1、Chapter-1: IN-AMP BASICS 第一章第一章第一章第一章： ： ：仪表放大器基本原理 仪表放大器基本原理仪表放大器基本原理仪表放大器基本原理 1. INTRODUCTION 简介简介简介 简介 Instrumentation amplifiers (in-amps) are sometimes misunderstood. Not all amplifiers used in instrumentation applications are instrumentation amplifiers, and by no means are all in-amps used onl
2、y in instrumentation applications. In-amps are used in many applications, from motor control to data acquisition to automotive. The intent of this guide is to explain the fundamentals of what an instrumentation amplifier is, how it operates, and how and where to use it. In addition, several differen
3、t categories of instrumentation amplifiers are addressed in this guide. 仪表放大器经常被误解。并非所有用在仪器仪表应用中的放大器都是仪表放大器；也绝非所有仪表放大器只能应用于仪器仪表应用中。仪表放大器应用于多种应用领域，从马达控制到汽车业的数据采集，等等。本文目的就是解释什么是仪表放大器、仪表放大器如何工作、又如何应用，等等一系列基础知识。此外，本文附有不同种类的仪表放大器供查询。 2. IN-AMPS vs. OP AMPS: WHAT ARE THE DIFFERENCES? 仪表放大器仪表放大器仪表放大器仪表放大器
4、- 运算放大器运算放大器运算放大器运算放大器： ： ：区别何在 区别何在区别何在区别何在？ ？ ？ An instrumentation amplifier is a closed-loop gain block that has a differential input and an output that is single-ended with respect to a reference terminal. Most commonly, the impedances of the two input terminals are balanced and have high values
5、, typically 109 , or greater. The input bias currents should also be low, typically 1 nA to 50 nA. As with op amps, output impedance is very low, nominally only a few milliohms, at low frequencies. 仪表放大器为闭环增益模块，为差动输入、单端输出（相对于参考端）。通常，两个输入端阻抗平衡，并且为高阻输入，典型值为 109 甚至更高；输入偏置电流低，典型值 1nA 50nA；输出阻抗极低，如同运算放大器
6、，在低频范围内一般只有数 m。 Unlike an op amp, for which closed-loop gain is determined by external resistors connected between its inverting input and its output, an in-amp employs an internal feedback resistor network that is isolated from its signal input terminals. With the input signal applied across the tw
7、o differential inputs, gain is either preset internally or is user set (via pins) by an internal or external gain resistor, which is also isolated from the signal inputs. 运算放大器，闭环增益由连接在反相输入端和输出端之间的外部电阻决定；仪表放大器则不同，利用内部电阻反馈网络，并且该电阻网络与输入端之间是电气隔离的。输入信号施加于两个差动输入端之间。增益可内部预置，也可用户设置（引脚设置），即通过内部或外部增益电阻实施增益设置
8、，增益电阻网络与输入端之间也是电气隔离的。 Figure 1-1 shows a bridge preamp circuit, a typical in-amp application. When sensing a signal, the bridge resistor values change, unbalancing the bridge and causing a change in differential voltage across the bridge. The signal output of the bridge is this differential voltage,
9、 which connects directly to the in-amp s inputs. In addition, a constant dc voltage is also present on both lines. This dc voltage will normally be equal or common mode on both input lines. In its primary function, the in-amp will normally reject the common-mode dc voltage, or any other voltage comm
10、on to both lines, while amplifying the differential signal voltage, the difference in voltage between the two lines. 图 1-1 所示，为一电桥前置放大电路，也是典型的仪表放大器应用。信号检测过程，构成电桥的电阻值变化，使得电桥不平衡，电桥两端的差分电压发生变化。电桥的输出信号为一差分电压，直接连接到仪表放大器的输入端。此外，在两条输入线上，还存在一个恒定的直流电压，在两条输入线上，这个直流电压一般是相等的，或者说，对两条输入线为共模电压。仪表放大器的一个最基本的功能，就是抑制直
