Understand_Capacitor_Soakage_to_Optimize_Analog_Systems(理解电容的浸润效应--翻译版).doc

Understand Capacitor Soakage to Optimize Analog Systems(理解电容器浸润优化模拟系统)
Dielectric absorption can cause subtle errors in analog applications(在模拟应用中电解质吸收可能导致微妙的错误) such as those employing S/H circuits(例如使用采样保持电路), integrating ADCs (集成ADC)and active filters(和有源滤波器). But knowing how to measure this soakage(但是知道如何测量这些浸润) and compensate for it helps you minimize its effects(并且补偿它能帮助你减小它的影响). 
Veteran circuit designers often got a shocking introduction to dielectric absorption (老的电路设计者对电容器浸润经常有令人震惊的介绍)when supposedly discharged high-voltage oil-filled paper capacitors reached out and bit them(当很可能是高压油浸纸电容器放电并且击穿电容). Indeed, the old oil-filled paper capacitors were notorious for what was once called soakage(实际上老的油浸纸电容器是因为“浸润”而声名狼藉) -- a capacitor's propensity to regain some charge after removal of a momentary short(浸润——电容在短路放电后，开路时倾向于恢复原有电荷(也就是电路中的“长尾”现象)). Today, you won't find very many of these capacitors in use(今天你会发现这种电容应用的不是太多), but you will still encounter soakage(但是仍然会遭遇浸润). Do you know how to deal with it? (你知道怎样解决吗？)
Nowadays, you're more likely to notice the effects of dielectric absorption in some more subtle way(当今，你更可能在一些更微妙的方式下注意到电介质吸收的影响), perhaps in the performance of an integrator that can't be reset to zero(积分器表现出拒绝复位至0) or a sample/hold that refuses to work correctly(或采样保持器表现出变化不定的误差). But whether you literally feel its effects or merely observe them in a circuit's behavior(但是不管你真正感受到它的影响或是仅仅是观察它们在电路中的响应), dielectric absorption is an undesirable characteristic that every capacitor possesses to some degree(电介质吸收这种不良特征每种电容或多或少都会有). This characteristic is inherent in the dielectric material itself(这种特征天生存在于电介质材料当中), although a poor manufacturing procedure (即使不良的制造工序)or inferior foil electrodes can contribute to the problem(或者劣质箔电极都能导致这种问题). 

Fig 1 - A simple test fixture lets you evaluate dielectric absorption at low speeds(一个简单的让你在低速下评估电介质吸收的测试固件). To use the one shown here(举个例子), start with all switches off and throw S1 and S2 on for 1 min(首先将所有开关打到OFF并且S1和S2打到ON一分钟); throw S1 and S2 off and wait 6 sec(S1和S2打到OFF并且等待6S), throwing S3 on during the wait period(在等待周期内将S3打到ON). Next, turn S2 on and watch VOUT for 1 min(接下来将S2打到ON并且观察Vout一分钟). To compensate for leakage(为了弥补泄露), leave all switches off for 1 min and then throw S2 and S3 on(将所有开关打到OFF一分钟并且将S2和S3打到ON). Monitor VOUT for 1 min and subtract this value from the VOUT value obtained earlier(监测Vout一分钟并且减去早先获得的Vout值). (View a larger version of the image.) Indeed, soakage seems an apt term for dielectric absorption (实际上，浸润似乎是描述介电吸收一个恰当的词)when you note what the capacitor seems to be doing(当你注意到电容表现出的特性时). Consider a typical example(考虑到一个特殊的例子): A capacitor charges to 10V for a long time T (一个电容在一个长的时间T内充电到10V)and then discharges through a small-value resistor for a short time t(然后通过一个小的电阻放电一小会时间t). If you remove the short circuit and monitor the capacitor terminals with a high-impedance voltmeter(如果你在断开放电回路之后用一个高阻抗的电压表测量电容的一端), you see the capacitor charge back to 0.1%, 1% or as much as 10% of the original voltage(你会观察到电容被重新充电到原电压的0.1%，1%甚至是10%). For example, a 1-µF Mylar capacitor charged to 10V for 60 sec (TCHARGE)(例如一个1uF的聚酯薄膜电容器充到10V需要60s(Tcharge)) and discharged for 6 sec (TDISCHARGE) charges to 20 or 30 mV after 1 min (THOLD)(并且放电6s(Tdischarge)，断开一分钟后会重新充电到20-30mv). Fig 1 shows a simple evaluation circuit for measuring this characteristic(图1显示了一个简单的测量这一特点的评估电路). 

