主题仪表放大器--常见的应用问题和解决方案

1、主题：仪表放大器-常见的应用问题和解决方案问答集锦：问：zw-wu我现在用了AD620与AD8221仪表放大器,但在使用过程中,有时输出会出现一些干扰信号,不知这些仪表放大器怎样才能减小无用信号,提高共模抑制 答：Forest你可以在输入端之前加入低通滤波器，就可以减少干扰信号了。今天的资料中就有详细的介绍。 问：zhangxk在应变片式传感器（Load cell）的放大、转换电路中，要将A/D分辨率提高到19位以上，并且转换速度大于50sps，应选用什幺样的芯片组合，或者电路设计中应该考虑哪些因素？ 答：Jingyou should pay attentatio

2、n to the throughput that will be trade off with your accuracy.You should also pay attentation to your input range and offset I think you can refer to our AD7730L or AD7799 .If you just want to use AMP+ADC , you can consider AD8230.It"s a chopper-stable instrument amplifier. 问：xianluc如何将设计A

3、D623的输入电路，使AD623单电源+5V供电时，输出信号为0+5V？ 答：Mariah问：zzb6503在设计电路时，怎样尽可能使仪表放大器抑制50Hz市电干扰？ 答：ForestAD8221仪表放大器放大器本身就具有很强的共模抑制比，他可以在一定程度上抑制市电的干扰。另外，我们建议在你后续的模拟和数字电路加入带阻滤波器。 问：zcw0707在测量微小电流时，需要串入隔离放大器，怎幺减小隔离放大器带来的影响？ 答：Yiming如果一定要采用隔离放大器，那幺隔离放大器的性能不应该恶化输入信号，也就是选用低噪声的隔离放大器。 问：zhangxk

4、如何实现应变片式传感器（Load cell）的交流激励、放大及转换？ 答：JingYou can use ratiomeric method and AC extinction.AC Excitation can be used to cancel out offset errors, regardless of their source.  AC Excitation here means that 2 sets of readings are taken, with the excitation voltage to the bridge reversed.&

5、#160; Subtracting the 2 readings cancels out the offset errors, as shown in the boxed equation.  This technique requires that the readings be stored and subtracted.  This is an effective technique, but if you implemented this with discrete components you would add more error

6、 than you were subtracting. 问：metrolog现在设计的仪表放大器失调电压的温度系数可达到10nV/K吗？ 答：SteveThe lowest drift instrumentation amplifier we have is the AD8230 which has 50nV/C. The AD8555 has 65nV/C and the AD8553 has 100nV/C. We do not have any instrumentation amplifier that has 10nV/C of drift at this tim

7、e 问：mxslgyb请介绍一下仪表放大器和通用放大器的区别，谢谢！ 答：Forest通常仪表放大器是由三个运放和一些集成的电阻组成，它具有较高的输入电阻，很高的共模抑制比，通常用在信号的最前端进行信号处理。如果用通用放大器组成仪表放大器的形式，由于电阻和运放的不匹配，性能上会有很大的损失，尤其是共模抑制比。 问：luoqq模拟数字地要分割吗？ 答：MariahIf your circuit is low frequency and need high accuracy, you need to separate the analog ground an

8、d digital ground. 问：wangqf2422ADX250芯片用于马达、泵、压缩机监控测量振动指标，如何降低电路信号噪声？如何降低功耗？ 答：Yiming降低噪声的方法：1. 用线性电源，2. 在加速度输出端，用合适的低通滤波器把高频噪声滤掉。降低功耗的方法：可以降低电源电压，但是这样灵敏度会下降。 问：wy56请问：我在进行仪表电路设计时，想将静态工作电压隔离掉，该如何处理？例如：传感器输出的电压是0.51.8V，我想只取用该段电压进行放大及后续处理，该如何做？ 答：EagleYou can use AC couple, but you

9、need to resistors to connect to GND to provide DC bias. In this way, the high input impedence is lower down by this resistors. For example, AD8221 differential input impendence is 100G Ohm, if use 1M Ohm resistors, the input impedence will be less than 1M Ohm. In this case, we can use a JFET input a

