Demystifying Auto-Zero Amplifiers Part 1 They essentially eliminate offset, drift, and 1/f noise. How do they work? Is there a downside? by Eric Nolan INTRODUCTION Whenever the subject of auto-zero or chopper-stabilized amplifiers comes up, the inevitable first question is How do they really work? Beyond curiosity about the devices inner workings, the real question in most engineers minds is, perhaps, The dc precision looks incredible, but what kind of weird behavior am I going to have to live with if I use one of these in my circuit; and how can I design around the problems? Part 1 of this article will attempt to answer both questions. In Part 2, to appear in the next issue, some very popular and timely applications will be mentioned to illustrate the significant advantages, as well as some of the drawbacks, of these parts. CHOPPER AMPLIFIERS HOW THEY WORK The first chopper amplifiers were invented more than 50 years ago to combat the drift of dc amplifiers by converting the dc voltage to an ac signal. Initial implementations used switched ac coupling of the input signal and synchronous demodulation of the ac signal to re-establish the dc signal at the output. These amplifiers had limited bandwidth and required post-filtering to remove the large ripple voltages generated by the chopping action. Chopper-stabilized amplifiers solved the bandwidth limitations by using the chopper amplifier to stabilize a conventional wide-band amplifier that remained in the signal path 1. Early chopper-stabilized designs were only capable of inverting operation, since the stabilizing amplifier s output was connected directly to the noninverting input of the wide-band differential amplifier. Modern IC chopper amplifiers actually employ an auto-zero approach using a two-or-more-stage composite amplifier structure similar to the chopper-stabilized scheme. The difference is that the stabilizing amplifier signals are connected to the wide-band or main amplifier through an additional nulling input terminal, rather than one of the differential inputs. Higher-frequency signals bypass the nulling stage by direct connection to the main amplifier or through the use of feed-forward techniques, maintaining a stable zero in wide-bandwidth operation. This technique thus combines dc stability and good frequency response with the accessibility of both inverting and noninverting configurations. However, it may produce interfering signals consisting of high levels of digital switching noise that limit the usefulness of the wider available bandwidth. It also causes intermodulation distortion (IMD), which looks like aliasing between the clock signal and the input signal, producing error signals at the sum and difference frequencies. More about that later. Auto-Zero Amplifier Principle Auto-zero amplifiers typically operate in two phases per clock cycle, illustrated in Figures 1a and 1b. The simplified circuit shows a nulling amplifier (A A ), a main (wide-band) amplifier (A B ), storage capacitors (C M1 and C M2 ), and switches for the inputs and storage capacitors. The combined amplifier is shown in a typical op-amp gain configuration. In Phase A, the auto-zero phase (Figure 1a), the input signal is applied to the main amplifier (A B ) alone; the main amplifier s nulling input is supplied by the voltage stored on capacitor C M2 ; and the nulling amplifier (A A ) auto-zeros itself, applying its nulling voltage to C M1. In Phase B, with its nulling voltage furnished by C M1, the nulling amplifier amplifies the input difference voltage applied to the main amplifier and applies the amplified voltage to the nulling input of the main amplifier and C M2. SB SA A B A A V NB V NA C M1 EXTERNAL FEEDBACK a. Auto-Zero Phase A: null amplifier nulls its own offset. SB SA A B A A V NB V NA C M1 C M2 EXTERNAL FEEDBACK b. Output Phase B: null amplifier nulls the main amplifier offset. Figure 1. Switch settings in the auto-zero amplifier. Both amplifiers use the trimmable op-amp model (Figure 2), with differential inputs and an offset-trim input. A B V N C M2 = A( ) +BV N A = DIFFERENTIAL GAIN B = TRIM GAIN Figure 2. Trimmable op amp model. In the nulling phase (Phase A Figure 1a), the inputs of the nulling amp are shorted together and to the inverting input terminal (common-mode input voltage). The nulling amplifier nulls its own inherent offset voltage by feeding back to its nulling terminal whatever opposing voltage is required to make the product of that 1 Edwin Goldberg and Jules Lehmann, U. S. Patent 2,684,999: Stabilized dc amplifier. Analog Dialogue 34-2 (2000) 1