Fig 2 - To model the soakage characteristic of a 1-µF Mylar capacitor(用一个1-uF的聚脂薄膜电容模拟浸润), consider a circuit that incorporates a 0.006-µF capacitor to represent the dielectric's charge-storage characteristics(用0.006-uF的电容代表电介质的电荷存储特性).
A capacitor exhibiting dielectric absorption(一种具有介电吸收的电容器)acts as if during its long precharge time the dielectric material has soaked up some charge that remains in the dielectric during the brief discharge period(好像在一个长的预充电时间内介电材料已经吸收了一些电荷，这些电荷在短暂的放电期间内仍然保留在电介质当中). This charge then bleeds back out of the dielectric during the relaxation period and causes a voltage to appear at the capacitor terminals(这些电荷在张弛周期内由电介质中释放并且引起电容两端的电压的改变). Fig 2 depicts a simple model of this capacitor(图2描述了这种电容的简单模型): When 10V is applied for 1 min, the 0.006-µF capacitor gets almost completely charged(10V电压在1分钟内几乎可以将0.006-uF电容充饱), but during a 6-sec discharge period it only partially discharges(但是在6秒放电周期内只有一部分电荷放掉). Then, over the next minute(接下来在下一分钟内), the charge flows back out of the 0.006-µF and charges the 1-µF capacitor to a couple of dozen millivolts(电荷从0.006uF电容回流并且重新给1-uF电容充电至几十mV). This example indicates that a longer discharging time reduces soakage error (这个例子表明一个长的放电时间可以减小浸润效应)but that discharging for only a small fraction of that time results in a larger error(但是放电一小段时间会导致更大的问题). Illustrating this point(图解这一要点), Fig 3 shows the results of conducting Fig 1's basic test sequence(图3显示了按照图1的流程基本测试结果1/6/12s的放电时间) for 1-, 6- and 12-sec discharge times. Note that the capacitor tries to remember its old voltage(注意到电容在尝试记住以前的电压), but the longer you hold it at its new voltage, the better it forgets(但在新的电压上持续的时间越长，它的记忆性越差) -- in the Fig 3 case, soakage errors equal 31 mV at tDISCHARGE=l sec, 20 mV at tDISCHARGE=6 see and 14 mV at tDISCHARGE=12 sec.(在图3，在T放电=1s时，浸润电压=31mv；在T放电=6s时，浸润电压=20mv；在T放电=12s时，浸润电压=14mv)

Fig 3 - Obtained using Fig 1's test circuit, these dielectric-absorption-measurement results for a 1-µF capacitor shown that longer tDISCHARGE times reduce soakage-caused errors(用图1电路得到，对于1uF电容的电介质吸收测试结果表明：越长的放电时间越能减缓浸润效应). 
High-speed tests predict S/H performance (高速测试预测采样/保持电路的性能)
You might now ask whether these low-speed tests have any bearing on a capacitor's suitability in fast millisecond or microsecond sample/hold applications(你会问是否这种低速测试结果在ms或us级的采样保持电路中有同样的影响). If you repeat the Fig 1 experiment for TCHARGE = THOLD = 1000 µsec and tDISCHARGE = 100 µsec(如果你重做电路1的实验，T充电=T保持=1000us并且T放电=100us), you see very similar capacitor-voltage waveforms but with about 10-times-smaller amplitudes(你可以看到同样的电容-电压波形，但是幅度只有原来的1/10). In fact, for a constant T(事实上，对于恒定不变的T(这里指充电时间)):t ratio, the resulting soakage error decreases only slightly in tests ranging in length from minutes to microseconds(对于T的，浸润结果误差仅仅在测试时常从分钟到毫秒细微的减少). ？？？？？

