#3
文章发表于：2007-09-27 20:30
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，连接两级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级增益的一个。电路由IC1的两个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，所有三仪表放大器增益误差是独立和服从高斯分布的，δ2=3δ1出现的可能性约为单运放出现δ1最大增益误差的1/20。
#4
文章发表于：2007-09-27 20:30
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 reasonable 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.