voltage and the incremental gain of the nulling input approximately equal to A A s input offset (S ). The nulling voltage is also impressed on C M1. Meanwhile, the main amplifier is behaving like a normal op amp. Its nulling voltage is being furnished by the voltage stored on C M2. During the output phase (Phase B Figure 1b) the inputs of the nulling amplifier are connected to the input terminals of the main amplifier. C M1 is now continuing to furnish the nulling amplifier s required offset correction voltage. The difference input signal is amplified by the nulling amplifier and is further amplified by the incremental gain of the main amplifier s nulling input circuitry. It is also directly amplified by the gain of the main amplifier itself (A B ). The op amp feedback will cause the output voltage of the nulling amplifier to be whatever voltage is necessary at the main amplifier s nulling input to bring the main amplifier s input difference voltage to near-null. Amplifier A A s output is also impressed on storage capacitor C M2, which will hold that required voltage during the next Phase A. The total open-loop amplifier dc gain is approximately equal to the product of the nulling amplifier gain and the wide-band amplifier nulling terminal gain. The total effective offset voltage is approximately equal to the sum of the main-amplifier and nullingamplifier offset voltages, divided by the gain at the main amplifier nulling terminal. Very high gain at this terminal results in very low effective offset voltage for the whole amplifier. As the cycle returns to the nulling phase, the stored voltage on C M2 continues to effectively correct the dc offset of the main amplifier. The cycle from nulling to output phase is repeated continuously at a rate set by the internal clock and logic circuits. (For detailed information on the auto-zero amplifier theory of operation see the data sheets for the AD8551/AD8552/AD8554 or AD857x amplifiers). Auto-Zero Amplifier Characteristics Now that we ve seen how the amplifier works, let s examine its behavior in relation to that of a normal amplifier. First, please note that a commonly heard myth about auto-zero amplifiers is untrue: the gain-bandwidth product of the overall amplifier is not related to the chopping clock frequency. While chopping clock frequencies are typically between a few hundred Hz and several khz, the gain bandwidth product and unity-gain bandwidth of many recent auto-zero amplifiers is 1 MHz 3 MHz and can be even higher. A number of highly desirable characteristics can be easily inferred from the operating description: dc open-loop voltage gain, the product of the gains of two amplifiers, is very large, typically more than 10 million, or 140 db. The offset voltage is very low due to the effect of the large nulling-terminal gain on the raw amplifier offsets. Typical offset voltages for auto-zero amplifiers are in the range of one microvolt. The low effective offset voltage also impacts parameters related to dc changes in offset voltage dc CMR and PSR, which typically exceed 140 db. Since the offset voltage is continuously corrected, the shift in offset over time is vanishingly small, only 40 nv 50 nv per month. The same is true of temperature effects. The offset temperature coefficient of a welldesigned amplifier of this type is only a few nanovolts per C! A less obvious consequence for the amplifier s operation is the low-frequency 1/f noise characteristic. In normal amplifiers, the input voltage noise spectral density increases exponentially inversely with frequency below a corner frequency, which may be anywhere from a few Hz to several hundred Hz. This lowfrequency noise looks like an offset error to the auto-correction circuitry of the chopper-stabilized or auto-zero amplifier. The auto-correction action becomes more efficient as the frequency approaches dc. As a result of the high-speed chopper action in an auto-zero amplifier, the low-frequency noise is relatively flat down to dc (no 1/f