运放中不使用引脚正确处理方式

一块集成运算放大器通常包括多个分离的运放单元，一般我们不能全部用到，那么这些不使用的运放单元怎么处理？图1示意了一种元件最少的正确处理方式，图2是一些常见的错误方式，这些方式因为浮空输入或者地平面浮动等等可能会引起振荡。

图1 不使用运放单元的正确处理方式

 图2 不使用运放单元的错误处理方式

以下是对于错误处理方式的原文解释

A. This is the worst thing to do with an uncommitted op amp. Both inputs are floating and willpick up noise and the output may switch from rail to rail unpredictably. This configuration willdraw varying amounts of supply current and will couple noise into the other op amps and mayadd noise to the power and ground traces.

B. These two circuits are not recommended because they are stressful to the op amp inputs.This circuit applies the maximum potential across the input and can damage certain op ampssuch as those with diode clamps between the inputs. The op amp output will be in a knownstate and will be railed low in the first circuit and railed high in the second. Railing an op amp's output low causes the op amp to consume more supply current.

C. These two circuits are bad because the output will be railed in an unknown state. The outputstate will be determined by the value of the input stage's offset voltage. Due to the randomness of input offset voltage it cannot be predetermined if the output will be railed high or low. Inaddition to the output's unknown state there is a likely chance the output will change states over
www.maxim-ic.com/an1957 Page 4 of 5 temperature inducing unnecessary noise in the circuit. Railing an op amp's output low causes the op amp to consume more supply current.

D. These two circuits are the most common mistake made when terminating an uncommittedop amp. They are equally bad because the op amp's output will be railed low, causing the opamp to consume more supply current.

 

Avoiding The Uncommitted Amplifier Yields Many Benefits

In many cases it is beneficial to avoid having an uncommitted amplifier. The next-generation of sub-micron technology op amps are affordable and are available in tiny packages. Instead of using three of four op amps in a TSSOP package consider using only what you need. Using multiple single op amps in small SC70 packages or a combination of a dual op amp in a tiny SOT23 package and a single op amp offers the following benefits:
·  Avoids the issue of the uncommitted amplifier
·  Reduces power dissipation by eliminated extra amplifiers
·  Reduces PCB area by using smaller components (Figure 4)
·  Eases PCB layout and reduces traces length
·  Less expensive