tag 标签: 电阻

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  • 2020-3-28 18:54
    1033 次阅读|
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    【AD封装】插件电阻贴片电阻排阻封装(带3D)
    包含了我们平时常用的插件电阻,贴片电阻封装型号。以及我们平时常用的插件排阻,贴片排阻封装,都带有精美3D模型。 添加小助手回复编号: 0016免费获取 ↓↓↓
  • 2020-3-10 15:04
    643 次阅读|
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    四年前,由于内存价格的飙涨,买卖DRAM(动态随机存取存储器)被称为是“比炒房还要赚钱的生意”。 四年后,存储元器件的涨价潮似乎又开始卷土重来,并且蔓延至多个领域。 自今年年初开始,受供需不平衡影响,全球存储芯片的市场开始出现波动,CMOS影像感测器(CIS,互补金属氧化物半导体图像传感器)、被动元件等关键电子元器件也出现了不同程度的缺货。 此外,由于日本和韩国承载了大量电子上游核心元器件及材料的生产,疫情如果不能得到很好的控制,业内人士分析,将会给整个电子产业的供给造成很大紧缺,从而进一步扩大涨价趋势。 群智咨询首席分析师陈军在接受记者采访时表示,疫情给很多元器件涨价提供了理由,但长期来看价格的波动仍然取决于市场需求能不能得到满足。 “受5G等因素影响 ,从去年四季度我们就判断产能吃紧,疫情只是涨价其中的一个原因。 ” 日韩疫情加速元器件价格波动 作为仅次于中、美、日、德的制造业大国,韩国和日本的疫情发展走势直接影响着全球制造业供应链的稳定。 以存储为例,存储在手机中的成本通常达到25%~35%,已超过屏幕、CPU,成为手机最大的成本,而三星、SK海力士两家韩系厂商均在存储器领域占据重要地位。 根据集邦咨询半导体研究中心(DRAMeXchange)调查,从2019年第四季度全球NAND品牌厂商营收来看,三星排名第一,市场份额达到35.5%; SK海力士排名第六,市场份额为9.6%。 2019年第四季度全球DRAM厂自有品牌内存营收排名中,三星位居第一,市场份额43.5%,SK海力士为第二名,市场份额29.2%。 也就是说,在NAND领域两家韩系厂商的市场占有率已经达到45.1%,而在DRAM领域,占比更是高达72.7%。 申港证券在报告中指出,存储市场在经历了去年 7 月西部数据断电事故,以及今年年初三星华城厂断电事故,价格已有趋稳之势。 此次又受疫情影响,特别是韩国作为全球存储重地, 疫情形势不容乐观,可能影响三星、SK 海力士工厂正常运营,从而影响全球 存储供应导致价格上涨。 业内最新的报价显示,内存及SSD硬盘的现货价已开始出现波动。 以8Gb DDR4标准型DRAM现货价为例,1月以来涨幅超过10%,4Gb DDR4标准型DRAM现货价涨幅更是接近20%。 日本企业则在半导体材料领域占据重要地位。 在 2019 年前 5 个月,日本生产的半导体材料占全球产量的 52%。 同期,韩国从日本进口的光刻胶价值就达到 1.1 亿美元。 韩国贸易协会报告显示,韩国半导体和显示器行业在氟聚酰亚胺、光刻胶及高纯度氟化氢对日本依赖度分别为 91.9%、43.9%及 93.7%。 在硅片领域,日本的信越化学和SUMCO两家就占据全球 53%的市场份额。 在去年的日韩贸易战中,日本限制含氟聚酰亚胺、光刻胶,以及高纯度氟化氢这三种材料的对韩出口,引起了整个半导体领域的震动。 上游产能不足恐延续至年中 在多家分析机构看来,电子元器件涨价潮维持多久一方面取决于疫情能否在全球得到有效控制,另一方面则是上游产能的供给是否能够满足需求。 随着去年下半年市场对5G需求的增加,加上苹果iPhone11系列销售优于预期,半导体生产链订单触底回升。 里昂证券表示,亚洲8英寸晶圆代工供不应求,不仅是台积电,联电、中芯国际等厂商也面临相同情况,包括超薄型屏下指纹辨识、5G手机拉货等都在让整体晶圆需求大增。 此外,TWS耳机带来的蓝牙主控芯片需求增长,CMOS 影像感测器需求的上升也给上游晶圆厂商的产能带来挑战。 根据群智咨询(Sigmaintell)《全球智能手机摄像头供需报告》数据,2019年全球智能手机摄像头传感器出货量约47亿颗,同比增长约15%。 得益于华为、三星、小米、OPPO领先发力多摄的贡献,使得销量主力的中低端陆续地搭载四摄。 同时,伴随着定制化和大像素需求的上升,摄像头传感器销售额也是逐年递增。 从传感器出货量来看,全球智能手机摄像头传感器供应链集中化程度非常高。 按照地区分布来看,中国大陆约占30%,韩国约占40%。 “摄像头在上半年依旧会有一个较好的发展趋势。 ”陈军对记者表示,2019年随着多摄的渗透率加速,大像素需求快速增加,导致摄像头市场供需关系出现结构性紧张,部分像素传感器在第四季度出现紧缺现象。 “部分产品一季度的数据相比去年四季度已经有5%左右的价格涨幅。 ” 除了摄像头外,被动元件等产品的涨幅已开始扩大。 被动元件主要用于电路中,控制信号传递、增益信号大小等功能,对于芯片、通信、面板等高新技术行业不可或缺。 受需求激增以及疫情影响,从今年年初开始,MLCC(片式多层陶瓷电容器)、芯片电阻等被动元件已多次计划调涨价格,近期涨价主要由电阻龙头国巨带动,调价幅度在30%左右。 早在春节之前,部分MLCC厂商就因供需紧张而宣布涨价: 华新科2019年12月27日宣布涨价20%~25%,风华高科2020年1月2日宣布涨价20%~30%,三星电机1月2日宣布涨价10%~15% 。 来源:第一财经
  • 热度 9
    2020-3-7 17:17
    1862 次阅读|
    4 个评论
    小阻值电阻的几个应用1
    对应做电子产品的从业人员,电阻是天天见的小东西。用好电阻可不是件容易的事,特别是随着工龄的增长,就越感觉电阻的玄妙。我想论坛里藏龙卧虎,有很多电阻应用的大家,这里就不做什么理论、分类的讨论,直接用自己实际的应用来作为文章的内容,应该更加容易接受。 如果想用好电阻,大家可以参考《电子元器件的选择与应用》——图解使用电子电子技术丛书。该系列丛书都是日本一线大师所写,每个电路图都有实际电路和示波器截图,远比我这种画个框图用嘴瞎说来的真实有效。我不是精日分子,不发表任何政治见解,只是如果想超过他们,就要踏实的学习,努力的工作。 书归正传,我想大家的工作台都会有一些电阻盒或者电阻本,看着密密麻麻的排列的电路,无论是E24的,还是E96的,很多同仁都能背下来,这里由衷的佩服。当然做射频电路的工程师会嗤之以鼻,毕竟射频电路用电阻匹配会被别人笑掉大牙的。射频部分会在后续讨论,我也在慢慢构思。对于1K,4.7K,5.1K,10K等这些常用阻值的,可能会是电阻盒里消耗最快的,毕竟上下拉电阻,放大电路,分压电路用的都很多。大阻值的1M欧的跟PMOSFET也是一组合适的搭档。120欧也是485和CAN总线常备之选。51欧也是以太网变压器的匹配之选——不差钱的可以用49.9欧。33欧在USB电路很常见。提到10几欧电阻,做开关电源工程师立刻举手,我们用它做启动电阻。0欧不乐意了,确实它的应用太广。毫欧级别的电阻在电流采样里还有大用处。还有一些高速电路会用到线路匹配的小阻值组排,这也不说了。 那我说什么呢,是那些阻值小的单身狗——就像我,没有固定搭配,用量的不多,但是特殊情况却又大作用。 以前的一个产品,电路框图如下(瞎画的,难如各位大家法眼)。 类似一个动作感应电路,无线发送电波,遇到物体反射回来后经过检波、放大之后再ADC采集。原理很简单(其实不简单),电路也不复杂(这是真的),做完样机之后,测试也没多大问题。但多个样机之后,还是出现了有个别电路出现震荡。想想也很简单,毕竟是个闭环系统,而且还有模拟放大电路,稳定裕度不够很容易就震荡,加上有还有射频信号,高低频混在一起,还有MCU程序,没问题才是最大的问题。 正常的查错应该是一个部分一个部分的找,确定每个部分都符合指标之后,再看整体是否有问题。但是这在企业里基本上会被定义为磨洋工、划水、说好听的也是效率低,一大堆帽子给你扣上,让你永世不得翻身,跟宫斗戏似的。时间紧任务重似乎是企业里一种再正常不过的不正常现象。没办法,人家给钱,还有房贷要还。 怎么办,既然样机没问题,那就是原理没错,增加稳定裕度也成为唯一的选择。改变放大器的电路会带来整个电路的改变,问题更大。射频电路虽然有干扰,有问题也应该是都有问题,而不是一个两个。程序和ADC也是一起搭配,不见得一起跑飞。算了,在放大电路后面增加个10欧电阻,在放大环路外增加稳定裕度吧。反正是解决了这个问题,换回Boss一句有问题不怕,就是解决问题。 同样电路,单个负反馈放大电路虽然可以做的很稳定,但是放在另一个闭环系统中,特别是容型负载,外加上其他干扰,很容易造成稳定裕度的下降,导致系统的不稳定。这时候莫不如在后端加上个小阻值电阻,也花不了多少钱,但能省很多事情。 说了这多废话,就为说一个小电阻的事。其实说着轻松,电阻虽小,那可不是小事。下回再说几个其他的应用。
  • 2018-8-23 16:59
    771 次阅读|
    0 个评论
    bourns电位器属于进口电位器,熟悉电位器的参数意义对我们选择电位器有很大帮助,那么一起来了解下bourns电位器几个主要参数。 (1) 额定功率 电位器的两个固定端上允许耗散的最大功率为电位器的额定功率。使用中应注意额定功率不等于中心抽头与固定端的功率。 (2) 标称阻值 标在产品上的名义阻值,其系列与电阻的系列类似。 (3) 允许误差等级 实测阻值与标称阻值误差范围根据不同精度等级可允许20%、10%、5%、2%、1%的误差。精密电位器的精度可达 0.1%。 (4) 阻值变化规律 指阻值随滑动片触点旋转角度(或滑动行程)之间的变化关系,这种变化关系可以是任何函数形式,常用的有直线式、对数式和反转对数式(指数式)。 (5)分辨率 电位器的分辨率也叫做分辨力,对于线绕电位器来讲,当动接点每移动一圈的时候,输出电压就会不连续的发生变化,这个变化量与输出电压的比值为分辨率。直线式线绕电位器的理论分辨率为绕线总匝数N的倒数,并以百分数表示。电位器的总匝数越多,分辨率越高。 (4)动噪声 bourns电位器的动噪声是指当电位器在外加电压作用下,其动接触点在电阻体上滑动时,所产生的电噪声。动噪声是滑动噪声的主要参数,其大小与转轴速度、接触点和电阻体之间的接触电阻、电阻体的电阻率不均匀变化、动接触点的数目以及外加电压的大小有关。 秦晋电子供应电位器,需要了解可到 www.fuse-tech.com
  • 热度 3
    2018-5-31 15:19
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    无源元件并非真的“无源”:第1部分——电容 摘要:晶体管、集成电路等有源器件利用来自电源的能量对信号进行转换,而电阻、电容、电感以及连接器等无源元件则不消耗电能——或许是我们的假设。由于无源元件均具有寄生参数,它们实际上会以不可预知的方式改变信号。本文分为3部分,这里为第1部分,讨论寄生电容的影响。 引言 有源元件和无源元件——在工程设计领域真的是非白即黑吗? 晶体管和集成电路由于利用来自电源的能量改变信号,所以被认为是有源元件。基于这个依据,我们将电容、电阻、电感、连接器,甚至是印刷电路板(PCB)称为无源元件,因为它们看起来不耗电。然而,由于无源元件均具有寄生参数,它们实际上也会以不可预知的方式改变信号。所以,许多所谓的无源元件并非真的“无源”。本文分为3部分,这里为第1部分,专注于讨论电容的有源特性。 并非完全无源的电容 无源可定义为惰性和/或不活跃,但无源电子元件会以不可预知的方式成为有源电路的一部分。所以,纯容性电容实际上是不存在的。所有电容在本质上都存在一定的寄生成分( 图1 )。 图1. 电容(C)及其最大的寄生元件。 我们进一步观察图1所示寄生元件。标有“C”的电容是我们的考察对象,其它所有元件则是不希望存在的寄生元件1。并联电阻RL引起泄漏,从而改变有源电路的偏置电压、滤波器的Q因子,并影响采样-保持电路的保持能力2。等效串联电阻(ESR)则会降低电容抑制纹波和通过高频信号的能力,因为等效串联电感(ESL)形成谐振电路(即自谐电路)。这意味着,在自谐频率以上时,电容呈现为电感,不再具备电源与地之间的高频噪声去耦作用。电容介质可能是压电介质,增加振动产生的噪声(AC),就好像电容C内部嵌入了电池(未绘出)。冷焊应力造成的压电效应可以改变电容值。压电电解电容也具有等效的串联寄生二极管(未绘出),这些二极管会对高频信号进行整流,改变偏置或增大信号失真。 较小的电池SB1至SB4表示塞贝克(Seebeck)结3,是由不同金属(寄生热电偶)在此形成的电压源。当我们连接测试设备时,需要考虑共用连接器的塞贝克效应。Jim Williams在参考文献4中指出,BNC和橡胶插头连接器对的热电势范围为0.07µV/°C至1.7µV/°C (附录J,图J5)。这一变化只适合我们日常在实验室内部的简单连接。将看起来较小的失调增益乘以1000,就达到1.7mV——这是我们尚未实际开始操作就存在的。 SB2和SB3可能是电容内部连接引线的箔,或连接至焊盘或表贴元件焊料的金属化物。SB1和SB4表示器件通过焊料到PCB铜线的结。以往的焊料是63%的铅和37%的锡,但现在使用的符合RoHS标准的无铅焊料成分变化很大,会影响电容附近的电压,所以必须查询合金成分。 可对介质吸收(DA)或Bob Pease所称的“渗透”进行建模,等效为无数个RC时间常数:DA1至DAINFINITY,其中每个时间常数由电阻RDA和电容CDA组成。Bob Pease列举了一些“渗透”非常重要的实例,本文附录中介绍了一段关于 吸收 的有趣经历。 “如果您关闭彩色电视机,然后打开后盖,那么在您开始操作之前首先必须要做的是什么?在螺丝刀上连接一条地线,然后接触高压插头上的橡胶垫圈下方,对CRT放电。那好,现在电容已经放电了,如果让这一过程持续大约10分钟,那么有多少电压将“渗透”回显像管的“电容”?当您第二次放电时,足以造成可见的电弧....这就是我所说的介质吸收5。” 由此可见,电容会随着作用电压的改变而改变。然后再加上老化、温度的影响,以及其它可能造成电容器物理损坏的众多因素6,这种简单的无源元件就变得非常复杂。 现在,我们应该讨论一下与自激有关的因素,这是去耦电容以及接地不良的电容最常见的问题。如果接地不良,任何电容都不能正常工作。电容自激主要受图1所示ESL的影响,当然,PCB过孔也会产生一定的影响。工作在射频频段时,这些过孔将影响小电容的自激点。以 图2 为例,讨论了1µF电容的曲线。 图2. 三个电容的自激频率(曲线的最低点),图示表明,电容的性能并不完全一致。在左侧,当曲线(阻抗)向下移动时,电容表现为电容。当达到其最低点时,电容呈现为电感(ESL),不再是有效的去耦电容。 1µF曲线在4.6MHz时达到最小,高于该频率时,ESL占支配地位,电容的工作特性表现为电感。由此,去耦电容在高频下称为一个双向导体:对于电源总线上的高频信号而言,电源线与地短接,反之亦然。电容模糊了电源和地之间的差异。 随着对信号频率和电容的深入考察,我们可能忘记了所产生的谐波或边带。例如,一个50MHz方波的SPI时钟,具有无限次的奇次谐波。大多数系统(并非所有系统)会忽略5次以上的谐波,因为这些谐波的能量已经非常低,在噪底以下。如果谐波在半导体器件中经过整流,仍可造成负面的影响,因为它们会转换成新的低频干扰。 