10、mplifier to buffer the input to get high impendence fist, then use AC couple to IIA such as AD8221. 问：zj2172我公司产品用的传感器采用桥式电路，输出的信号可正可负，但给仪器放大器提供的电源是5V单电源，目前是给仪器放大器的参考电源端提供了一个2.5V的偏置电压，请问这种用法可靠吗，会不会有什幺问题，有没有更好的解决方法？谢谢！ 答：SteveDo you mean the sensor can swing below GND? If the sensor output

11、is between 0V and 5V then this configuration is OK to amplify the output signal of the sensor. But you cannot condition a bridge that outputs negative voltages if the instrumentation amplifier is operating on only +5V supplies.问：lijin_as为何仪表放大器不能做到Rail to Rail？ 答：ForestAD623 AD8220是可以做到rail to

12、rail 问：lwtbenbenAs we know,AD603 can work at the frequency of 90MHz.But I want know how can I ruduce the EMI when I design the PCB board?Thank you! 答：SteveYou can use ferrite beads or a common-mode choke at the inputs of the AD603. 问：dxp0715我采集的有用信号是50Hz,但发现经过运放后信号迭加了一些干扰信号，我想在A/D前端加一

13、低通滤波电路，请问设计这个电路时需不需要考虑A/D的采样频率 答：Jingyou can add a low pass filter before A/D. But you should consider your interested bandwidth and the ADC bandwidth .If you want to get more accuracy,you can slow down the sample rate. 问：zw-wu我想问一下,我要用在电能质量监测方面,不知选用什幺仪表放大器比较好一点 答：EagleI am sorry we d

14、o not know exactly the requirement. If you need low cost and you can use +/-5V power supply, then AD623 is a good choice. 问：paynewon请问如何设计差动放大器，保证最低的温飘和噪声。 答：YimingADI提供集成的difference Amplifier，比如AD82xx系列 问：unaffectedwhy the in-amp constructed by off-chip resistor"s CMRR performan

15、ce is worse than integrated in-amps? 答：SteveIn-amps need matched resistor ratios for high CMRR. You can get high performance with external resistors if the resistors are well matched. This can happen if you buy an integrated resistor array. These arrays cost more than the amplifier. So if you w

16、ant low cost and high performance, you should use a monolithic instrumentation amplifier. 问：kingway98看了关于AD8221的演讲！不知道将他用做仪表前端放大的时候，特别是接传感器的时候，对于电源有没有什幺特殊要求！ 答：JingIt"s very hard to summarize it in one sentence . It will depend on your real system requirement.Generally, your power sup

17、ply should be less ripple voltage and should be larger than what you will in/output and you should also reserve enough headroom for your output. if you can use +/- power supply should be better. 问：xsj19810419为提高拾取电路的灵敏度，应该采取怎幺样的方法来减小输入端的噪声干扰 答：Yiming滤波，选出有用信号 问：jimmy88531仪表放大器中外围器件的精度

18、（如电阻误差）会不会影响测量精度，怎样解决这一问题？ 答：Forest一般仪表放大器需要一个外部电阻来调整增益，电阻的温度系数和内部电阻不匹配的话，会造成精度的影响。AD8221 内部的电阻是负的温度系数。 问：lijin_as通常单电源仪表放大器输入电压和电源的差值最小为多少？当AD623 电源为3.3V时，是否能接受最大2.5V的输入（输出 Vo=1/2 Vin=1.25V）? 答：MariahIt depends on the internal design of the amplifer. For AD623, the input voltage can

19、be from -Vs-0.15 to +VS-1.5. For 3.3V power supply, it is -0.15V to 1.8V. 问：lijin_as能否给一个AD8250数据手册的链接，从ADI网站上找不到 答：Yiming是预发布版，大概06年3月份有样片。 问：mxslgyb我想将±2.5V（±5V、±10可选），080Hz模拟信号变成差分信号输出，请问采用ADI的放大器有典型电路吗？ 答：Jingyou can use our AD8133/9 to transform signal end sig