Fig 4 - More precise than Fig 2's equivalent circuit(比电路2更准确的等效电路), a capacitor model employing several time constants(使用几个时间常数的电容模型)proves valid for a wide range of charge and discharge times(对于大范围的充放电时间是有效的). This model approximates a Mylar capacitor(这个模型近似聚酯薄膜电容器). 
Fig 4's circuit approximates this capacitor characteristic(图4电路近似这种电容特征), which you can observe on actual capacitors by using Fig 5's test setup(你可以用图5的测试设置观察实际电容器). Here, a sample/hold IC exercises the capacitor under test at various speeds and duty cycles(这里，一个采样保持电路练习对电容在多种速率和占空比下进行测试), and a limiter amplifier facilitates close study of the small residual waveforms(并且一个限幅放大器帮助你更深入学习小的残余波形), without over-driving the oscilloscope when the capacitor is charged to full voltage(当电容被充满时不会过驱动示波器). 

Fig 5 - Capable of automatically sequencing the dielectric-absorption tests(可以自动排序电介质吸收测试), a circuit employing timers(电路采用定时器), a sample/hold and limiting stages allows you to make measurements for a wide range of TCHARGE, THOLD, and tDISCHARGE values(采样/保持和限制阶段允许对充电/保持/放电有广泛时间的测试(也就是可以在很多时常数下测试)). Fig 7 shows the results obtained using the circuit shown here(图7示出此电路的测试结果). (View a larger version of the image.) 

Notes:
1. ALL DIODES = 1N914    
2. IC5, IC6 = LM301A    
3. IC7 = MM74C04    
4. USE R4 OR -10 GAIN TO KEEP SCOPE WAVEFORM BELOW 200mV SO AS TO AVOID DISTORTION OR FALSE ATTENUATIONS(用R4或-10倍的增益来保持波形的范围低于200mv以至于避免失真或错误的衰减)

Such experiments illustrate that if you put a certain amount of charge into a less-than-ideal capacitor(这个实验例子说明如果你给一个不太理想的电容充电), you will get out a different amount of charge(电路中将会出现额外电荷), depending on how long you wait(电荷的多少取决于你等多久(也就是放电时间的长短)). Thus, using low-soakage capacitors proves important in applications such as those involving high-resolution dual-slope integrating ADCs(因此在高分辨率的双积分ADC中应用低的浸润效应的电容是十分重要的). And sure enough, many top-of-the-line digital voltmeters do use polypropylene (a low-soakage dielectric) devices for their main integrating capacitors(果不其然，许多数字电压表确实使用的聚丙烯电容).
But dielectric-absorption characteristics are most obviously detrimental in applications involving sample/holds(但是电介质吸收特点很明显在采样/保持电路中是有害的). Manufacturers guarantee how fast these devices can charge a capacitor in their Sample mode(厂商们会保证他们的器件(电容)在采样模式下充电是有多快) and how much their circuits' leakage causes capacitor-voltage droop during the Hold mode(和电路泄露泄露导致电容上电压在保持模式时掉了多少), but they don't give any warning about how much the capacitor voltage changes because of soakage(但是他们不会给出任何的关于电容电压因为浸润效应会改变多少的警告). This factor is especially important in a data-acquisition system(这个因素在采集系统中尤为重要), where some channels might handle small voltages while others operate near full scale(在一些通道工作于满量程状态时，另外一些通道可能有感应电压). Even with a good dielectric, a sample/hold can hurt your accuracy(即使是好的电介质，采样保持电路也会影响精度), especially if the sample time is a small fraction of THOL) .(特别是采样时间只占保持时间的一小部分) For example, although a good polypropylene device can have only 1-mV hysteresis per 10V step if T/t=100 msec/10 msec, this figure increases to 6 mV if the T/t ratio equals 100 msec/0.5 msec(例如，虽然好的聚丙烯器件能达到只有没10V有1-mv的滞后，当T采样/T保持=100msec/10msec时.这个指标可以达到6mv当T采样/T保持=100msec/0.5msec时)(滞后，指的是输入信号实际值与采样值的误差，如图2-1所示). Because most sample/hold data sheets don't warn you of such factors(因为大多数采样/保持数据手册对这一因素没有警告你), you should evaluate capacitors in a circuit such as Fig 5's(你应该用电路5测量电容), using time scaling suited to your application(时间常数依你的应用定). 