控制生产误差 从图2可以看出,电容在生产过程中存在不一致的问题。一般而言,高质量电容的重复性非常好,而一些廉价电容则会受成本控制而存在较大的生产误差。有些厂商按照严格的误差等级或标准筛选电容( 图3 ),并收取高额费用。这对用于设置系统时间或频率的电容并不适合。 图3. 生产误差等级或筛选,会以不同方式影响电容性能。 图3中的实线(黑色)为一个好的生产过程的标准方差,尽管该图在Maxim Integrated应用笔记4301“ 零晶体管IC,IC设计领域的又一里程碑 ”中用于表示电阻特性,但也同样适用于电容。当生产误差变化时,每个“盒子”内的器件数量也随之变化。误差曲线可向右移动(绿色虚线),结果是没有符合1%容限的元件;统计概率也可以是双峰曲线(灰色虚线),得到较多的符合5%和10%容限的元件,而符合1%和2%容限的元件数量很少。 从分布特性看,“似乎”能够保证2%容限的元件只有-1到-2,或+1到+2 (没有满足1%容限的器件);“好像”从5%容限的“盒子”里移除了1%和2%容限的器件。我们之所以用“看起来”和“好像”是因为销售量、人为因素也会影响分配比例。例如,工厂经理可能急需发货5%容限的电容,但又没有足够的产品满足本月的需求。而库房又存放了过多的2%容限元件。于是,他将这些元件划分到5%容限的“盒子”里,然后发货。很容易解决了上述问题,人为干预(也确实这么做了)会“歪曲”统计数据和方法。 这样做对于无源电容意味着什么?我们必须了解所预期容限,比如±5%,其统计分布可能在±2%中心位置有一个缺口。电容用于控制关键频率或定时,我们需要预先考虑到这点。这也意味着我们需要规划,通过校准来修正较宽变化范围。 焊接对无源器件性能的影响 焊接会对电容造成应力,尤其是表贴元件。应力将随着振动产生压电电压,甚至损害电容,存在系统故障隐患。 大家对回流焊流程并不陌生,液体焊料的表面张力使元件整齐排列滚动,好像被磁铁吸住一样。如果焊料的温度特性较差,则有可能损坏器件。您可能在现场看到过,电容像墓碑一样单脚直立?如果焊料温度变化出现问题,既有可能引发这种情况。请务必遵守制造商的焊接建议。有些元件对温度更为敏感,所以可能需要用两种或多种不同温度的焊料进行焊接。首先用高熔点焊料对电路中的大多数元件进行焊接,然后再用低温焊接“敏感”元件。必须以正确的顺序使用焊料,避免前期焊接的器件不会随后“溶化”掉。 总结 当我们讨论电容等无源元件时,必须注意这些元件均具有寄生效应,从改变了信号。当然,这种影响取决于信号强度。当测量微伏级信号时,需要谨慎考虑以下因素:接地(星形连接点)、屏蔽去耦电容、保护线、布局、塞贝克效应、电缆结构,以及连接器。我们的原理图上往往忽略了这些因素,但当我们排查微弱的噪声干扰或信号时,将不得不考虑这些因素。 注意,无源电容不仅仅是一个无源元件,要比表面看起来“活跃”得多,寄生成分、误差、校准、温度、老化,甚至组装方法和操作规范都会对电路产生微妙的影响,从而影响器件性能。了解到这一点,我们还需要理解电容器的累积误差。在本文的后续部分,我们还将讨论其它类型的无源元件:电阻、电位器、开关,甚至是不引人注意的PCB。 参考文献 For information on distortions caused by capacitors, see “Capacitor Distortion Mechanisms,” TWEM (The Electric Web Matrix of Digital Technology), www.co-bw.com/Audio_Capacitor_Distrotion_Mechanisms.htm . Note: the author realizes that the word “distrotion” in this URL is misspelled, but the URL is correct as shown. Bob Pease, “What's All This Capacitor Leakage Stuff, Anyhow?,” Electronic Design, March 29, 2007, http://electronicdesign.com/analog/whats-all-capacitor-leakage-stuff-anyhow . Jim Williams used an “x” to indicate a Seebeck junction. He would count the junction in parallel paths and purposely cut the PC trace and solder them back together to make equal numbers of junctions. See Jim Williams et al, application note 86, “A Standards Lab Grade 20-Bit DAC with 0.1ppm/°C Drift,” http://cds.linear.com/docs/en/application-note/an86f.pdf . See also Bob Pease, “Understand capacitor soakage to optimize analog systems,” www.datasheetarchive.com/files/national/htm/nsc03883.htm . For more general information on the Seebeck effect, you can start at http://en.wikipedia.org/wiki/Thermoelectric_effect . Williams et al, “A Standards Lab Grade 20-Bit DAC.” Bob Pease, “What's all this soakage stuff, anyhow?,” Electronic Design, May 13, 1998, http://electronicdesign.com/analog/whats-all-soakage-stuff-anyhow . John Maxwell, “TECHNICAL INFORMATION, CRACKS: THE HIDDEN DEFECT,” AVX Corporation, www.avx.com/docs/techinfo/cracks.pdf . Spice tools for Kemet® can be found near the bottom of the page at www.maximintegrated.com/cal . Keith Snook, “WHAT’S ALL THIS TRAPPED CHARGE AND DIELECTRIC COMPRESSION STUFF ANYHOW?,” www.keith-snook.info/capacitor-soakage.html . 附录 无源电容的介质吸收、渗透和电压放电 我对第一次看到介质渗透的经历印象深刻,与我第一次测量功率变压器场景大不相同。 在我十几岁时,当地一位“火腿族”(20世纪中期的一个称呼,指业余无线电爱好者——糟糕,我可能暴露了我的年龄。)在他的车库中维修电视机。我从他那里学到了很多东西,有些是手把手教的。在他的工作台上有一个断开的功率变压器,引线裸露在外。我说我能够用欧姆表测量出电阻,于是,我非常幼稚地抓住两个探头,然后将每个探头按到裸露的引线上。嗖!即使欧姆表仅由3V电源供电,电感产生的反冲也足以使我牢记这次教训。 他同情地看着我(希望我牢记教训,并非要我死)。于是,他像Bob Pease所说的那样,把CRT接地,然后向我展示电荷仍会停留几分钟。我照样子做了,急于弄清电荷到底能够停留多久——结果发现电荷似乎无休止地保持着(直到我觉得无聊,停止了试验)。Keith Snook 8对DA理论进行了深入讨论,这是值得关注的一个好课题。 答案就在我们学过的知识中:我们不可能对电容完全充电,除非我们等待无限长时间。实际应用中,对于大多数电路,我们认为达到时间常数的5倍之后,即充电完毕,此时电压达到所加总电压的99.3%。电容放电的过程亦如此。就CRT而言,从高压开始,在较长的时间内都能产生令人痛苦的电击。 无源元件并非真的“无源”:第2部分——电阻 电阻可不是简单的角色 Passive components don't draw power but even resistors can, and do, modify signals in unexpected ways. A resistor's reaction to temperature, voltage, and signal frequency can often catch the inexperienced engineer by surprise. Tolerances may not be as they seem and simple resistors may provide nonlinear signal response, introducing harmonics where there were none. Capacitors, resistors, inductors, connectors, and even the PCB are called passive because they don’t have gain or control power like semiconductors or other active devices. But these apparently passive components can, and do, change the signal in unexpected ways because they all contain parasitic portions. In Part 1 of this series on “passives”we talked about capacitors. Now in Part 2 we look at resistors. Indeed, resistors are simple, benign, passive devices—right? Wrong. As we will see, resistors really do some unexpected things. In Part 3 we discuss how PCB flaws and errors that are usually hidden, or at least disguised, can introduce passive errors into IC performance. The Simple Resistor, Really Ain’t that Simple How many times have we walked down a street and seen concrete that is lumpy, bumpy, and horrible. It reveals someone’s inexperience and overconfidence because pouring concrete looked so simple. Resistors have the same basic issue: they seem simple until one looks closely. There is a superb book on resistors1 and the authors Dr. Zandman et al. have the same lament, “This work attempts to demonstrate that the design and fabrication of resistive components require the application of particularly complex physical phenomena and are no longer based on the traditional empirical methods generally associated with the ‘kitchen recipe’ approach.”2 Ahhh, the kitchen. While cooking, so many of our mothers use to say, “a little of this and a pinch of that.” Fine for cookies, but this kitchen-recipe approach to manufacturing resistors is a serious issue. There are vendors that prioritize price over quality. Other vendors accept a large variation in tolerances as if they were formulating batches using a kitchen recipe. A little difference in food can add variety and interest, but the kitchen recipe has no place in manufacturing of close tolerance parts. The late Dr. Zandman, inventor of a zero-tempco resistor and founder of Vishay Intertechnology, certainly underscores his work with mathematics and material science. His book delineates the formulas and reasons behind the many variations in resistors. He devotes three wonderful chapters to Ohm’s law, first to Ohm’s law itself and its limitations, then to reversible and irreversible phenomena associated with the law. Reversible conditions include a rise in temperature that changes the resistance, but the resistance returns to the starting point when the temperature is reduced. An irreversible effect means that the resistor’s change becomes permanent like those caused by diffusion or oxidation. Set Tolerances to Match the Application Let’s admit an important fact at the outset: resistors introduce error. Our initial