11、流共模电压，以及两条输入线上的其它一切共模电压；同时放大差分信号，即两条输入线上的电压差。 In contrast, if a standard op amp amplifier circuit were used in this application, it would simply amplify both the signal voltage and any dc, noise, or other common-mode voltages. As a result, the signal would remain buried under the dc offset and noise
12、. Because of this, even the best op amps are far less effective in extracting weak signals. Figure 1-2 contrasts the differences between op amp and in-amp input characteristics. 作为对比，本应用如果采用标准运算放大器电路，它将只是一个简单的放大作用，除了信号电压，还对直流、噪声及其它一切共模形式的电压进行放大。结果，信号被掩埋在直流失调电压和噪声中。由于这个原因，即使应用最好的运算放大器进行微弱信号提取，也远远不能奏效
13、。图 1-2，为运算放大器、仪表放大器输入特性的比较。 1） ） Signal Amplification and Common-Mode Rejection 信号放大与共模抑制信号放大与共模抑制信号放大与共模抑制信号放大与共模抑制 An instrumentation amplifier is a device that amplifies the difference between two input signal voltages while rejecting any signals that are common to both inputs. The in-amp, theref
14、ore, provides the very important function of extracting small signals from transducers and other signal sources. 仪表放大器就是一种用来放大两个输入信号之间的的电压差、抑制两个输入端的一切共模信号的器件。因此，仪表放大器的一个非常重要的功能，就是从换能器或其它信号源提取微弱信号。 Common-mode rejection (CMR), the property of canceling out any signals that are common (the same potent
15、ial on both inputs), while amplifying any signals that are differential (a potential difference between the inputs), is the most important function an instrumentation amplifier provides. Both dc and ac common-mode rejection are important in-amp specifications. Any errors due to dc common-mode voltag
16、e (i.e., dc voltage present at both inputs) will be reduced 80 dB to 120 dB by any modern in-amp of decent quality. 共模抑制（ CMR），即在放大差模信号（即两个输入端之间的电势差）的同时，消减共模信号（两个输入端上相等的电势）的特性，是仪表放大器最重要的一个功能。直流共模抑制、交流共模抑制是仪表放大器的两项重要指标。任选一款质量不错的现代仪表放大器，对直流共模电压（即同时出现在两个输入端的直流电压）造成的信号误差，都能提供 80 120 dB衰减。 However, inade
17、quate ac CMR causes a large, time-varying error that often changes greatly with frequency and, therefore, is difficult to remove at the IAs output. Fortunately, most modern monolithic IC in-amps provide excellent ac and dc common-mode rejection. 然而，交流共模抑制能力不足，会导致随频率剧烈变化的、较大的时变误差，很难从仪表放大器输出端去除。但多数现代单
18、片集成仪表放大器都具有极佳的交流、直流共模抑制能力。 Common-mode gain (ACM), the ratio of change in output voltage to change in common-mode input voltage, is related to common-mode rejection. It is the net gain (or attenuation) from input to output for voltages common to both inputs. For example, an in-amp with a common-mode
19、 gain of 1/1000 and a 10 V common mode voltage at its inputs will exhibit a 10 mV output change. The differential or normal mode gain (AD) is the gain between input and output for voltages applied differentially (or across) the two inputs. The common mode rejection ratio (CMRR ) is simply the ratio
20、of the differential gain, AD, to the common-mode gain. Note that in an ideal in-amp, CMRR will increase in proportion to gain. 共模增益（ ACM），即输出电压的变化与共模输入电压的变化之比，与共模抑制能力有关。是两共模电压从输入端到输出端的净增益（或者净衰减）。比如，一个共模增益为 1/1000 的仪表放大器，那么，输入端的 10V 共模电压，会使输出端电压呈现 10mV 的变化。差模增益或者说常模增益（ AD），是施加在两个输入端的差模电压，从输入到输出的增益。共模
