提高電流測量精度的拉動了電流檢測放大器的輸入失調電壓

摘要：某些應用程序要求輸入偏移電壓（V 操作系統(tǒng) ）的電流檢測放大器進行校正 ， 以提高測量精度。 This is not a straightforward task because of interactions between the output voltage low (V OL ) and the input V OS . 這不是因為輸出電壓之間的低（五職等 ）和輸入的 V OS相互作用簡單的任務。 This application note outlines a simple method to artificially "skew" the input V OS of unidirectional current-sense amplifiers. 本應用說明概述了一個簡單的方法 ， 人為地“傾斜”的輸入電壓V OS的單向電流檢測放大器。 The method allows measurement of total-input V OS without the normal limitations from V OL , and improves the overall accuracy of current measurements. 該方法可以測量總投入的 V OS從V 職等不正常的限制，提高了整體的電流測量精度。 The MAX4080 current-sense amplifier serves as the example of the technique. 在MAX4080電流檢測放大器作為該技術的例子。

A similar version of this article was published on March 23, 2009 on the Planet Analog website.這個類似文章發(fā)表在2009年3月23日在地球模擬網站。

Introduction導言

Current-sense amplifiers are sophisticated ICs, popular in electronic equipment that monitors load currents in real time.電流檢測放大器是復雜的集成電路，電子設備，實時監(jiān)測，負載電流受歡迎。 System controllers use this load information to implement power-management algorithms that modify the load-current characteristic itself, and to implement flexible overcurrent protection schemes.系統(tǒng)控制器使用此負載信息來實現(xiàn)電源管理算法，修改負載電流特性本身，并實施靈活的過電流保護計劃。

Current-sense amplifiers magnify a small differential voltage while rejecting the input common-mode voltage.電流檢測放大器放大小差分電壓，同時拒絕輸入共模電壓。 In this role they operate like traditional op amp-based differential amplifiers.在這個職位上工作一樣，他們的傳統(tǒng)運算放大器的差分放大器。 There is, however, an important difference between these two amplifier architectures.然而，這兩個之間的放大器架構的重要區(qū)別。 The input common-mode voltage for current-sense amplifiers is allowed to exceed the power-supply (V CC ) voltage.輸入共模電流檢測放大器電壓可以超過電源，電源（V CC）的電壓。 When, for example, the MAX4080 current-sense amplifier is powered from V CC = 5V, it can still withstand an input common-mode voltage of 76V.時，例如，MAX4080電流檢測放大器是由V CC = 5V供電電源，它仍然可以承受的76V輸入共模電壓。 By using unique amplifier architectures, current-sense amplifiers are not hampered by the common-mode rejection limitations (CMRR) that arise from mismatched resistors.通過使用獨特的放大器架構，電流檢測放大器不是由共模抑制限制（共模抑制比），從匹配電阻產生的阻礙。 The MAX4080, for example, has 100dB (min) DC CMRR.在MAX4080，例如，為100dB（最?。┲绷鞯腃MRR。 In contrast, the performance of traditional op amp-based differential amplifiers is negatively impacted by CMRR, and their effective input V OS is magnified through the signal chain.相比之下，傳統(tǒng)運算放大器的性能是基于差分放大器的CMRR負面影響，其有效投入的 V OS是通過信號鏈放大。


Figure 1. 圖1。 The MAX4080 is a precision unidirectional current-sense amplifier. 在MAX4080是精密單向電流檢測放大器。

Using Calibration to Improve Precision使用校準精度提高

The MAX4080 has a precision ±0.6mV (max) input offset voltage (V OS ) at 25°C, and ±1.2mV (max) V OS over the -40°C to +125°C temperature range.在MAX4080具有精度± 0.6mV（最大值）在25 ° C輸入失調電壓（V OS），然后在-40℃± 1.2mV（最大值） 的 V OS至+125 ° C溫度范圍。 Some applications, however, need to further calibrate input V OS to enhance precision of the final current measurement.一些應用程序，但是，需要進一步校準輸入的 V OS，以加強最后電流測量精度。 To do this calibration, V OS is typically measured during production and stored in firmware.要做到這一點校準，第五操作系統(tǒng)通常是在生產過程中測量和固件中。 This V OS is then adjusted digitally in real time when the equipment is in the field and in use.這一V 操作系統(tǒng) ，然后調整實時數字設備時，在外地和正在使用中。

A preferred method of calibration, intended for the convenience of manufacturing, would measure the V OS when there is zero load current (zero input differential voltage).阿標定的首選方法，對擬生產的方便，將測量的 V OS為零時，負載電流（零輸入差分電壓）。 In this approach one could measure the output V OS and subtract this voltage from all future measurements.在這種方式人們可以測量輸出V OS和減去所有未來的測量這個電壓。 Unfortunately, this method has a drawback.不幸的是，這種方法有一個缺點。 The V OL (output voltage low) and input V OS specifications interact, causing the output voltage not to reflect the input V OS accurately. 其他職等的V（輸出電壓低）和輸入的 V OS規(guī)格的互動，導致輸出電壓沒有體現(xiàn)出輸入的 V OS準確。 This interaction is, in fact, characteristic of all single-supply amplifiers.這種互動，實際上，所有的特征單電源放大器。