reaction may be to ignore resistor inaccuracy as “too small to matter.” After all, a pinch of salt in an Olympic-sized swimming pool does not make it salt water. True, but adding a ton of salt would be a different issue. Obviously, an application dictates the acceptable error. The more precision required, the tighter the component tolerances must be. Knowing this, we should define what magnitude of error is acceptable. We will examine an example system with 12-bit resolution, one-half the least significant bit (LSB) is one part in 8192, or 0.012%, or 122 parts per million (ppm).3 A quick look at Digi-Key® and Mouser® catalogs finds thousands of 1% tolerance resistors with typically a ±100ppm/°C temperature coefficient (tempco). Consequently, barely more than a 1°C temperature change causes more than one LSB deviation. Not so good. So let’s try 0.1% tolerance resistors with a ±25ppm/°C, which means if the temperature changes 5°C, our error is one LSB. Remember that this is just one resistor and most systems have many resistors. We can draw some important conclusions from this example. To bracket the resolution numbers: A. For 8-bit (1 part in 256) resolution, one-half LSB is 0.195% or 1953ppm; and B. At 16-bit resolution (1 part in 65536), one-half LSB is 0.0015% or 15ppm. Clearly the need for smaller tolerances and tempco is more important at higher resolutions. That is actually true for many systems, but there are two extreme cases to note. First, a completely open-loop use, such as an arbitrary waveform generator, needs the DAC output and amplifier to have nearly perfect linearity. Second, a system with a feedback loop, such as a mechanical movement generated in a process controller, has servo action that is always driving the action toward the center to null out any error. As long as the servo is directed in the proper direction (the system is, by definition, monotonic), small nonlinearity errors will be ignored. Resistor Parasitic Components Figure 1 illustrates the parasitic components that are present with resistors. Inside the dotted Resistor box is the resistor. The inductors and capacitors on either side are the PC board (PCB) connections and traces. The R is what we want; the additional factors inside that box are unavoidable parasitics. To illustrate the effects of these parasitics, we drive the left side of the resistor with a low-impedance signal generator. That will swap out the left capacitor to ground, CG. Figure 1. We may only want a resistor, the R above, but we also have all the other unavoidable parasitic components. We can minimize some parasitics, but they are always present. To the right of the resistor we see the composite of all the network components. A frequency sweep of a sine wave shows the dominant RC high-frequency rolloff caused by resistor R and the right-side capacitor, CG. The series inductors cause additional, but minor, high-frequency attenuation. Capacitor C and inductor L inside the resistor cause a minor frequency peaking. Yes, each of the parasitic components is small. Still, we need to consider them when designing circuits so we can decide whether or not to ignore them. For example, at audio frequencies we can choose to disregard the parasitics, but at radio frequencies we may have to adjust for them. The piezoelectric element, P, is interesting as it influences performance during stress and vibration. (It also could represent a magnetostriction response to a magnetic field.) Stress can change the resistance depending on the resistor chemistry, and vibration can be converted into small AC voltages, which then add to the electrical noise. The solder stress is probably dominant and is important especially with surface-mount parts. Older designs with through-hole resistors allowed the leads to twist to absorb and mitigate most of the stress. Surface-mount parts, however, are held against a relatively rigid PCB. As the solder solidifies, these parts capture the change in thermal expansion between the resistor and PCB. To minimize the stress we must carefully follow the manufacturer’s recommendations for solder time/temperature profile. Let’s now talk briefly about wire-wound resistors, often chosen because they have very low temperature coefficients (tempcos). These resistors also have an important, unique characteristic: its structure can react to magnetic fields. Because they are essentially a coil of wire, they magnify a magnetic field that a single conductor might pick up. As a coil, they also have more inductance than other types of resistors. We have seen circuits with transformers, inductors, and wire-wound resistors cross talk over small magnet fields. To mitigate these effects, careful layout, rotating components 90 degrees, increasing the spacing, and shielding may be necessary. Finally, do not forget our friend Seebeck. Any dissimilar metal connections such as at the solder-to-board interface can cause small temperature-dependant offset voltages. Manufacturing Tolerance, Power Rating, and Temperature Coefficient Two other parameters, manufacturing tolerance and power rating (wattage), also impact resistor operation. In our last article4 on capacitors we explained how sorting and binning can distort manufacturing tolerances. This can also happen with some kinds of resistors. As a general rule, binning can cause performance problems for both manufacturers and customers if there is a process shift and then there is a large demand for the most precise item. A manufacturer can always ship more precise parts in place of low-precision parts, but the reverse is not true. For example, a 5% tolerance resistor could actually contain resistors with a tolerance between -5% to -2% and +2% to +5%. This is clearly not the full range between -5% through +5% that one might expect. If not enough high-precision devices are binned or if the customers only want the high-precision parts, then the manufacturer faces parts shortages. Power rating is simple, right? Voltage times the current (V × I) tells you what wattage rating to choose so the resistor does not burn up from self-heating. Right? No, wrong (or, maybe)? The answer, of course, depends on the application. A series resistor to limit current in a light-emitting diode (LED) can be a plain “vanilla” circuit where little additional concern is necessary. If the resistor has a negative tempco the resistance is reduced as the temperature increases. This, in turn, causes the resistor to draw more current at higher temperature, and that can contribute to overheating. At the other extreme, bias and modulation currents are critical in radio and laser communications