研討翻譯：大海撈針----存在于高共模電壓的小差分電壓測(cè)量
Finding the Needle in a Haystack: Measuring small differential voltages in the presence of large common-mode voltages 大海撈針：存在于高共模電壓的小差分電壓測(cè)量
by Scott WAYNE
譯注：本文下載地址已經(jīng)不記得了，根據(jù)文內(nèi)文字，可能是［ANALOG Dialogue 34-1 (2000)］的文章，如因此造成不便，請(qǐng)讀者原諒。
導(dǎo)讀：這篇文章描述風(fēng)格獨(dú)特，有的地方講得非常好，值得一讀。
Introduction ||概述
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In applications such as motor CONTROL, POWER-supply current monitoring, and BATTERY cell-voltage monitoring, a small differential voltage must be sensed in the presence of a high common-mode voltage. Some of these applications require galvanic isolation, others do not. Some applications use ANALOG CONTROL, others use DIGITAL CONTROL. Four cases of such measurements will be considered, each requiring unique considerations. They are:
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對(duì)象馬達(dá)控制、電源電流監(jiān)視、電池電壓監(jiān)視之類的應(yīng)用，必須把一個(gè)存在于高共模電壓的小差分電壓提取出來(lái)。部分應(yīng)用要求galvanic隔離，而其它的則沒(méi)有。有的應(yīng)用是模擬控制，有的則是模擬控制。這種測(cè)量有4種情況要考慮，each requiring unique considerations。它們是：
［譯注］從詞典上看，galvanic首先應(yīng)該指意大利的醫(yī)師和生理學(xué)家Luigi Galvani (1737～1798)，可能是他發(fā)現(xiàn)的直流刺激療法后才有這個(gè)詞用到電學(xué)領(lǐng)域中來(lái)。在這里，我認(rèn)為galvanic就是直流或直流電的意思。不知道對(duì)不對(duì)？ |
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1) galvanic isolation with ANALOG OUTPUT;
2) galvanic isolation with DIGITAL OUTPUT;
3) no galvanic isolation, ANALOG OUTPUT;
4) no galvanic isolation, DIGITAL OUTPUT.
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有模擬輸出的galvanic隔離
有數(shù)字輸出的galvanic隔離
無(wú)galvanic隔離，但有模擬輸出
無(wú)galvanic隔離，但有數(shù)字輸出 |
Differential Signals Versus Common-Mode Signals || 差分信號(hào)vs共模信號(hào)
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Figure 1 shows the input of a measurement SYSTEM. VDIFF represents the differential voltage, the signal of interest. VCM represents the common-mode voltage, which contains no useful information about the measurement and could in fact reduce the measurement accuracy. The common-mode voltage could be an implicit PART of the measurement SYSTEM, as in a BATTERY cell-voltage monitoring application, or it could be created by a fault condition where the sensor accidentally comes in contact with a high voltage. In either case, that voltage is unwanted, and it is the job of the measurement SYSTEM to REJECT it, while responding to the differential-mode voltage.
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如圖1所示的測(cè)量系統(tǒng)的輸入，VDIFF表示差分電壓，它是有用信號(hào)。VCM表示共模電壓，它攜帶的信號(hào)對(duì)測(cè)量沒(méi)有用處，且會(huì)降低測(cè)量精度。象電池電壓監(jiān)視的應(yīng)用，共模電壓是測(cè)量系統(tǒng)所隱含的。它會(huì)增加傳感器附帶包括有高電壓的不利條件。另外的原因是該電壓沒(méi)有用,是測(cè)量系統(tǒng)工作時(shí)所抑制的，而只響應(yīng)差模電壓。
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Figure 1. Measurement SYSTEM with differential and common-mode voltages.