Consider the example of the MAX4080T with a gain of 20, and hypothetical zero input V OS .考慮MAX4080T為20增益的例子，假設零投入的 V OS。 One would then expect to measure a true zero at the amplifier's output.由此便期待著在測量放大器的輸出真正的零。 However, even at zero input differential voltage, the amplifier is not guaranteed to output a voltage below 15mV (with 10μA sink current).然而，即使在零輸入差分電壓，放大器不能保證輸出電壓低于15mV（帶10μA的吸收電流）。 In fact, if the output voltage measurements were used directly for V OS calibration, the amplifier would appear to have a 0.75mV input V OS (15mV/20 = 0.75).事實上，如果輸出電壓測量被用于校準的 V OS直接，放大器似乎有0.75mV輸入的 V OS（15mV/20 = 0.75）。

Similarly, if a MAX4080T did have V OL = 0, then a positive input V OS will produce a positive output V OS as expected.同樣，如果MAX4080T確實有V 職等 = 0，則積極投入的 V OS將產生積極輸出V 操作系統(tǒng)預期。 However, a negative input V OS will not be "reflected" in the output voltage measurement, since the amplifier cannot really output a voltage below ground.然而，消極的輸入電壓V OS將不會是“反映在輸出電壓的測量”，因為不能真正放大器輸出電壓低于地面。 Thus to summarize, one cannot "directly" use an output voltage measurement with zero input differential voltage to calibrate the input V OS .因此，要總結，不能“直接”使用零輸入差分電壓的輸出電壓測量校準輸入的 V OS。

There are two methods typically used to calibrate V OS during production:通常有用于校準在生產過程中的 V OS兩種方法：

1. A bidirectional current-sense amplifier like the MAX4081 is used with a reference voltage of about 1.5V.阿雙向電流，如檢測放大器MAX4081使用約1.5V的參考電壓。 This effectively translates the output voltage measurements by 1.5V, so a zero input differential voltage will output 1.5V ±V OS -induced errors.這有效地轉換了1.5V的輸出電壓測量，所以零輸入差分電壓輸出1.5V的± 的 V OS引起的誤差。 Since this 1.5V voltage is well above the amplifier's V OL , it does not affect error analysis.由于這1.5V的電壓遠高于放大器的V 職等 ，它不會影響誤差分析。 V OS errors can thus be calculated by measuring the difference between the output voltage and the ideal 1.5V input reference voltage. 的 V OS錯誤，因此可以通過測量計算之間的輸出電壓和1.5V的理想的輸入參考電壓差異。 There is a drawback to this method: reduced dynamic range.目前對這一方法的缺點：降低動態(tài)范圍。 The 0 to 5V input dynamic range of the device's ADC is now reduced 30% to 1.5V to 5V).在0至5V的輸入動態(tài)范圍的器件的ADC現(xiàn)在降低了30％至1.5V至5V）。 Additionally, the method requires a more expensive bidirectional current-sense amplifier to be used for unidirectional measurements.此外，該方法需要更昂貴的雙向電流檢測放大器，用于單向測量。 Finally, generating a low-drift 1.5V reference voltage or spending a second channel to measure this 1.5V reference voltage is not attractive.最后，產生一個低漂移1.5V的參考電壓或支出第二個通道來衡量這一1.5V的參考電壓是沒有吸引力。
   
   
2. A two-point measurement method is employed in which two known values of a differential input voltage (load currents) are applied to the current-sense amplifier.阿二點的測量方法是采用在兩個已知的差分輸入電壓（負載電流）值應用到電流檢測放大器。 First, straight-line approximation is applied to the output voltage measurements to calculate the input V OS by extrapolating to a zero-sense voltage.首先，直線近似適用于輸出電壓測量計算推算為一個零檢測電壓輸入的 V OS。 Secondly, that voltage measurement is then used for calibration.其次，電壓測量然后用于校準。 This method has its drawback: it uses two "known" exact values of currents during production, which is inconvenient and increases test time.這種方法有它的缺點：它使用了兩個“之稱的”生產過程中的電流的精確值，這是不便并增加測試時間。 Finally, accurate measurements close to a zero input differential voltage are still not achieved because V OL limitations will cause errors at small sense voltages.最后，精確的測量接近零輸入差分電壓仍然沒有實現(xiàn)因為V 職等限制，將導致在檢測電壓誤差小。