systems. Many systems including radio and laser communications systems need to remain stable over the operational temperature extremes. Centering the feedback loop over temperature and voltage changes requires deeper study. How much power is dissipated and how each of the components reacts, including the resistor, are important. In such circuits the laser must be cooled to keep it on frequency; the heat of the surrounding components (self-heating) must also be removed. What do you do? There are questions to ask and answer before your design goes any further. Is there air flow across the circuit? What is the temperature of the air at the critical circuit (not just the air entering the enclosure)? Was the air heated by another circuit? Many times the air is first routed to the circuit boards and then exits through the power supply. Now does the power-supply voltage change with temperature? Are there other systems racked together in the same enclosure? Are there fans? How does dust and dirt collect, and what happens if one or more fans fail? Most resistors have a negative tempco, meaning that the resistance is reduced at higher temperatures. This also means that the resistor draws more power when heated. Each of us needs to carefully read a resistor’s data sheet because the different chemistries and manufacturers may have different ways of specifying the tempco. The tempco curves can be just about any shape and they may be specified by the “box method,” which is common for integrated circuits (ICs).5 Even a factory-trimmed part with thousands of transistors will show a family of curves over temperature and process variation. Simulation and correlation allow us to define a box that contains all the possible curves. The box “x axis” is the total operating temperature and the “y axis” the total magnitude of the error. Statistically we guarantee that the error of all the parts is within the box, but we do not know the shape of the curve for any individual part.6 Specialized resistors called thermistors can have negative (NTC) or positive (PTC) tempcos, and the curves tend to be very nonlinear. Basic Chemistry and Voltage Coefficient of Resistance (VCR) What is inside IC resistors, i.e., the chemistry, is very important for understanding resistance. Designers and process engineers need to understand how chemistry in the manufacturing process affects resistor performance.7 In chemistry there are two broad classifications for things made of one or more chemicals. A compound is two or more chemicals that react to make something new. A mixture is multiple mixed chemicals that retain their original properties. Remember that the resistors with color bands on a brown shell are carbon composition resistors, CC. The CC resistors are mixtures and some of the contact points inside form semiconductors. They change resistance with heating, cooling, vibration, and applied voltage. The high-voltage vacuum tubes (“valves” in the U.K.) remembered from our past history (and that still “resonate” for some audiophiles today) created “resistor distortion”8 that some people actually find pleasant. The distortion is caused by a voltage coefficient of resistance (VCR), a reduction in resistance value with an increase in voltage. In an audio system with a sine-wave signal of 75V peak-to-peak (VP-P), biased at 50V, the resistor that sets gain will be at a higher resistance (gain) on the lower half of the sine wave and have a lower resistance and gain on the positive peak. This adds second-harmonic errors to the signal. This “resistor distortion” is soft and smooth in the onset of distortion, which, as mentioned above, some find pleasant. For most resistors the voltage-coefficient error only becomes measurable over 25V. Today most circuits are lower voltage so the resistor distortion tends to be ignored. VCR is an important characteristic of high-voltage thick-film resistors.9 Typical thick-film ink consists of conductive material suspended in an insulating matrix. As the voltage across the ink is increased, new conducting paths are opened. The result is a drop in resistance. This means that the VCR is always negative in value. Thick-film resistors can be used as series-protection resistors in electrocardiogram (ECG) input circuits. These resistors help protect the ECG input from the 3kV to 5kV from defibrillator pulses.10 Obviously, we want the resistors to maintain their value and survive multiple voltage pulses. There are resistors with a VCR of < -1ppm to 5ppm over a large working voltage. With ECG, the humidity and the temperature performance of the resistor are critical. The resistor also must dissipate the heat energy of the defibrillator pulses. Thus, while the IC designer does not define the internal chemistry of a circuitry, it is important to understand how chemistry affects the tolerance and tempco of the part. Once again, this speaks to the importance of studying the data sheet. Thermal, White, or Johnson Noise Thermal noise, also called Johnson noise, is present in all passive resistive elements and is caused by the random thermal motion of electrons. The thermal noise level is unaffected by DC current. Resistors always generate noise, even when floating outside a closed circuit. This is white noise, which has a uniform spectral density and increases with temperature and resistance. Because some resistors are made of semiconductors, they can have other types of noise, such as shot, avalanche, flicker (1/f), and popcorn noise11 There is a free Thermal Noise Calculator and a User’s Guide that further explain the different noise types.12 Conclusion Good engineering is about the details and we are fortunate to be standing on the shoulders of engineering “giants.” Pioneering engineers like Dr. Zandman have struggled while researching physics and materials science, executing the careful work that produced the understanding that we rely on every day. As he observed, the seemingly small insignificant factors in an IC are many times taken for granted. This is certainly true for resistors that seem to be benign and passive, until their performance in a circuit startles us awake. That little resistor, in fact, dominates the circuit’s error budget. Tempco and manufacturing tolerance are just the start. That passive resistor can change value with voltage and actually lowpass filter a signal. The effect is unexpected and surprising until we look closely and realize that there is more to a resistor. Ultimately, the resistor that we tried to ignore is just following the laws of physics and we need to pay it special attention. References Zandman, Felix, et al., Resistor