21、抑制比（ CMRR）就是差模增益与共模增益的比值。请注意，一个理想的仪表放大器，共模抑制比随增益正比增加。 Common-mode rejection is usually specified for full range common-mode voltage (CMV) change at a given frequency and a specified imbalance of source impedance (e.g., 1k source imbalance, at 60 Hz). 共模抑制比，通常是在给定频率、给定源阻抗不平衡度条件下（比如 60Hz、 1K），对满度共模电压的
22、变化规定的。 Mathematically, common-mode rejection can be represented as 数学上，共模抑制比表示如下 OUTCMD VVACMRR = where: AD is the differential gain of the amplifier; VCM is the common-mode voltage present at the amplifier inputs; VOUT is the output voltage present when a common-mode input signal is applied to the
23、amplifier. 式中， AD 为放大器的差模增益； VCM 为放大器输入端的共模电压； VOUT 为放大器仅施加共模输入信号时，呈现的输出电压。 The term CMR is a logarithmic expression of the common-mode rejection ratio (CMRR). That is, CMR = 20 log10 CMRR. 术语 CMR 为共模抑制比（ CMRR）的对数表示，亦即 CMR = 20 log CMRR。 To be effective, an in-amp needs to be able to amplify microvo
24、lt-level signals while rejecting common-mode voltage at its inputs. It is particularly important for the in-amp to be able to reject common-mode signals over the bandwidth of interest. This requires that instrumentation amplifiers have very high common-mode rejection over the main frequency of inter
25、est and its harmonics. 电路工作的有效性，要求仪表放大器能够在放大 uV 级信号的同时，还要抑制输入端的共模电压。特别重要的，仪表放大器必须能够在所关心的信号带宽内有效地抑制共模信号，（应用中）要求仪表放大器在所关心的信号主频及其谐波范围内，具有极高的共模抑制能力。 For techniques on reducing errors due to out-of-band signals that may appear as a dc output offset, please refer to the RFI section of this guide. 对带外信号导致的
26、、呈现为直流输出偏差的误差信号的抑制技术，请参阅本文射频干扰一节。 At unity gain, typical dc values of CMR are 70 dB to more than 100 dB, with CMR usually improving at higher gains. While it is true that operational amplifiers connected as subtractors also provide common-mode rejection, the user must provide closely matched externa
27、l resistors (to provide adequate CMRR ). On the other hand, monolithic in-amps, with their pretrimmed resistor networks, are far easier to apply. 单位增益时， CMR 典型值为 70dB 到 100dB 以上；高增益工作时， CMR 一般会更高。事实是，运算放大器连接为减法器电路时，也具有较好的共模抑制能力，但是，用户必须使用精密匹配的外部电阻（以得到足够高的 CMRR）；另一方面，单片仪表放大器包含经过预校准的电阻网络，使之更便于使用。 2） ）
28、Common-Mode Rejection: Op Amp vs. In-Amp 共模抑制能力共模抑制能力共模抑制能力共模抑制能力： ： ：运算放大器与仪表放大器 运算放大器与仪表放大器运算放大器与仪表放大器运算放大器与仪表放大器之比较 之比较之比较之比较 Op amps, in-amps, and difference amps all provide common-mode rejection. However, in-amps and diff amps are designed to reject common-mode signals so that they do not appe
29、ar at the amplifiers output. In contrast, an op amp operated in the typical inverting or noninverting amplifier configuration will process common-mode signals, passing them through to the output, but will not normally reject them. 运算放大器、仪表放大器、差动放大器都具有共模抑制能力，然而，仪表放大器和差动放大器专为抑制共模信号而设计，使共模信号不出现在放大器输出端。
30、作为对比，运算放大器配置为典型的反相放大器、同相放大器，可以处理（放大）共模信号，一般不会抑制共模信号，而让它们出现在输出端。 Figure 1-3a shows an op amp connected to an input source that is riding on a common-mode voltage. Because of feedback applied externally between the output and the summing junction, the voltage on the “” input is forced to be the same a
