标签：
测放电路
来源：EDN/作者：Mariᮠ?tofka, Slovak University of Technology, Bratislava, Slovakia 发表时间：2007-10-08
没有反馈电阻的仪表放大器提
供精确增益，无论增益多大，放大器的同向和反向输入端提供高输入阻抗。典型仪表放大器IC中，连接两路增益调整引脚的单一电阻决定电路整体增益。绝大多数
仪表放大器的完整译本说明管脚为一致增益而保持断开，但是需要设置大于1的有限增益值。虽然增益调整电阻也许由微小的表贴器件组成，但是它的电极和内部电
阻层扩展了连接IC增益调整引脚的传导表面。扩展的表面起到天线作用，从而使放大器更容易受到外部杂散磁场的影响。
图1显示了不使用任何外部电阻的仪表放大器提供两路增益的一个。电路由IC1的两路通道形成的一系列不对称差分输出放大器组成。Analog Devices公司的AD8222仪表放大器和由一半IC2第二个AD8222的差分放大器组成。电路中所有三路仪表放大器部分提供单机增益。因为第一级的差分输出有相反的趋势，它们的差别输出是输入信号差的两倍。
电路的最大增益误差不超过δ2=3δ1的值，而增益为1时，δ1表示AD8222部分的最大增益误差。对B级IC，δ2≤ 0.06%时计算δ2的值（参考文献1）。典型的，δ2的值很少达到最大值。给出合理的假设，所有三路仪表
放大器增益误差是独立和服从高斯分布的，δ
2=3δ
1出现的可能性约为单运放出现δ
1最大增益误差的1/20。
英文原文：
Gain-of-two instrumentation amplifier uses no external resistors
Mariᮠ?tofka, Slovak University of Technology, Bratislava,
Slovakia; Edited by Brad Thompson and Fran Granville -- EDN, 2/15/2007
An instrumentation amplifier offers precise gain without feedback
resistors, and, at any value of gain, it provides high input impedances
at its noninverting and inverting inputs. In a typical IC
instrumentation amplifier, a single resistor that connects across two
gain-adjustment pins determines the circuit's overall gain. Integrated
versions of most instrumentation amplifiers allow the pins to remain
open for unity gain but require finite-value gain-setting resistors for
gains exceeding one. Although the gain-adjustment resistor might
comprise a tiny surface-mounted device, its electrodes and internal
resistive layer extend the conductive surface connected to the IC's
gain-adjustment pins. The extended surface acts as an antenna and thus
makes the amplifier more susceptible to stray external electromagnetic
fields.
Figure 1 shows an instrumentation amplifier that offers a gain of
two without using any external resistors. The circuit comprises a
cascade of asymmetrical, differential-output amplifier, formed by two
channels of IC1; an Analog Devices AD8222 instrumentation amplifier;
and a difference amplifier comprising one half of IC2, a second AD8222.
All three instrumentation-amplifier sections in the circuit provide a
stand-alone gain of one. Because the differential outputs of the first
stage have opposite signs, their difference is twice that of the
difference of the input signals.
The circuit's worst-case gain error does not exceed the value of
δ2=3δ1, where, at a gain of one, δ1 represents the maximum gain error
of one section of the AD8222.
For B-grade ICs, you calculate the value of δ2 as δ2≤ 0.06% (Reference
1). Typically, the value of δ2 rarely reaches its maximum value. Given
the r
easonable
assumptions that all three amplifiers' gain errors are independent and
obey a gaussian distribution, the probability of occurrence of δ2=3δ1
is about 1/20 the probability of encountering a single amplifier that
has a maximum gain error of δ1.
Reference
"AD8222 Precision, Dual-Channel Instrumentation Amplifier," Analog Devices Inc.
英文原文地址：http://www.edn.com/article/CA6413786.html
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