MAX4278EPA中文资料(10)

330MHz, Gain of +1/Gain of +2 Closed-Loop BuffersMAX4178/MAX4278

__________Applications Information

Grounding, Bypassing, and PC Board Layout

In order toobtain the MAX4178/MAX4278’s full 330MHz/310MHz bandwidths, Micro-Strip and Stripline tech-niques are recommended in most cases. To ensurethat the PC board does not degrade the amplifier’s per-formance, it’s a good idea to design the board for a fre-quency greater than 1GHz. Even with very short traces,it’s good practice to use these techniques at criticalpoints, such as inputs and outputs. Whether you use aconstant-impedance board or not, observe the follow-ing guidelines when designing the board:

Do not use wire-wrap boards. They are too inductive. Do not use IC sockets. They increase parasiticcapacitance and inductance.

In general, surface-mount components have shorterleads and lower parasitic reactance, giving betterhigh-frequency performance than through-hole com-ponents. The PC board should have at least two layers, withone side a signal layer and the other a ground plane. Keep signal lines as short and straight as possible.Do not make 90°turns; round all corners. The ground plane should be as free from voids aspossible.On Maxim’s evaluation kit, the ground plane has beenremoved from areas where keeping the trace capaci-tance to a minimum is more important than maintainingground continuity.

capacitor combine to add a pole and excess phase tothe loop response. If the frequency of this pole is lowenough and if phase margin is degraded sufficiently,oscillations may occur.

A second problem when driving capacitive loadsresults from the amplifier’s output impedance, whichlooks inductive at high frequency. This inductanceforms an L-C resonant circuit with the capacitive load,which causes peaking in the frequency response anddegrades the amplifier’s gain margin.

The MAX4178/MAX4278 drive capacitive loads up to100pF without oscillation. However, some peaking (inthe frequency domain) or ringing (in the time domain)may occur. This is shown in Figures 2a and 2b and thein the Small- and Large-Signal Pulse Response graphsin the Typical Operating Characteristics.

To drive larger-capacitance loads or to reduce ringing,add an isolation resistor between the amplifier’s outputand the load, as shown in Figure 1.

The value of RISOdepends on the circuit’s gain and thecapacitive load. Figures 3a and 3b show the Bodeplots that result when a 20 isolation resistor is usedwith a voltage follower driving a range of capacitiveloads. At the higher capacitor values, the bandwidth isdominated by the RC network, formed by RISOand CL;the bandwidth of the amplifier itself is much higher.Note that adding an isolation resistor degrades gainaccuracy. The load and isolation resistor form a dividerthat decreases the voltage delivered to the load.

Driving Capacitive Loads

The MAX4178/MAX4278 provide maximum AC perfor-mance with no output load capacitance. This is thecase when the MAX4178/MAX4278 are driving a cor-rectly terminated transmission line (e.g., a back-termi-nated 75 cable). However, the MAX4178/MAX4278are capable of driving capacitive loads up to 100pFwithout oscillations, but with reduced AC performanceDriving large capacitive loads increases the chance ofoscillations in most amplifier circuits. This is especiallytrue for circuits with high loop gains, such as voltagefollowers. The amplifier’s output resistance and the load

搜索“diyifanwen.net”或“第一范文网”即可找到本站免费阅读全部范文。收藏本站方便下次阅读，第一范文网，提供最新小学教育MAX4278EPA中文资料(10)全文阅读和word下载服务。