圖1：有差分和共模電壓的測(cè)量系統(tǒng)Common-Mode Rejection (CMR) || 共模抑制
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The measurement SYSTEM has both a differential-mode gain and a common-mode gain. The differential-mode gain is usually greater than or equal to one, while the common-mode gain is ideally zero. RESISTOR mismatches cause the dc gain from the inverting input to differ slightly from that of the noninverting input. This, in turn, results in a dc common-mode gain that is nonzero. If the differential gain is G = R2/R1 , the common-mode gain will be (%mismatch/100)×［G/(G+1)］ . The common-mode REJECTion ratio (CMRR) is the differential-mode gain divided by the common-mode gain, or (100/%mismatch )×(G + 1). The logarithmic equivalent (CMR—in dB), is 20log10［(100/%mismatch )×(G + 1)］ |
測(cè)量系統(tǒng)是共模增益和差模增益并存的。差模通常大于等于1；而理想的共模增益應(yīng)當(dāng)為0。電阻失配會(huì)引起反相輸入和同相輸入有輕微的差異，從而導(dǎo)致直流共模增益不為0。若差模增益為：G = R2/R1，則共模增益為(%mismatch/100)×［G/(G+1)］：。共模抑制比CMRR等于差模共模增益除以共模增益，或(100/%mismatch )×(G + 1)。CMR用對(duì)數(shù)等效表示為分貝（dB）：20log10［(100/%mismatch )×(G + 1)］。
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In real-world applications, external interference sources abound. Pickup will be coupled from the ac POWER LINE (50/60 Hz and its harmonics), from EQUIPMENT switching on and off, and from radio-frequency transmission sources. This type of interference is induced equally into both differential inputs, and therefore appears as a common-mode signal. So, in addition to high dc CMR, instrumentation amplifiers also require high ac CMR, especially at LINE frequencies and their harmonics. DC common-mode errors are mostly a function of RESISTOR mismatch. In contrast, ac common-mode errors are a function of differences in phase shifts or time delays between the inverting and noninverting inputs. These can be minimized by using well-matched high-speed components, and they can be trimmed with a CAPACITOR. Alternatively, in low-frequency applications, OUTPUT filtering can be used if necessary. While dc common-mode errors can usually be removed through calibration or trimming. AC common-mode errors, which can reduce the resolution of the measurement, are generally of greater concern. All ANALOG Devices instrumentation amplifiers are fully specified for both dc and low-frequency ac common-mode REJECTion.
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在實(shí)際應(yīng)用中，外部干擾源大量存在。電源線的50/60Hz干擾及其諧波、用電設(shè)備的開或關(guān)、無(wú)線頻率發(fā)射源會(huì)耦合進(jìn)系統(tǒng)中來(lái)。這種類型的干擾一并來(lái)到兩個(gè)差分輸入端，因而出現(xiàn)一個(gè)共模信號(hào)�？梢�，儀表放大器除了有高的直流共模抑制還要有高的交流共模抑制，尤其是電源頻率及其諧波。直流共模誤差主要是電阻失配的函數(shù)，對(duì)應(yīng)地，交流共模誤差則是同相反相輸入端之間的相移、延時(shí)差異的函數(shù)。(這些誤差)，用匹配良好的高速元件可以最小化，也可以用電容進(jìn)行修調(diào)，此外（Alternatively），對(duì)低頻應(yīng)用，如果需要，還可以進(jìn)行輸出濾波。直流共模增益誤差通常可以通過(guò)校準(zhǔn)或修調(diào)(trimming)技術(shù)消除，而降低測(cè)量精度的交流共模誤差卻更加從令人關(guān)注。AD公司所有的儀表放大器都對(duì)直流共模抑制和低頻的交流共模抑制有完整的詳細(xì)規(guī)定。
［譯者語(yǔ)］Alternatively指的應(yīng)該是前面說(shuō)的元件匹配和后面說(shuō)的濾波。 |
Galvanic Isolation || Galvanic隔離
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Some applications require that there be no direct ELECTRICal connection between the sensor and the SYSTEM ELECTRONICS. These applications require galvanic isolation in order to protect the sensor, the SYSTEM, or both. The SYSTEM ELECTRONICS may need to be protected from high voltages at the sensor. Or, in applications requiring intrinsic safety, the sensor excitation and POWER circuitry may need to be isolated to prevent sparks or the ignition of explosive gases that could be caused by a fault condition. In medical applications, such as electrocardiograms (ECG), protection is required in both directions. The patient must be protected from accidental ELECTRIC shock. If the patient’s heart stops beating, the ECG machine must be protected from the very high voltages applied to the patient by emergency use of a defibrillator in an attempt to restore the heartbeat.
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有的應(yīng)用，傳感器和系統(tǒng)（器件）之間要求不能有直接的電氣連接。這些 |
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>>參與討論 |
作者： ic921 于 2006/1/17 0:48:00 發(fā)布：
研討翻譯：大海撈針----存在于高共模電壓的小差分電壓測(cè)量 -2/2-
High Impedance Versus Galvanic Isolation || 高阻抗vs Galvanic隔離
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Many applications need the ability to sense a small differential voltage in the presence of a high common-mode voltage, but do not require the intrinsic safety or the ability to break ground loops that are provided by galvanic isolation. These applications require a high-CMR AMPLIFIER that can accept high common-mode voltage. This type of AMPLIFIER, sometimes called a “poor man’s isolation AMPLIFIER,” isolates the sensor from the SYSTEM with a high impedance, rather than with a galvanic isolation barrier. While not isolation in the true sense, it can serve the same purpose in some applications at much lower cost. In addition, a dc-to-dc converter is not required, since the whole SYSTEM is POWERed from the same POWER supply.