noise!). This lack of 1/f noise can be a big advantage in low-frequency applications where long sampling intervals are common. Because these devices have MOS inputs, bias currents, as well as current noise, are very low. However, for the same reason, wideband voltage noise performance is usually modest. The MOS inputs tend to be noisy, especially when compared to precision bipolarprocessed amplifiers, which use large input devices to improve matching and often have generous input-stage tail currents. Analog Devices AD855x amplifiers have about one-half the noise of most competitive parts. There is room for improvement, however, and several manufacturers (including ADI) have announced plans for lower-noise auto-zero amplifiers in the future. Charge injection [capacitive coupling of switch-drive voltage into the capacitors] occurs as the chopping switches open and close. This, and other switching effects, generates both voltage and current noise transients at the chopping clock frequency and its harmonics. These noise artifacts are large compared to the wideband noise floor of the amplifier; they can be a significant error source if they fall within the frequency band of interest for the signal path. Even worse, this switching causes intermodulation distortion of the output signal, generating additional error signals at sum and difference frequencies. If you are familiar with sampleddata systems, this will look much like aliasing between the input signal and the clock signal with its harmonics. In reality, small differences between the gain-bandwidth of the amplifier in the nulling phase and that in the output phase cause the closed-loop gain to alternate between slightly different values at the clock frequency. The magnitude of the IMD is dependent on the internal matching and does not relate to the magnitude of the clock noise. The IMD and harmonic distortion products typically add up to about 100 db to 130 db plus the closed-loop gain (in db), in relation to the input signal. You will see below that simple circuit techniques can limit the effects of both IMD and clock noise when they are out of band. Some recent auto-zero amplifier designs with novel clocking schemes, including the AD857x family from Analog Devices, have managed to tame this behavior to a large degree. The devices in this family avoid the problems caused by a single clocking frequency by employing a (patented) spread-spectrum clocking technique, resulting in essentially pseudorandom chopper-related noise. Since there is no longer a peak at a single frequency in either the intrinsic switching noise or aliased signals, these devices can be used at signal bandwidths beyond the nominal chopping frequency without a large error signal showing up in-band. Such amplifiers are much more useful for signal bandwidths above a few khz. 2 Analog Dialogue 34-2 (2000)
Some recent devices have used somewhat higher chopping frequency, which can also extend the useful bandwidth. However, this approach can degrade S performance and increase the input bias current (see below regarding charge injection effects); the design trade-offs must be carefully weighed. Extreme care in both design and layout can help minimize the switching transients. As mentioned above, virtually all monolithic auto-zero amplifiers have MOS input stages, tending to result in quite low input bias currents. This is a very desirable feature if large source impedances are present. However, charge injection produces some unexpected effects on the input bias-current behavior. At low temperatures, gate leakage and input-protection-diode leakage are very low, so the dominant input bias-current source is charge injection on the input MOSFETs and switch transistors. The charge injection is in opposing directions on the inverting and noninverting inputs, so the input bias currents have opposing polarities. As a result, the input offset current is larger than the input bias current. Fortunately, the bias current due to charge injection is quite small, in the range of 10 pa 20 pa, and it is relatively insensitive to common-mode voltage. As device temperature rises above 40 C to 50 C, the reverse leakage current of the input protection diodes becomes dominant; and input bias current rises rapidly with temperature (leakage