Theory and Technology , Vishay Intertechnology, Inc., ©2001. Ibid. , Introduction, p. 3. Maxim Integrated tutorial 5060, “ ADC/DAC Accuracy Calculator Tutorial ,” Free “ Accuracy Calculator ” (ACCU). Laumeister, Bill, “ Passive Components Aren’t Really So Passive (Part 1): Capacitors ,” Electronic Design, online June 4, 2013, also as Maxim application note 5663 . Maxim Integrated application note 4419, “ Understanding Voltage-Reference Temperature Drift .” Maxim Integrated tutorial 5062, “ Bandgap Reference Calculator Tutorial ,”. The free calculator includes the User’s Guide in the calculator documentation zip file, and explains the theory and practical operation in detail. Thei, K. B., et al., Characteristics of Polysilicon Resistors for Sub-Quarter Micron CMOS Applications , Inst. Of Microelectronics, Dept .of Electrical Engineering, National Cheng-Kung University, Taiwan. Keen, R.G., Using the Carbon Comp Resistor for Magic Mojo , Copyright 2002 R.G. Keen. Birkett, M., “ VCR Characteristics of High Voltage Thick Film Resistors ,” Resistor Business Unit, TT electronics Welwyn , July 2007. Oxley, Stephen, “ High reliability passive components in three broad areas: Contact, imaging, analysis ,” TT electronics, Fixed Resistors Business Unit , 7 April 2011. Edwards, Steve, “Managing Noise in the Signal Chain, Part 1: Annoying Semiconductor Noise, Preventable or Inescapable?” Maxim Integrated application note 5664 . Maxim Integrated tutorial 5059, “ Thermal Noise Calculator Tutorial ”. 无源元件并非真的“无源”:第3部分——PCB Abstract: Active components like transistors and integrated circuits change signals using energy from the power supply. However, passive components like resistors, capacitors, inductors, and connectors actually can, and do, change the signal in unexpected ways. This happens because all these passive components contain parasitic components. This application note, the last in a 3-part series, discusses printed circuit boards and the errors that can occur because passive components aren't really so passive. Introduction Sometimes the best way to hide something is in plain view. Magicians use this technique along with some distraction to amaze an audience ( Figure 1 ). It is simple actually: our experience leads us to expect certain norms and to see what we expect. Thus boxes are square, not squished parallelograms; spheres are symmetrical, not hemispheres or with elongated portions on the back where it is unseen. In that same sense, printed circuit boards (PCBs) seem straightforward. You think that you can see everything going on, but you are really only looking at the circuitry on the exterior surface. In fact, if you delve deep enough down to the board itself, you find complex layers and structures and a myriad of things that can go wrong here. When high-precision op amps and high-resolution data converters fail to perform as expected, we need to closely examine all the surrounding active and passive components, including the PCB. Into this context we also insert the PCB vendor who has an understated role that is, in fact, critical for IC performance. This article is Part 3 of a series on passive components in ICs. In Part 1 we talked about capacitors. In Part 2 we looked at resistors and explained that they are not seemingly simple, benign, passive devices. Here in Part 3 we are going to discuss how PCB flaws and errors that are usually hidden, or at least disguised, can introduce passive errors into IC performance. To understand how PCBs can introduce passive errors, we must first examine the composition of a typical board. Four examples of PCB problems and efforts to solve those hidden errors will help us appreciate the contribution that a good reliable PCB vendor makes to successful products. We admit here that our articles on passives have generated some lively discussion about the definition of “passive.” In our search for more knowledge and better-informed engineers, we are quite pleased about this. See our Sidebar: Defining Passives for some summary comments on this discussion. Figure 1. A magician and his assistant provide distractions to help “sell” illusions. Passive Viewing—Seeing What We Expect Let’s see how well, how carefully we viewed Figure 1. Did you notice the PCB assembly? Yes? No? It is in the shadows just to the woman’s left side. Yes, we see what we expect to see. The same is true when we examine a PCB. When you look at a typical board directly ( Figure 2 ), what do you see? Figure 2. A PCB assembly with various components. If you are like most of us, we see an Ethernet connector, another RJ-45 connector with the label “settings sensor”, “UPS data”, and “RS-232”. We see an inductor and electrolytic capacitors for a switching supply, several large-scale integrated circuits (ICs) and a bunch of decoupling capacitors. Put all this together and it is probably a digital board with several options because we can also see unstuffed components. Right? Yes, but we did not really see the bare PCB itself, and that is where this story starts. As we said at the outset, myriad things can go wrong with something as complex as a PCB. Experience has taught us that a good, reliable PCB vendor is very important to us now. There are many choices in the materials, the density of the weave in the FR4, the polymer, via construction, minimum trace structure for a given etch method, tin plate and solder mask choices. We might specify a hard-to-find FR4 (a common fiberglass PCB) material because we prefer it, but lack of available FR4 materials could delay production and even double the board cost. Our respected PCB vendor will know about resources, what via construction methods are available, or what assembly methods are recommended for our application. There is definitely nothing passive about this relationship. When we tell the vendor that we care about board quality, he will reciprocate in like manner. No Magic Wand Building a Board Yes, the board—you start with fiberglass. The top and bottom layers (typically industry-type FR4) have copper on what will become the outside of the PCB. The center layer is copper with FR4 on both sides, thus comprising the two inside conductive layers. Prepreg is effectively the glue that holds the stack together; it can be just adhesive or it can be a combination of FR4 fiberglass and thermal-setting adhesive. During the fabrication process the stack in Figure 3 will be compressed under heat and pressure to bond the layers together. Figure 3. Is a typical four-layer PCB stackup. The order of construction can differ depending on many things. Our favorite reference resource, the handbook that most