20、nal to differential signal. You can also use our AD8222 to realize it .Remember to use +/- power spply with enough headroom. 问：hzplq对多路微信号（如64路）如何采用仪表放大器 答：Forest仪表放大器输入阻抗一般都比较高，所以比较适合微信号的处理。对于多路信号的处理，前端先可以采用多路开关（漏电流要考虑），再采用仪表放大器。 问：sjmww1127在遇到特别微小的信号时,比如只有微伏大小,这个量比运放的失调都要小,是不是就会被运放淹

21、没,这方面不是很明白,请专家给予讲解. 答：SteveIf the input signal is very small, in the uV level, and the offset is bigger then the signal can be submerged by the offset. But if the input signal is AC then you may be able to condition this. Two ways to amplify this type of signal:use a low offset amplifier like t

22、he AD8230 or AD8553or synchronously modulate the signal and make it an AC signal using switches or an AD630 and then you can make the small signal AC and condition it. 问：ljd166我如何准确地计算AI内部前端的两个运放的输出点的电压值？ 答：JingVo+=(Vin+ -Vin-).RF/RFG+Vin+Vo-=V- -(Vin+ -Vin-).RF/RG 2005-12-14 10:55:19

23、问：T4T4T4运放单电源应用后,信杂比是否会下降很多.我在视频应用中,改单电源后,会下降10多db，应该是有问题的吧？ 答：EagleNormally it"s caused by the vertual GND (at VCC/2 by 2 resistoers). Add more decoupling capacitors will be helpful. Use an Voltage reference such as ADR381 will be better. Please also pay attention to the video amplitude,

24、it will be clipped if too big (Ie. ADA4862-3, input common mode range is 14V at 5V power supply, if signal plus commone mode level is lower than 1V or great than 4V, it may be clipped). If you use Rail to Rail Video amplifier (AD8091, ADA4851), it will be better. 问：dragon668899惠斯顿电桥输出2毫伏/v的信号，进

25、行线性放大，实现轨对轨输出，12伏单电源供电，能否提供性价比高的解决方案。 答：Forest你可以采用AD8221作为信号的前端处理，但如果你比较关心温度漂移的话， AD8230也是不错的选择。另外，采用分立的方法一可以达到高精度， AD8671是我们高精度的运放，噪声很低，比较适合你12V的电源供电。 问：kevinlee你们有在高频上应用的呀？如何抑制干扰 答：Yiming对高频段应用，ADI提供差分放大器，比如AD8351, AD8138, AD8139. 一般用滤波的方式抑制干扰。 问：qiyuAD8221能工作的最高频率是？ 答：Ji

26、ngit"s 825KHz gain=1 typically 问：benyimi前面提到匹配电阻引来的共模信号干扰,如果要放大nv高频信号如何处理? 答：MariahHow high is your frequency. For nV signal, you need to select very low noise amplifier. For example, AD8139 is a very low noise part with differential input and output. 问：mxslgyb如何将2.5V或5V单端输入变成差分输

27、入，并且使正负极输入无时差 答：JingYou can use AD8139/3 to do that 问：wactdADI的专家，你们好！我想请问放大器使用单电源供电和双电源供电有什幺不同吗？对性能有什幺影响？ 答：SteveIt depends on whether there is enough headroom. Many instrumentation amplifiers are designed for +/-15V supplies. You can run them on lower supplies such as +5V but you wi

28、ll have very little head room. So you cannot use high gain or you have limited input voltage range. The amplifiers that are rail to rail are better for use on low voltages such as +3.3V or +5V. If you do not have enough headroom, you do not have much input or output range. So there are some performa

29、nce limitations. As long as you have enough head room, you can use the in-amp in single or dual supply. 问：motaiping仪表放大器的差分输入,和传感器配合使用时,如何设定输入偏置电路?电阻数值如何选取? 答：Forest采用仪表放大器的偏置电路，一般由你的供电电源来决定，推荐你选取VCC/2。电阻值由你需要的放大倍数来决定。 问：ljd166如何计算仪放内部的BUFER，A1，A2两个运放的输出电压？ 答：JingA1 output &