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許多應(yīng)用需要這樣的能力，以感測(cè)存在于高共模電壓中的小差分電壓，但它并沒(méi)有本質(zhì)安全的要求或在提供galvani隔離的情況下允許切斷地環(huán)路。此類應(yīng)用就需要能接受高共模電壓的高共模抑制的放大器。這種類型的放大器，有時(shí)稱之為“窮人的放大器”，以高阻抗將傳感器與系統(tǒng)隔離而不用galvanic隔離柵。如果實(shí)際的感測(cè)不隔離就在某些情況下可以以較低的費(fèi)用獲得同樣的效果。不要DC-DC轉(zhuǎn)換器的另一種情形，是因?yàn)檎麄�(gè)系統(tǒng)都由同一電源供電。
［譯者語(yǔ)］While not isolation in the true sense 中，true一詞似乎有特殊的意義，那種情景/情形實(shí)在想象不出！ |
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Figure 2 shows the AD629, a high-common-mode-voltage difference AMPLIFIER that was designed for these types of applications. It seems simple enough. It’s “just” an op amp and five resistors. Can’t users “roll their own?” Yes, but the resistors would have to be matched to better than 0.01% and would have to track to better than 3 ppm/℃. RESISTOR self-heating would degrade dc CMR, while capacitive strays would degrade ac CMR. Performance, size, and cost would all be sacrificed compared to what could be obtained in an 8-lead DIP or SOIC.
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如圖2所示的高共模電壓差分放大器AD629就是為這種類型的應(yīng)用而設(shè)計(jì)的。它顯得足夠簡(jiǎn)單，正好是1個(gè)運(yùn)算放大器加5個(gè)電阻。Can’t users “roll their own?”是的，但電阻的匹配度優(yōu)于0.01%，且有優(yōu)于3ppm/℃的(溫度循跡效果)。電阻自熱會(huì)降低直流共模抑制DC-CMR，而雜散電容(capacitive strays)則會(huì)降低交流共模抑制AC-CMR。犧牲性能、尺寸、價(jià)格compared to what could be obtained in an 8-lead DIP or SOIC。
［譯者語(yǔ)］Can’t users “roll their own?”這句不會(huì)譯。
另，雜散電容一般是stray capacitive，這里capacitive strays有什么不同嗎？ |
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Applications such as simple industrial process-CONTROL loops with ANALOG inputs and OUTPUTs that require galvanic isolation could use the AD202/AD204. These are complete isolation AMPLIFIERs with galvanic isolation between the input and OUTPUT stages. Transformer coupling means that they can also provide isolated POWER to the input stage, eliminating the need for an external dc-to-dc converter. The AD202/AD204 provide an uncommitted op amp for input signal conditioning, have CMR of 130 dB at a gain of 100, and 2000-V-peak CMV isolation. Figure 3 shows an AD202 CIRCUIT* to measure a ± 5-V full-scale signal riding on a common-mode voltage of up to 2000 V. For applications that require isolated BRIDGE excitation, cold-junction compensation, linearization, and other signal-conditioning functions, the 3B, 5B, 6B, and 7B series provide a family of complete, well-isolated signal conditioners.
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象有需要模擬galvanic隔離輸入輸出的簡(jiǎn)單工藝控制環(huán)這樣的應(yīng)用，可以用AD202/AD204。這些是輸入級(jí)和輸出級(jí)之間完全隔離的galvanic隔離放大器。變壓器耦合意味著給輸入級(jí)提供電源隔離，免去了額外的DC-DC轉(zhuǎn)換器。The AD202/AD204 provide an uncommitted op amp for input signal conditioning，在增益為100時(shí)有130dB的共模抑制和2000V的峰值電壓隔離。如圖3的AD202電路*測(cè)量±5V的滿度信號(hào)所承受(riding)的共模電壓高達(dá)2000V。
在需要隔離橋激勵(lì)、冷端補(bǔ)償、線性化和其它信號(hào)調(diào)理功能的應(yīng)用，the 3B, 5B, 6B, and 7B series provide a family of complete, well-isolated signal conditioners.