currents approximately double per 10 C increase). The leakage currents have the same polarity at each input, so at these elevated temperatures the input offset current is smaller than the input bias current. Input bias current in this temperature range is strongly dependent on input common-mode voltage, because the reverse bias voltage on the protection diodes changes with common-mode voltage. In circuits with protection diodes connected to both supply rails the bias current polarity changes as the common-mode voltage swings over the supply-voltage range. Due to the presence of storage capacitors, many auto-zero amplifiers require a long time to recover from output saturation (commonly referred to as overload recovery). This is especially true for circuits using external capacitors. Newer designs using internal capacitors recover faster, but still take milliseconds to recover. The AD855x and AD857x families recover even faster at about the same rate as normal amplifiers taking less than 100 µs. This comparison also holds true for turn-on settling time. Finally, as a consequence of the complex additional circuitry required for the auto-correction function, auto-zero amplifiers require more quiescent current for the same level of ac performance (bandwidth, slew rate, voltage noise and settling time) than do comparable nonchopped amplifiers. Even the lowest power auto-zero amplifiers require hundreds of microamperes of quiescent current; and they have a very modest 200-kHz bandwidth with broadband noise nearly 150 nv/ Hz at 1 khz. In contrast, some standard CMOS and bipolar amplifiers offer about the same bandwidth, with lower noise, on less than 10 µa of quiescent current. APPLICATIONS Notwithstanding all of the differences noted above, applying auto-zero amplifiers really isn t much different from applying any operational amplifier. In the next issue, Part 2 of this article will discuss application considerations and provide examples of applications in current shunts, pressure sensors and other strain bridges, infrared (thermopile) sensors, and precision voltage references. Analog Dialogue 34-2 (2000) 3
易迪拓培训 专注于微波 射频 天线设计人才的培养网址 :http://www.edatop.com 射频和天线设计培训课程推荐 易迪拓培训 (www.edatop.com) 由数名来自于研发第一线的资深工程师发起成立, 致力并专注于微波 射频 天线设计研发人才的培养 ; 我们于 2006 年整合合并微波 EDA 网 (www.mweda.com), 现已发展成为国内最大的微波射频和天线设计人才培养基地, 成功推出多套微波射频以及天线设计经典培训课程和 ADS HFSS 等专业软件使用培训课程, 广受客户好评 ; 并先后与人民邮电出版社 电子工业出版社合作出版了多本专业图书, 帮助数万名工程师提升了专业技术能力 客户遍布中兴通讯 研通高频 埃威航电 国人通信等多家国内知名公司, 以及台湾工业技术研究院 永业科技 全一电子等多家台湾地区企业 易迪拓培训课程列表 :http://www.edatop.com/peixun/rfe/129.html 射频工程师养成培训课程套装该套装精选了射频专业基础培训课程 射频仿真设计培训课程和射频电路测量培训课程三个类别共 30 门视频培训课程和 3 本图书教材 ; 旨在引领学员全面学习一个射频工程师需要熟悉 理解和掌握的专业知识和研发设计能力 通过套装的学习, 能够让学员完全达到和胜任一个合格的射频工程师的要求 课程网址 :http://www.edatop.com/peixun/rfe/110.html ADS 学习培训课程套装该套装是迄今国内最全面 最权威的 ADS 培训教程, 共包含 10 门 ADS 学习培训课程 课程是由具有多年 ADS 使用经验的微波射频与通信系统设计领域资深专家讲解, 并多结合设计实例, 由浅入深 详细而又全面地讲解了 ADS 在微波射频电路设计 通信系统设计和电磁仿真设计方面的内容 能让您在最短的时间内学会使用 ADS, 迅速提升个人技术能力, 把 ADS 真正应用到实际研发工作中去, 成为 ADS 设计专家... 课程网址 : http://www.edatop.com/peixun/ads/13.html HFSS 学习培训课程套装该套课程套装包含了本站全部 HFSS 培训课程, 是迄今国内最全面 最专业的 HFSS 培训教程套装, 可以帮助您从零开始, 全面深入学习 HFSS 的各项功能和在多个方面的工程应用 购买套装, 更可超值赠送 3 个月免费学习答疑, 随时解答您学习过程中遇到的棘手问题, 让您的 HFSS 学习更加轻松顺畅 课程网址 :http://www.edatop.com/peixun/hfss/11.html `
易迪拓培训 专注于微波 射频 天线设计人才的培养网址 :http://www.edatop.com CST 学习培训课程套装该培训套装由易迪拓培训联合微波 EDA 网共同推出, 是最全面 系统 专业的 CST 微波工作室培训课程套装, 所有课程都由经验丰富的专家授课, 视频教学, 可以帮助您从零开始, 全面系统地学习 CST 微波工作的各项功能及其在微波射频 天线设计等领域的设计应用 且购买该套装, 还可超值赠送 3 个月免费学习答疑 课程网址 :http://www.edatop.com/peixun/cst/24.html HFSS 天线设计培训课程套装套装包含 6 门视频课程和 1 本图书, 课程从基础讲起, 内容由浅入深, 理论介绍和实际操作讲解相结合, 全面系统的讲解了 HFSS 天线设计的全过程 是国内最全面 最专业的 HFSS 天线设计课程, 可以帮助您快速学习掌握如何使用 HFSS 设计天线, 让天线设计不再难 课程网址 :http://www.edatop.com/peixun/hfss/122.html 13.56MHz NFC/RFID 线圈天线设计培训课程套装套装包含 4 门视频培训课程, 培训将 13.56MHz 线圈天线设计原理和仿真设计实践相结合, 全面系统地讲解了 13.56MHz 线圈天线的工作原理 设计方法 设计考量以及使用 HFSS 和 CST 仿真分析线圈天线的具体操作, 同时还介绍了 13.56MHz 线圈天线匹配电路的设计和调试 通过该套课程的学习, 可以帮助您快速学习掌握 13.56MHz 线圈天线及其匹配电路的原理 设计和调试 详情浏览 :http://www.edatop.com/peixun/antenna/116.html 我们的课程优势 : 成立于 2004 年,10 多年丰富的行业经验, 一直致力并专注于微波射频和天线设计工程师的培养, 更了解该行业对人才的要求 经验丰富的一线资深工程师讲授, 结合实际工程案例, 直观 实用 易学 联系我们 : 易迪拓培训官网 :http://www.edatop.com 微波 EDA 网 :http://www.mweda.com 官方淘宝店 :http://shop36920890.taobao.com 专注于微波 射频 天线设计人才的培养易迪拓培训官方网址 :http://www.edatop.com 淘宝网店 :http://shop36920890.taobao.com