Fig 6 - Soakage can present problems when you're designing a fast-settling amplifier or filter(当你设计一个快速建立的放大器或者滤波器时浸润效应可能会引起问题). In the circuit shown here, for example, C1 can be a Mylar or tantalum unit, but making C2 a polypropylene device improves performance(在这个电路中，C1可以用聚脂薄膜电容或坦电容，但是C2必须用聚丙烯电容以改善性能). 
Other applications in which soakage can degrade performance are those involving fast-settling ac active filters or ac-coupled amplifiers(其他的因为浸润效应降低性能的应用还包括：快速建立的有源滤波器或交流耦合放大器). In Fig 6's circuit, C1 can be a Mylar or tantalum unit because it always has 0V dc on it(在电路6种C1之所以可以用聚脂薄膜电容或者坦电容是因为它的直流电压一直是0V), but making C2 polypropylene instead of Mylar noticeably improves settling(但是在C2中用聚丙烯电容替代聚苯乙烯电容会显著地提高建立时间). For example, settling to within -0.2 mV for a 10V step improves from 10 to 1.6 sec with the elimination of Mylar's dielectric absorption(例如,以2mv每步建立到10V如果消除电解质吸收的话，可以从10s提高到1.6s). Similarly, voltage-to-frequency converters benefit from low-soakage timing capacitors, which improve V/F linearity(通常,电压/频率转换器受益于低侵润的定时电容能够提高V/F的线性度). 
Some dielectrics are excellent at all speeds (有些电介质在任何速率下都是很好的)
Fortunately, good capacitors such as those employing polystyrene, polypropylene, NPO ceramic and Teflon dielectrics perform well at all speeds(幸运地是，用聚苯乙烯/聚丙烯/NOP的陶瓷和特氟龙制成的电容在任何速率下都表现的很好). Fig 7 shows the characteristics of capacitors using these dielectrics and others such as silver mica and Mylar(图7画出了用这些优质电介质和其他像是镀银云母和聚脂薄膜制成的电容的特点). In general, polystyrene, polypropylene or NPO-ceramic capacitors furnish good performance, although polystyrene can't be used at temperatures greater than 80°C(尽管聚苯乙烯不能够在超过80℃以上工作，但是聚苯乙烯，聚丙烯或者NOP的陶瓷电容器能表现出很好的性能). And although NPO ceramic capacitors are expensive and hard to find in values much larger than 0.01 µF(尽管NOP的陶瓷电容器昂贵并且大于0.01uF的很难找到), they do achieve a low temperature coefficient (他们有着低的温度系数)(a spec not usually significant for a S/H but one that might prove advantageous for precision integrators or voltage-to-frequency converters(这种参数对采样保持电路来说意义不大，但是对精密积分器或者电压频率转换器来说很有用)). Teflon is rather expensive but definitely the best material to use when high performance is important(特氟龙材料虽然贵但是在高性能应用中是最好的材料). Furthermore, only Teflon and NPO ceramic capacitors suit use at 125°C(此外，只有特氟龙和NOP陶瓷电容适用于125℃). 