［譯者語(yǔ)］前一個(gè)保留未譯的一句，主要是對(duì)uncommitted的含意不解。后一句可能與產(chǎn)品系列有關(guān)，未詳查，故未譯。 |
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*These Figures are illustrative examples; they are not detailed schematics of tested applications. Please consult PRODUCT data sheets for more information. You will also find the online seminar notes, Practical ANALOG Design Techniques, and the book, Practical Design Techniques for Sensor Signal Conditioning (available from ADI), to be useful sources of design information. Use extreme caution when working with high-voltage CIRCUITs.
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*這些圖只是具體的實(shí)例，但不詳細(xì)描述它們的測(cè)試應(yīng)用原理。要獲取更多更詳盡的信息，請(qǐng)參考產(chǎn)品數(shù)據(jù)手冊(cè)。你也可以查找在線研討會(huì)筆記《Practical ANALOG Design Techniques》和索取《Practical Design Techniques for Sensor Signal Conditioning》一書(ADI有)的有用信息。Use extreme caution when working with high-voltage CIRCUITs.
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Figure 2. AD629 High-common-mode-voltage-difference AMPLIFIER.
Figure 3. AD202/AD204 used in application requiring galvanic isolation and ANALOG OUTPUTs.
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Some industrial sensor applications require galvanic isolation, combined with the DIGITAL OUTPUT of a SMART sensor. DIGITAL isolation, rather than ANALOG isolation, could be used more cost-effectively but an external dc-to-dc converter is required. An example of this sort of application is in motor CONTROL, where a fault condition in the motor could destroy the CONTROL ELECTRONICS. The AD7742 synchronous voltage-to-frequency converter could be used, together with an opto-coupler and a dc-to-dc converter, as shown in Figure 4. A remote AD7742 can be interfaced with a SYSTEM microprocessor or microCONTROLler to complete the A/D conversion. For stand-alone applications the serial-OUTPUT AD7715 ANALOG front end, a 16-bit sigma-delta A/D converter, could be used, but it has five DIGITAL lines to isolate, rather than the SINGLE DIGITAL OUTPUT from the V/F converter. However, instead of five opto-couplers and a dc-to-dc converter, an AD260 five-channel high-speed LOGIC isolator with its own on-board transformer could be used. Figure 5 shows the AD7715 and AD260.
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部分工業(yè)傳感器應(yīng)用要求有g(shù)alvanic隔離，并和智能傳感器的數(shù)字輸出結(jié)合到一起。用數(shù)字隔離而不用模擬隔離可以更節(jié)省但需要外加DC-DC轉(zhuǎn)換器。以馬達(dá)控制一類的應(yīng)用為例，馬達(dá)故障情況下就會(huì)損壞控制器件。如圖4，將AD7742同步電壓頻率轉(zhuǎn)換器和光耦合器、DC-DC轉(zhuǎn)換器用到一起。由系統(tǒng)微處理器或微控制器接口就能完成A/D轉(zhuǎn)換�？捎糜心M前端（處理的）串行輸出的16位Σ-Δ模數(shù)轉(zhuǎn)換器AD7715，但它有5個(gè)數(shù)字線要隔離，不單是V/F轉(zhuǎn)換那樣只有一條數(shù)據(jù)信號(hào)的輸出。然而，用AD260則可以代替5個(gè)光耦和一個(gè)DC-DC轉(zhuǎn)換器。
［譯者提示］同步VFC（AD7742）和異步VFC（如ADVFC32）是不同的。請(qǐng)閱讀器件數(shù)據(jù)手冊(cè)加以區(qū)別。
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Figure 4. AD7742 used in application requiring galvanic isolation and DIGITAL OUTPUTs.
Figure 5. AD7715/AD260 used in application requiring galvanic isolation and DIGITAL OUTPUTs.
Figure 6 |
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>>參與討論 |
作者： ic921 于 2006/1/17 0:50:00 發(fā)布：
這是前天上班時(shí)譯的
今晚整理一下發(fā)給大家評(píng)點(diǎn)，請(qǐng)不紊指教!
