in the frequency

response with and without an isolation resistor.

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 capacitive load of the transmis-

sion line is essentially eliminated.

ADC Input Buffer

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 Driving

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

ADC’s input impedance often changes very rapidly dur-

ing the conversion cycle, measurement accuracy must

Ultra-Low-Distortion, Single-Supply,

300MHz Op Amps with Enable

______________________________________________________________________________________

11

PART

MAX4265

MAX4266

MAX4267

24

500

500

∞

500

125

GAIN (V/V)

1

2

5

MAX4265

MAX4266

MAX4267

Figure 1. Noninverting Configuration