Fig 7 - Soakage-measurement results for a variety of capacitors illustrate the effects of tDISCHARGE values on dielectric-absorption-caused errors. Note the curves for two different samples of NP0 ceramic capacitors intersect(各种各样的电容随Tdischarge的值不同电解质吸收引起的误差浸润测试结果). 
If you look at Fig 7's dielectric-absorption values, you can see wide differences in performance for a given dielectric material(如果你看图7电解质吸收值，你可以不同电解质材料的巨大差异). For example, polypropylene sample A is about as good as B at t=6 see,( polypropylene sample A和B差不多) but B is four times better at high speeds(但是B在高速下比A好4倍). Similarly, NP0-ceramic sample A is slightly worse than NP0-ceramic sample B at low speeds(同样地，NP0-ceramic sample A在低速下比NP0-ceramic sample B更差), but A is definitely better at high speeds(但很明显A在高速下性能更好). And some Mylar capacitors (sample A) get better as speed increases from 1000 to 100 µsec(一些聚酯薄膜电容(sample A)当速度从1000us上升到100us时性能会变好), but others (sample B) get worse(但是另一些(sample B)会变差). So if you want consistently good performance from your capacitors(所以如果你想让你的电容在性能上一直保持稳定), evaluate and specify them for the speed at which they'll be used in your application(在他们被应用的特定速度的电路中，评估并指定这些电容). Keep in mind that because most sample/holds are used at much faster speeds than those corresponding to the 1- or 5-min ratings usually given in data sheets(大多数采样/保持被用在相当于它们数据手册中1-5倍的速率下), a published specification for dielectric absorption has limited value(一个已发布的对电解质吸收值的限制的说明书). 
In addition, other dielectrics furnish various levels of performance(此外，其他的电解质): 
· Because any long word that starts with poly seems to have good dielectric properties, how about polycarbonate or polysulfone? No -- they are about as bad as Mylar(似乎聚酯的电容有好的介电性能，那聚碳酸酯或聚丙烯呢？不是这样的—--他们像聚酯薄膜一样烂).  
· Does an air or vacuum capacitor have low soakage? (是不是空气或真空介质电容器有低的浸润)Well, it might, but many standard capacitors of this type are old designs with ceramic spacers, and they might give poor results because of the ceramic's hysteresis(也许是，但因为陶瓷的迟滞使得有的结果很差).  
· If a ceramic capacitor is not an NP0 device(如果陶瓷电容器不是NP0材料的), is it any good? (它性能更好吗？)Most of the conventional high-K ceramics are just terrible(大多数传统的高K陶瓷是很差的) -- 20 to 1000 times worse than NP0 and even worse than tantalum(比NP0差20—1000倍，甚至比钽电容还差).  
· Is silicon dioxide suitable for small capacitances(二氧化硅小电容合适吗)? Although Fig 5's test setup, used in preparing Fig 7's chart(用图5的测试电路绘制出图7样的图), only measures moderate capacitances (500 to 200,000 pF), silicon dioxide appears suitable for the small capacitors needed for fast S/Hs or deglitchers(它只能制造出中等数值容量的电容 (500-200000pF,)二氧化硅表现出适合于低电容需求快速采样/保持电路或变阻器).
Cancellation circuit improves accuracy (浸润消除电路和提升精度)
A practical method of getting good performance with less-than-perfect capacitors is to use a soakage-cancellation circuit such as one of the form shown in Fig 8(常用的使不怎么好的电容变得性能更好的办法，就是像电路8一样使用浸润消除电路), in which a capacitor of the type modeled in Fig 4 serves as an integrator. (Only the first two soakage elements are shown.) （这个电路中电容像图4一样，是它的实际等效模型(其中只有只有其中两路)） The int