engineers call the “PC Bible,” is by Coombs.1 He details the PCB fabrication processes, outlining literally hundreds of variations and possibilities. Just when you are thoroughly intimidated, you get to the Appendix. The knowledge in the Appendix is massive, a list of industry standards pertaining to everything PCB. It takes you from components including surface mount, general and passives, to printed boards, materials, design activities, then to component mounting and soldering, and through quality assessment, test methods, and repair. At this point we begin to appreciate and understand why we need our best board vendor to guide and advise us. Still, mistakes do happen with boards, and it seems that they always occur just before a firm deadline. The four PCB examples below happened either before a bed-of-nails test of the bare board was available or after that test was eliminated to save time—always a bad practice that will punish us. Can you guess the errors that we found in each example? Example 1: Over-etching We received PCBs and assembled six boards. Oddly, the boards all had different issues. Normally when you fix one board that same fix applies to all the boards. But not this time, which was the key to understanding the problem. We found that some of the errors were tiny shards of copper that shorted random things. Simultaneously, we were seeing a massive “passive” problem (at least we usually think that a PCB is passive) in the circuit’s performance. No circuit can function with dozens of random shorts. Because these shorts were random and different on each board, it was a troubleshooting nightmare. We sectioned the PCB and looked under a microscope. The board was over-etched, as shown in Figure 4 . Figure 4. PCB section with over-etched, thin copper edges that break off as long thin shards and short to adjacent traces. Figure 4A has flat sides under photo resist. If the chemistry and temperature are not correct or the board is in the etching solution too long, the effect is etching “around the corner” that undercuts the copper (Figure 4B). Long thin shards can break off the top edge, stay connected on one end, and short to the adjacent traces. Looking closely at our board, we saw two abrasive scratch-mark patterns at 90 degree angles. The vendor had used a polymer grinding wheel with an embedded abrasive. They attempted to scrub off the shards by grinding the board in two passes on each side. They did remove the majority of the shards, but then they solder-coated the board, which made the remaining shards solid random shorts. Adding solder mask hid the shorts and most grind marks. Example 2: Orientation We received a two-sided PCB with solder mask and top silkscreen and assembled a board by hand with through-hole parts. Nothing worked. We had a serious problem with the so-called passive PCB: all three power supplies were shorted in multiple places. Nothing made any sense; not one circuit block out of dozens functioned at all. The technician tried, but finally called the engineer for help. The technician managed to insert the parts in some strange ways. For example, the three leads of a transistor, which would normally form a triangle in the silkscreen outline, were distorted and twisted. Looking closely under the solder mask we realized that the silkscreen and the bottom side of the board were oriented properly, but the top component copper side of the board was a mirror image. The film used to make the topside image was upside down when the solder resist was exposed. Example 3: Find Your Way We received a four-layer board as in Example 2 above with similar issues. Again many traces were connected to the wrong things, power supplies were shorted at multiple places, and nothing (no circuit block) worked. Usually when there is a board error, at least some of the circuits function. We had implemented a complete bed-of-nails test and were confused when it did not identify the issues. Then we found out that the purchasing department skipped the bed-of-nails test to expedite board delivery. That test would have saved us days of effort. The wasted time was a costly error. We found that the board layers were assembled in the wrong order. Many blind vias were attached to the wrong layers. As a result, we added an edge code ( Figure 5 ) so we could inspect the boards. Figure 5. The copper layers of the PCB with a staggered edge code on the right. With the code implemented, we could quickly inspect the board layer order before we wasted time by populating the board with components. The code of Figure 5 extends to the edge of the PCB. The boards are typically fabricated in larger panels made of many smaller PCBs to ease handling during fabrication. The individual boards are separated using a router, thereby exposing the Figure 5 code on the board’s edge. A microscope lets us measure the copper spacing and see that it meets the board specification. This assures us that the stripline will be the correct impedance. Example 4: the Right Thickness, but Not the Right Answer We received a four-layer board. Most of it worked, but the striplines had huge ringing and reflections. Striplines are the equivalent of coaxial cables embedded into the PCB. A coax is a center conductor inside an insulating dielectric, surrounded with a circular ground shield. In addition to shielding the signal from external contamination, the coax and the stripline provide a known impedance signal path that, when terminated in its characteristic impedance, does not reflect energy. If the PCB is not constructed properly, the impedance change causes reflections and ringing which destroys analog signals and can even confuse digital signals. Sectioning the board permitted us to measure the thickness of the various layers. We found that the PCB vendor had a shortage of some thicknesses of the board material. Their untrained employee tried to meet our delivery deadline and substituted extra prepreg layers from something in stock, thus making the total thickness correct. This might sound like a good “fix,” but it definitely was not so. Look back at Figure 4. Let’s say that the center layer with copper on both sides was substituted with thinner material. The capacitance between those two layers will rise because the dielectric is thinner. To keep the layout and the final board the same total thickness, we can compensate by increasing the upper prepreg layer thickness. This will lower the capacitance between the top copper and the nearest copper layer in the center. Note, however, that this also assumes that the prepreg has the same dielectric constant in both cases, which may