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>>參與討論 |
作者： rfid2005 于 2006/1/17 12:10:00 發(fā)布：
謝謝摟主，頂
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>>參與討論 |
作者： rxl8888 于 2006/1/18 8:57:00 發(fā)布：
請(qǐng)教上述翻譯中
電阻失配會(huì)引起反相輸入和同相輸入有輕微的差異，從而導(dǎo)致直流共模增益不為0。若差模增益為：G = R2/R1，則共模增益為(%mismatch/100)×［G/(G+1)］。
請(qǐng)問(wèn)此處共模增益的推導(dǎo)方法
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>>參與討論 |
作者： IC921 于 2006/1/18 9:37:00 發(fā)布：
自己先試推導(dǎo)一下看看？
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>>參與討論 |
作者： rxl8888 于 2006/1/18 14:13:00 發(fā)布：
我推導(dǎo)的結(jié)果是共模增益是1+G
推導(dǎo)的過(guò)程我一會(huì)發(fā)上來(lái)，ic921幫我查查錯(cuò)
因?yàn)槭莣ord文檔做了公式編輯，粘貼不上來(lái)
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>>參與討論 |
作者： rxl8888 于 2006/1/18 14:57:00 發(fā)布：
推導(dǎo)
我不會(huì)貼圖，就借用IC921的圖1
其中定義VDIFF的負(fù)端為Vf,正端為Vz,
定義運(yùn)放的反相輸入端為V1,正相輸入端V2，運(yùn)放的輸出定義為V0
列出下面連個(gè)等式：
(Vf-V1)/R1=(Vf-Vo)/R2 1)式
(Vz-V2)/R1=V2/R2 2)式
2)式-1)式得
[(Vz-Vf)-(V2-V1)]/R1=(V2-V1+Vo)/R2 3)式
由于現(xiàn)在是考慮V2-V1到輸出的增益，所以令Vz-Vf=0即沒(méi)有差分輸入，并另Vm=V2-V1
則
(-Vm)/R1=(Vm+Vo)/R2 4)式
于是得到
Vo/Vm=-(R1+R2)/R1=-(1+G)
所以我考慮共模增益應(yīng)該是-（1+G）
以上推導(dǎo)請(qǐng)指正
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>>參與討論 |
作者： iC921 于 2006/1/18 17:32:00 發(fā)布：
共模增益不能用差模電壓計(jì)算VDIFF
只能計(jì)算VCM的增益。
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你分?jǐn)?shù)有500分(我沒(méi)看)帖圖的方法是
1 點(diǎn)擊編輯框下面的“普通”
2 等待片刻后，粘貼復(fù)制好的內(nèi)容進(jìn)去就可以了。
我現(xiàn)在就這樣做。
Figure 1. Measurement SYSTEM with differential and common-mode voltages.
圖1：有差分和共模電壓的測(cè)量系統(tǒng)
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另一種方法是：將圖片鏈接放到下面的“鏈接圖片”欄中，這不受分?jǐn)?shù)限制。
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>>參與討論 |
作者： sheepyang 于 2006/1/18 18:10:00 發(fā)布：
不知道上面怎么推導(dǎo)的
結(jié)果是正如文章所寫的。
推導(dǎo)如下。
考慮電阻值失配，則上面的R1和R2用R1'和R2'來(lái)替代。
故根據(jù)基爾霍夫電流方程有
(Vcm-Vdm-V-)/R1'+(Vout-V-)/R2'=0 (1)
V+=R2*Vcm/(R1+R2) (2)
V+=V- (3)
差模：Vcm=0,
有
Vout/Vdm=R2'/R1'=G
共模： Vdm=0 由上面三個(gè)方程得到
Vout/Vcm=(R2*R1'-R2'*R1)/(R1'*(R1+R2)) (4)
從4式看出如果電阻嚴(yán)格匹配，則共模放大為0。
雖然存在失配，但是R2/R1～R2'/R1'=G.
所以(4)上下各除以R1,有
(R2/R1-R2'/R1')/(1+R2/R1) (5)
分子和分母都存在放大倍數(shù)的失配，但仍是比較小的量，在分母中可以忽略，因此有
G*(失配率)/(1+G)
* - 本貼最后修改時(shí)間：2006-1-19 10:02:41 修改者：sheepyang
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| 11樓： |
>>參與討論 |
作者： rxl8888 于 2006/1/19 9:34:00 發(fā)布：
謝謝樓上和ic921
我推導(dǎo)的思路錯(cuò)誤，我將電阻失配引起的共模增益用加在運(yùn)放正負(fù)輸入極上的差模電壓來(lái)計(jì)算了,基本功還要加強(qiáng)
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| 12樓： |
>>參與討論 |
作者： wahahacat 于 2006/1/19 10:39:00 發(fā)布：
re
呵呵.
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| 13樓： |
>>參與討論 |
作者： iC921 于 2006/1/28 9:56:00 發(fā)布：
水王老鄉(xiāng)，請(qǐng)進(jìn)
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| 14樓： |
>>參與討論 |
作者： hotpower 于 2006/1/28 11:39:00 發(fā)布：
廣西老鄉(xiāng)謝了,不過(guò)現(xiàn)在忽悠成ADS7871了
本想直接用模擬的直接采樣,看來(lái)還得用ADC了...
祝新年快樂(lè),水到渠成.
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