Using Input Resistors to Introduce Input V OS使用輸入電阻將推出輸入的 V OS

This application note presents a third method for measuring the input V OS of a current-sense amplifier.本應用筆記介紹了測量輸入的 V OS的電流檢測放大器三分之一的方法。 Once again, the MAX4080 serves as the example.再次MAX4080作為例子。 This approach applies a zero input differential voltage and overcomes the interactions between V OL and V OS —making it easy to use on the production line.這種方法適用于一零輸入差分電壓和克服這是由V和V 操作系統(tǒng)職等的相互作用，從而易于使用的生產線。

All current-sense amplifiers have input bias currents.所有的電流檢測放大器的輸入偏置電流。 Therefore the use of input resistors (for input filtering, as an example) should be carefully studied since the resistors can introduce unplanned gain and offset errors.因此，使用的輸入電阻（用于輸入過濾，為例）應小心，因為電阻可以引進計劃外增益和失調誤差研究。 These issues are detailed in application note 3888, " Performance of Current-sense Amplifiers with Input Sense Resistors ."這些問題的詳細應用筆記3888“ 的電流檢測電阻與輸入感覺放大器的性能 ?！?The method presented here uses similar techniques, but in this case, the input resistors are intentionally mismatched.這里介紹的方法使用類似的技術，但在這種情況下，輸入電阻是有意不匹配。 In this manner an intentional output V OS is introduced.在這種有意輸出V OS是提出的方式。 The MAX4080 has a temperature-compensated bias current of 5μA (typ) and 12μA (max) over process variation.在MAX4080有一個溫度補償偏置電流降至5μA（典型值）和12μA的（最大值超過過程變化）。 Using a 2kΩ resistor in series with RS- ( Figure 2 ) thus produces a typical input V OS of 10mV and 24mV, respectively, over process variation.使用與RS系列2kΩ電阻（ 圖2），從而產生對過程變化的典型輸入有10mV和24mV，分別的 V OS。 This additional input V OS then causes an output offset of 200mV (typ) and 480mV (max), which is adequate to override any V OL and V OS limitations in the basic MAX4080.這種額外的輸入電壓V 操作系統(tǒng)然后200mV的原因（典型值）抵銷了產量和480mV（最大），這足以覆蓋的基本MAX4080任意V 職等和V 操作系統(tǒng)的限制。 Error in this input-resistor-induced V OS will have a temperature dependence based both on the drift characteristics of the input resistor (usually 100ppm) and on the bias current (negligible).在此輸入錯誤電阻引起的 V OS將有一個溫度特性基礎上，輸入電阻（通常為100ppm）的漂移特性都和偏置電流（忽略不計）。


Figure 2. 圖2。 The MAX4080 configured to use an external 2kΩ resistor in series with RS-. 在MAX4080配置為使用外部2kΩ的與RS系列電阻。

The resistor drift characteristic of +100ppm causes a +1% change in the resistance value over a 100°C change (ie, +20Ω).在100分之電阻漂移特性導致超過100℃的變化在1％的電阻值的變化（即20Ω）。 Additional input V OS drift from the input resistor is then typically about +0.1mV, and +0.24mV max across the process variation of bias current.額外投入的 V OS漂移的輸入電阻，然后通常約為0.1 mV時，和0.24整個過程中的偏見壓變化最大電流。 This drift is still only 20% of the ±0.6mV bidirectional error in input V OS that one would usually expect from process variation if no calibration were used.漂移仍只有20％的± 0.6mV在雙向輸入錯誤的 V OS，一個通常會期望從工藝變化，如果沒有被用來校準。

Decreasing the size of the input series resistor further reduces this drift error.減少了輸入串聯(lián)電阻的大小，進一步降低此漂移誤差。 To account for the 15mV V OL and the ±1.2mV input V OS over temperature, the additional input V OS would need to be a minimum of 1.2mV + 15mV/20 = 1.95mV = 2mV, approximately.以考慮到15mV V 職等及以上溫度的± 1.2mV輸入的 V OS，增加投入的 V OS將需要最低限度的1.2mV + 15mV/20 = 1.95mV = 2mV，大約。 Table 1 shows the test results over temperature.表1顯示溫度的測試結果。 Here, the MAX4080 has negligible drift in V OS , and so all measured drift in V OS is due to the use of input resistor and its ppm drift.在這里，MAX4080在微不足道的 V OS漂移，因此所有測量漂移在V OS是由于輸入電阻和漂移的百萬分之使用。

Table 1. 表1。 Results of Temperature Tests With and Without Input Resistors 溫度測試結果有和沒有輸入電阻

Conclusion結論

This application note presents a method that introduces known input V OS by suitably sizing input resistors for current-sense amplifiers like the MAX4080.本應用筆記介紹了，介紹已知的適當大小的電流，如MAX4080檢測放大器輸入電阻輸入的 V OS方法。 Equipment manufacturers can thus use this methodology for production-line calibration of V OS with zero input current to enhance the accuracy of real-time measurements.因此，設備制造商可以使用輸入電流為零，以提高實時測量精度的方法，這種生產線的V 操作系統(tǒng)校準。