not be true. Thus, the change in capacitance changes the PCB and stripline impedance, and our supposedly “passive“ PCB is now ringing. You can say “Good bye” to signal integrity. PCB Problems Cause Passive Failures Clearly an unseen, taken-for-granted, whatever-you-want-to-call-it PCB exerts considerable influence on precision circuit performance. Moreover, we cannot take anything for granted nor assume that passive IC problems are unrelated to the PCB itself. Common IC performance problems and errors caused by a bad PCB include voltage drops and impedance in ground vias, planes or foils; leakage resistances and moisture absorption; and stray capacitance, with welcomed and dielectric absorption or soakage. Voltage Drop Voltage drop in ground vias, planes or foils is a commonly overlooked issue. Adding to the complexity of the problem, voltage drops at both DC and high frequencies require different remedies. Recall Coombs Handbook,2 Chapter 10 for trace versus capacitance and crosstalk and Chapter 13 for voltage and ground copper thickness versus sheet resistance. For via impedance we look to Sayre:3 L = 5.08h Where: L = inductance of the via, nH h = length of the via, inches d = diameter of the via, inches Using h = 0.0625 inch and d = 0.020 inch gives us a via inductance of 0.666µH. How can we reduce this inductance? Place two, four, or more vias in parallel. This is a good first-order approximation and is useful in thinking about signal integrity below a few hundred megahertz. For more details and consideration of the current return paths, we turn to Howard W. Johnson and his “Black Magic” series.4 Leakage Resistance The leakage resistance5 of the PCB can disturb sensitive high-impedance circuits. The sources of leakage include improper selection of a laminate material, fingerprints, skin oils, human breath, residual fabrication chemicals, improperly cleaned solder flux, and surface moisture and humidity. If this is a problem for your circuit, consider surface and subsurface contamination and moisture absorption to be everywhere, on, in, or under the solder mask; on, in, or under conformal coatings; on or in active or passive components. When troubleshooting an existing PCB, remember an experienced engineer blowing on the board through a soda straw. The straw localizes the moisture to help identify the sensitive area. Thorough board cleaning with the proper solvents is important. The wrong solvent, for example, cleaning a water-soluble flux with a polar solvent, can leave salts on the board. If deionized water is used to clean the boards, bake the boards to dry them. Even now you may not be done. Even the cleanest board may still cause problems. A PCB with a very sensitive circuit such as an op amp with a high impedance input and high gain likely needs additional attention. It might be necessary to guard or surround the sensitive pins on all board layers with a driven low-impedance circuit matching the DC level of the guarded pins.6 Stray Capacitance Capacitance is usually a problem when it is stray and unavoidable. It reduces bandwidth and slows high-speed signals. It is bad when dielectric absorption or soakage7 causes hooking, slew-rate errors, or under-/overshoot. However, capacitance is welcome when it is high-frequency power decoupling. We can specify a thinner than normal dielectric (even thin FR4) between the power and ground planes to increase the capacitance. Discrete capacitors smaller than 10pF (self-resonant at ~2GHz in surface mount) are easily compromised by trace and via inductance. Where the capacitance is distributed between power and ground planes, it has low series inductance and is repeatable if, yes, if we have a “golden” excellent PCB vendor. Summary Let’s think back to our opening magician’s mysterious box with hidden tricks. We expect certain norms and see what we expect. We simply cannot be that blind when it comes to the potential problems in a PCB. The manufacture and assembly of a board is far more complex than it appears to a casual examiner and in that complexity lies the potential for PCB flaws and errors. Now, most importantly for our discussion, those flaws and errors can introduce passive errors in ICs. We only examined voltage drop, leakage current, and stray capacitance, but the list of potential passive errors is indeed longer. Solving these passive problems inevitably means fixing the PCB, and each situation will demand its own solution. Finally, within this context we can all appreciate the contribution that a good reliable PCB vendor makes to our successful products. References Coombs, Clyde F. Jr., Printed Circuits Handbook , sixth edition, McGraw Hill, ©2008. Coombs, op cit. , Chapters 10 and 13. Sayre, Cotter, Complete Wireless Design , McGraw-Hill, IBSN 978-0-07-154452-8, page 421 in first edition. Johnson, Howard, High Speed Digital Design: A Handbook of Black Magic and High Speed Signal Propagation: Advanced Black Magic and Via Inductance Maxim Integrated tutorial 4429, “ Murphy's Law and the Risks of Designing ‘Off Data Sheet ,’” Figure 3 and the following text. Maxim Integrated application note 3264, “ Compact DWDM laser Temperature Control with the MAX8521 ,” Figure 2 and surrounding text. Laumeister, Bill, Passives Aren’t Really So Passive: Part 1, Capacitors , Electronic Design , June 4, 2013 article. Sidebar: Defining Passives When we started talking about “passives”, we stirred up a hornet’s nest? Several engineers1 inside Maxim Integrated and in the larger engineering community immediately challenged the definition of “passive.” We are still trying to find a short, accurate definition that is universally accepted. The most common definition is simply “not active.” Thus, a typical active device uses power to do something like create gain. But there are always exceptions. For example, an emitter follower is active, uses power, converts impedance, and has a gain just less than unity. The goal of these articles on passives, therefore, has been to warn people that what we think is a passive, can and does cause nonlinear responses that can change the signal. Thus resistor voltage dependence or capacitive absorption (soakage) can cause harmonic distortion. Hydroscopic PCBs can change offset. How does one define a passive component? It is also a tough question. Engineers in a chat room had some good suggestions. The IEEE® dictionary2 defines: Passive device, A device that does not require power and contains no active components. Passive Electric Network, An electric network containing no source of energy. Davor Vujatovic in the Encyclopedia of Life Support Systems (EOLSS) suggests a passive