31、s that on the “ +” input voltage. Therefore, the op amp ideally will have zero volts across its input terminals. As a result, the voltage at the op amp output must equal VCM, for zero volts differential input. 图 1-3a 所示，运算放大器电路连接到一个浮于共模电压之上的信号源。由于外部反馈，反相输入端的电压强被制等于同相输入端的电压，因此，运算放大器的两个输入端之间为电压零。结果，在零
32、差动输入时，运算放大器输出端电压必定等于 VCM。 Even though the op amp has common-mode rejection, the common-mode voltage is transferred to the output along with the signal. In practice, the signal is amplified by the op amps closed-loop gain, while the common-mode voltage receives only unity gain. This difference in gai
33、n does provide some reduction in common-mode voltage as a percentage of signal voltage. However, the common mode voltage still appears at the output, and its presence reduces the amplifiers available output swing. For many reasons, any common-mode signals (dc or ac) appearing at the op amps output i
34、s highly undesirable. 即使运算放大器具有共模抑制能力，共模电压还是随信号一起被传送到输出端。实际情况是，信号以运算放大器的闭环增益放大，共模电压仅获得单位增益。这两个增益的差别，使得共模电压在信号电压中的百分比降低。但共模电压依然出现在输出端，共模电压的存在又减小了放大器的可用输出摆幅。由于种种原因，共模信号（包括交流、直流）是非常不希望出现在输出端的。 Figure 1-3b shows a 3-op amp in-amp operating under the same conditions. Note that, just like the op amp circu
35、it, the input buffer amplifiers of the in-amp pass the common-mode signal through at unity gain. In contrast, the signal is amplified by both buffers. The output signals from the two buffers connect to the subtractor section of the IA. Here the differential signal is amplified (typically at low gain
36、 or unity) while the common-mode voltage is attenuated (typically by 10,000:1 or more). Contrasting the two circuits, both provide signal amplification (and buffering), but because of its subtractor section, the inamp rejects the common-mode voltage. 图 1-3b 所示，为同样工作条件下的三运算放大器型仪表放大器电路。请注意，仪表放大器内部的输入缓
37、冲放大器，与运算放大器电路一样，允许共模信号以单位增益通过。不同的是，信号被两个缓冲器放大，两个缓冲器输出的信号连接到仪表放大器内部减法器单元，再此，差模信号被放大（一般为低增益或单位增益），共模信号却被衰减（典型衰减率 10000:1 或更高）。比较这两个电路，二者均可实现信号放大（及缓冲），但由于减法器单元的存在，使仪表放大器可以抑制共模电压。 Figure 1-3c is an in-amp bridge circuit. The in-amp effectively rejects the dc common-mode voltage appearing at the two brid
38、ge outputs while amplifying the very weak bridge signal voltage. In addition, many modern in-amps provide a common-mode rejection approaching 80 dB, which allows powering of the bridge from an inexpensive, non-regulated dc power supply. In contrast, a self-constructed in-amp, using op amps and 0.1%
39、resistors, typically only achieves 48 dB CMR, thus requiring a regulated dc supply for bridge power. 图 1-3C 为仪表放大器电桥电路，在放大非常微弱的电桥输出信号电压时，对呈现在电桥两个输出端的直流共模电压进行有效抑制。许多现代仪表放大器可提供高达 80dB 的共模抑制比，允许电桥采用廉价的非稳压电源供电。相比之下，应用 0.1% 精度的外部电阻网络和运算放大器自行搭接的仪表放大器，所能达到的共模抑制比典型值为 48dB，因而要求电桥必须采用稳压电源供电。 Figure 1-3d shows
40、 a difference (subtractor) amplifier being used to monitor the voltage of an individual cell that is part of a battery bank. Here the common-mode dc voltage can easily be much higher than the amplifiers supply voltage. Some monolithic difference amplifiers, such as the AD629, can operate with common
