Output Capacitive Loading and Stability

The MAX4200–MAX4205 provide maximum AC per-

formance with no load capacitance. This is the case

when the load is a properly terminated transmission line.

However, these devices are designed to drive any load

capacitance without oscillating, but with reduced AC per-

formance.

Since the MAX4200–MAX4205 operate in an open-

loop configuration, there is no negative feedback to be

transformed into positive feedback through phase shift

introduced by a capacitive load. Therefore, these devices

will not oscillate with capacitive loading, unlike similar

buffers operating in a closed-loop configuration. However,

a capacitive load reacting with the buffer’s output imped-

ance can still affect circuit performance. A capacitive load

will form a lowpass filter with the buffer’s output resistance,

thereby limiting system bandwidth. With higher capacitive

loads, bandwidth is dominated by the RC network formed

higher. Also note that the isolation resistor forms a divider

that decreases the voltage delivered to the load.

Another concern when driving capacitive loads results

from the amplifier’s output impedance, which looks induc-

tive at high frequency. This inductance forms an L-C reso-

nant circuit with the capacitive load and causes peaking in

the buffer’s frequency response.

Figure 2 shows the frequency response of the MAX4200/

MAX4203 under different capacitive loads. To settle

out some of the peaking, the output requires an isola-

tion resistor like the one shown in Figure 3. Figure 4 is

a plot of the MAX4200/MAX4203 frequency response

with capacitive loading and a 10Ω isolation resistor.

In many applications, the output termination resistors

included in the MAX4201/MAX4202/ MAX4204/MAX4205

will serve this purpose, reducing component count and

board space. Figure 5 shows the MAX4201/MAX4202/

MAX4204/MAX4205 frequency response with capacitive

loads of 47pF, 68pF, and 120pF.

Coaxial Cable Drivers

Coaxial cable and other transmission lines are easily

driven when properly terminated at both ends with their

characteristic impedance. Driving back-terminated trans-

mission lines essentially eliminates the line’s capacitance.

The MAX4201/MAX4204, with their integrated 50Ω output

termination resistors, are ideal for driving 50Ω cables.

The MAX4202/MAX4205 include integrated 75Ω termina-

tion resistors for driving 75Ω cables. Note that the output

termination resistor forms a voltage divider with the load

resistance, thereby decreasing the amplitude of the sig-

nal at the receiving end of the cable by one half (see the

Typical Application Circuit).

MAX4200–MAX4205

Ultra-High-Speed, Low-Noise, Low-Power,

SOT23 Open-Loop Buffers

www.maximintegrated.com

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