definition:3 A passive component denotes a component that is unable to deliver more energy to an external circuit than it initially stores. To determine whether the component is passive, the total energy absorbed by it must be greater or equal to zero. In other words, a component that absorbs more energy than it delivers is passive. If the total energy delivered by the component is greater than the total absorbed energy, the component is active, i.e. the active component is capable of delivering energy to the outside world. The chat room engineers also suggested the Wikipedia entry under, “Passivity (engineering)”.4 It has an interesting perspective in the first two paragraphs: “Passivity is a property of engineering systems, used in a variety of engineering disciplines, but most commonly found in analog electronics and control systems. A passive component, depending on field, may be either a component that consumes (but does not produce) energy (thermodynamic passivity), or a component that is incapable of power gain (incremental passivity). A component that is not passive is called an active component. An electronic circuit consisting entirely of passive components is called a passive circuit (and has the same properties as a passive component). Used without a qualifier, the term passive is ambiguous. Typically, analog designers use this term to refer to incrementally passive components and systems, while control systems engineers will use this to refer to thermodynamically passive ones.” Then depending on one’s engineering discipline; Wikipedia says, Thermodynamic passivity In control systems and circuit network theory, a passive component or circuit is one that consumes energy, but does not produce energy. Under this methodology, voltage and current sources are considered active, while resistors, capacitors, inductors, transistors, tunnel diodes, glow tubes, metamaterials and other dissipative and energy-neutral components are considered passive.” Incremental passivity…In circuit design, informally, passive components refer to ones that are not capable of power gain; this means they cannot amplify signals. Under this definition, passive components include capacitors, inductors, resistors, diodes, transformers, voltage sources, and current sources. They exclude devices like transistors, vacuum tubes, relays, tunnel diodes, and glow tubes.” The Wikipedia article really sums it up in the second paragraph: “Used without a qualifier, the term passive is ambiguous.” We included the wording “seems to be passive” in our article definition in an effort to “weasel word” the definition to allow nonlinear distortion from something that we expect to be “inert” or “benign.” “Seems“ used above drew lightning for the engineers, so now adding inert or benign will probably add more fuel to the fire. We are still trying to find a short, accurate definition of “passive” that is universally accepted. The most common definition is “not active” and it is not sounding so bad after all. A similar version of this article appeared January 9, 2014 in Electronic Design . References www.electro-tech-online.com/threads/capacitors-more-active-than-you-think.135643/#post-1137754 The IEEE Standard Dictionary of Electrical and Electronics Terms, sixth edition, IEEE Std. 100-1996; terms are ”Passive device” and ”passive electric network.” Vujatovic, Davor, Electronic Engineering Vol I –Active Networks, ©2009 Encyclopedia of Life Support Systems, EOLSS Publishers Co Ltd, ISBN-13: 978-1848269774). Wikipedia: http://en.wikipedia.org/wiki/Passivity_(engineering) 应用笔记
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    面试问题噢,那个面试问题―萦绕在耳边的那个问题作者:BruceTrump,德州仪器(TI)2012年10月2日上周的博文在Facebook上引起了一些热议,我感觉有必要继续写下去。下面是让我纠结了41年多的面试问题:一个1VAC电源连接至一个与1Ω电抗电容器串联的1Ω电阻器,那么电容器的AC电压是多少?多年以来,我把这个问题告诉过许多工程师。最常见的回答是,“频率为多少?”但是,为什么你需要知道呢?我们已经知道电容器的电抗。频率就不那么必要了。其它一些人问我电源有没有可能是DC,但这并不是一道脑筋急转弯题。在我的图解法中,我清楚地表明电源为AC,并且电容器的电抗有限。它不可能是一个DC电源。一些老兄掉入了0.5V的陷阱。如果它只是一个电阻分压器(1Ω-1Ω),则其可能为输出电压。但是,此处不是这样。那个时候,我通过进行一些简单的相量数学计算,正确地回答了这个问题。这没有什么了不起的。但是,正如我上周所说,我原本认为我能够展示出更多新的观点。下面是我希望的答案:R/C电路形成一个简单的实极点。电阻和电容电抗相等的情况便为“角”频或者截止频率。45°延迟时该点的响应为-3dB(0.707V)。就这么简单。无需数学计算。下面是波特图:另一个观察角度:电阻器电压的大小与电容器一样―都为0.707V。当然,相位不同。现在,我敢说我最初的回答无法让我赢得那份工作。但毕竟它是正确的。那么,为什么它会让我纠结呢?我欣赏直觉理解力。我认为,它是创新的关键因素。附言:我利用1Hz角频率在TINA-TI中绘制的该波特图(上面)。由于使用一个1Ω电阻器,因此它要求一个大电容器值。它应该为一个非极性电容器。我有许多1uF聚酯电容器。它要求多少呢?……
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    构建属于你自己的差动放大器——有时1%电阻就已经足够了构建属于你自己的差动放大器——有时1%电阻就已经足够了作者:BruceTrump,德州仪器(TI)标签:分流器电流监控器、INA133、1%电阻值通过上一篇文章,我们知道,集成差动放大器的高精确匹配的电阻器对于获得需共模抑制至关重要。然而,在一种相对常见的情况下,1%电阻器和一个较好的运算放大器便可以构建一个完全合格的差动放大器。当我们在负载“低侧”的情况下使用一个分流器进行电流测量时,共模电压常常非常小。您可能会忍不住想要使用一个标准的非反相放大器来测量该分流器的电压,因为分流器电压为接地参考。但是,仍然可能会有较小的杂散接地电阻压降。您可能需要一种差动测量方法对该电压进行开尔文检测,从而实现分流器的四线连接。[pic]由于杂散或者寄生电阻的压降都很小,因此使用中等共模抑制比的差动放大器便已完全足够。正如我们在上周的文章中所讨论的那样,如果在这种自制差动放大器的电阻器中,有两个电阻器错配±1%,则杂散电阻误差电压衰减100x,也即40dB的共模抑制比。如果这种寄生杂散电阻的唯一电流为已测得的负载电流,则所产生的误差刚好为期望信号的增益误差。它可以为正或者负增益误差,具体取决于电阻器错配的方向。但是,电路板或者系统中常常会存在其他电流,这些电流可能会形成与已测得的负载电流无关的电压。[pic]另外,图2描述了一个低侧测量案例。在这种情况下,您可能还会需要高精确电阻器匹配。此时,输出电压为偏移电压,并且基准电压应用于差动放大器的“参考”端。这样做的目的一般是为了把输出电压升高至零以上,从而更加精确地处理接近零负载电流的信号。这种方法与我们上周介绍的方……
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    内部输入偏置电流消除InternalInputBiasCurrentCancellationBruceTrumpLastweekwereviewedtheuseofaninputbiascurrentcancellationresistortobalancethesourceresistanceatthetwoinputsofanopamp.Theconclusionwasthatthispracticeisoftennotnecessaryandmayevenbedetrimental.Thisdiscussionbuildsonthepreviousblogsoyoumaywanttoreviewit,first.[pic]Iendedlastweeksayingthattherearecertainopampsforwhichthispracticeisdefinitelynotrecommended.Theseareamplifierswithbipolarinputtransistors(BJTs)thathaveinternalinputbiascurrentcancellation.Theyhavecurrentsources,I1andI2thatsupplybasecurrentfortheinputtransistorpair.Thesecurrentsarederivedbymirroringcarefullymatchedbasecur……
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    内部输入偏置电流消除内部输入偏置电流消除作者:BruceTrump,德州仪器(TI)标签:偏置电流消除、OPA209上次,我们讨论了输入偏置电流消除电阻器的使用,使用它的目的是对运算放大器两个输入端的电源电阻进行平衡。讨论的结论是,通常情况下这种方法并无必要,甚至是有害的。本文将以前篇博文为基础,因此您可能需要首先阅读前一篇文章。[pic]在上次的文章结尾我曾说过,这种方法肯定不适合某些运算放大器。一些双极输入晶体管(BJT)放大器拥有内部输入偏置电流消除功能。它们都有电流源I1和I2,其为输入晶体管对提供基极电流。我们可以通过将匹配基极电流小心地映射到运算放大器输入端中来得到这些电流。[pic]尽管这些电流精确匹配输入晶体管的基极电流(通常在几个百分点以内),但是它们并不理想。它们留下一个可能为正也可能为负的小剩余输入偏置电流。两个输入端的这种剩余电流可能差异非常大,它们的极性甚至是相反的。电源电阻匹配(请参见图1)的任何潜在好处都取决于输入偏置电流近似匹配。内部输入偏置电流消除让这种方法变得毫无用处。哪些运算放大器拥有输入偏置电流消除呢?产品说明书有时并没有清楚地表明这一点。但是,我们一般可以通过查看输入偏置电流规范的详细内容来进行了解。图3a显示了OPA209的输入偏置电流规范,它是一款具有输入偏置电流消除功能的低噪声运算放大器。您首先需要注意的是,输入偏置电流前面会有一个±符号,其表示电流流动的方向。还要注意,输入偏移电流规范与输入偏置电流的量级相同(使用这种运算放大器时实际上完全一致)。这些规范表明,这款器件具有内部输入偏置电流消除功能。[pic]图……
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