41、-mode voltages as high as 270 V. 图 1-3d 所示，为用于检测电池组中单节电池电压的差动放大器（减法器）。这里的共模电压很可能高于放大器的电源电压，一些单片差动放大器，比如 AD629，可工作于高达 270 V 的共模电压。 Many difference amplifiers are designed to be used in applications where the common-mode and signal voltages may easily exceed the supply voltage. These diff amps typical
42、ly use very high value input resistors to attenuate both signal and common-mode input voltages. 有许多专门设计的差动放大器，目的就是用于共模电压、信号电压高于电源电压条件下。这些差动放大器一般采用非常大的输入电阻来衰减信号电压和共模输入电压。 3. Difference Amplifiers 差动放大器差动放大器差动放大器差动放大器 Figure 1-4 is a block diagram of a difference amplifier. This type of IC is a specia
43、l-purpose in-amp that normally consists of a subtractor amplifier followed by an output buffer, which may also be a gain stage. The four resistors used in the subtractor are normally internal to the IC, and, therefore, are closely matched for high CMR. 图 1-4 为一种差动放大器的方框图，此类集成电路为特种应用目的仪表放大器，一般由一个减法放大
44、器后跟一个输出缓冲器构成，输出缓冲器也可以是增益级。减法器的四个电阻一般内置于集成电路内部，因而可以做到精确匹配，得到较高的共模抑制能力。 4. WHERE are in-amps and Difference amps used? 仪表放大器和差动放大器用在哪里仪表放大器和差动放大器用在哪里仪表放大器和差动放大器用在哪里仪表放大器和差动放大器用在哪里？ ？ ？ 1） ） Data Acquisition 数据采集数据采集数据采集数据采集 In-amps find their primary use amplifying signals from low level output transd
45、ucers in noisy environments. The amplification of pressure or temperature transducer signals is a common in-amp application. Common bridge applications include strain and weight measurement using load cells and temperature measurement using resistive temperature detectors, or RTDs. 仪表放大器的基本应用是放大来自在噪
46、声环境中的低电平输出换能器的信号，仪表放大器普遍应用于压力、温度换能器信号的放大。通用电桥应用，包括应用荷重传感器的应力测量、重力测量，应用阻性温度传感器或温度遥测器的温度测量。 2） ） Medical Instrumentation 医学应用医学应用医学应用医学应用 In-amps are widely used in medical equipment such as EKG and EEG monitors, blood pressure monitors, and defibrillators. 仪表放大器广泛应用于医疗设备，比如心电图、脑电图监测，血压监测，电击除颤器等。 3） ）
47、 Monitor and Control Electronics 电子监控电子监控电子监控电子监控 Diff amps may be used to monitor voltage or current in a system and then trigger alarm systems when nominal operating levels are exceeded. Because of their ability to reject high common-mode voltages, diff amps are often used in these applications. 差
48、动放大器可用于某个系统的电压或电流监控，超过正常工作电平时触发报警系统。以其对高共模电压的抑制能力，差动放大器经常应用在这些领域。 4） ） Software-Programmable Applications 软件可编程应用软件可编程应用软件可编程应用软件可编程应用 An in-amp may be used with a software-programmable resistor chip to allow software control of hardware systems. 仪表放大器可结合软件可编程的电阻芯片应用，实现硬件系统的软件控制方式。 5） ） Audio Applic
49、ations 音频应用音频应用音频应用音频应用 Because of their high common-mode rejection, instrumentation amplifiers are sometimes used for audio applications (as microphone preamps, for example), to extract a weak signal from a noisy environment, and to minimize offsets and noise due to ground loops. Refer to Table 6-4 (page 6-26), Specialty Products Available from Analog Devices. 以其优异的共模抑制能力，仪表放大器有时用于音频应用（比如麦克风前置放大器），从噪声环境中提取微弱信号，使接地环路造成的噪声、失调最小化。请参阅表 6-4， AD公司（ Analog Device）专业化产品。 6） ） High Speed Signal Conditioning 高速信号调理高速信号调理高速信号调理高速信号调理 Because the speed and ac