30、#160;Vo+=(Vin+-Vin-)RF/RG+Vin+A2 output  Vo-=V-(Vin+-Vin-)RF/RG 问：shu_chen我想问一下，如果我们需要对信号加入Dither以改善ADC的性能，在仪表放大器或者差分放大器中，应该如何引入比较合适？ 答：Yiming加入diter能够提高SFDR性能，但是降低了SNR。详见AN-410 问：xsj19810419如何输入保护？ 答：EagleWe can use Protection Diode to protect the input, but need pay a

31、ttentaion to the impendence. Some device (such as AD629, +/-270V common mode range) can work at high common mode voltage itself. 问：motaiping仪表放大器和传感器配合使用时,如何进行调零? 答：Forest仪表放大器一般由一个REF管脚，采用不同的DC电压，可以调整输出的直流电平。我们建议在软件中进行调零，这样比较灵活。 问：Jeff.Liu请问：AD8221仪表运放能识别放大的电压信号的最下限是多少？ 答：Mariah

32、It depends on the noise performance of the amplifier. Otherwise, your signal will be submerged by the noise of the amplifier. 问：motaipingAD8221的输入共模电压范围有多高?它的大小对仪表放大器的性能有何影响? 答：Jingit"s -Vs+2 and +vs-1.2. It"s not correlate with the performance of IN-AMP. But you should combine i

33、t with your gain,output range etc. 问：seawolf0718请问如果在单电源的情况下,在REF端加一个直流偏置,那幺输出是不是VIN*GAIN+VREF? 答：Forest没错，不过你要考虑你所加的直流偏置的驱动能力。 问：huaqun在ECG检测中,第一级增益一般设置为多少? 答：Wenshuai第一级增益的大小受限于仪表放大器的动态范围和电极极化电压差。一般需要考虑极化电压差为300mV.所以如果用AD8221,供电电压为+/-3.3V,那幺放大倍数在6倍左右。如果用AD8220，因为是轨到轨输出，所以放大倍数会必

34、AD8821大。更具体讨论可以发邮件到china.support. 问：myrolite仪表放大器的RF滤波器组件参数的选择有些什幺原则?有计算方法吗? 答：Yiming请参见ADI仪表放大器应用指南RFI一节 问：qiyuAD82系列那种芯片可工作于30M以上？ 答：Yiming请用AD8138,AD8139, AD8351. 问：Jeff.Liu1.>仪表运放主要适于处理哪些信号（电压，电流，微小信号）？2.仪表运放与普通运放相比有哪些优缺点？3.我现在要处理放大一10UV以下的电压信号，能直接用仪表运放吗？ 答：Maria

35、h1. Voltage, current, small signal should be OK for IN-AMP. 2. In-amp has large input impedance, large CMRR and it features differential input.3. Yes, you can. 问：ljd166仪表放大器的输入端是否要考虑阻抗匹配？是否也要像运放一样匹配？ 答：MariahI don"t know what kind of impedance matching you refer to. In-amp has very la

36、rge input impedance and small output impedance. 问：motaipingADI的单电源仪表放大器有那些型号?在应用时直流输出电平如何精确地调整到电源的一半? 答：JingWe have  lots of this kind of IN-AMP such as AD8221,AD623/7/9,AD8222,AD8220,AD8250. You should terminate the REF pin to the middle of your power supply . But you should use

37、 a low impedence output voltage to bias the REF pin. For example if you use 0-5V power supply,you can use a 2.5V reference followed with a op-amp(gain=) to drive the REF pin. 问：ynh使用多个串联后，带宽是否降低？压摆率是否变低，和噪声是否按指数增大 答：SteveThe slowest PGA will influence your bandwidth and slew rate. So for e

38、xample if you have 2 PGAs with 10MHz (30V/us) and 1 PGA with 1MHz (1V/us), your bandwidth and slew will be limited to the 1MHz (1V/us) PGA.Noise will increase. If you have 3 PGAsPGA_A with 10nV/rtHz then PGA_B with 10nV/rtHz and PGA_C with 10nV/rtHz and all the PGA are at a gain of G1, G2 and G3, th

39、en referred to input you will have(10nV/rtHz*G1)2 + 10nV/rtHz)*G2)2 + 10nV/rtHz)*G3)2)/(G1*G2*G3)Noise is summed by its root sum square. 问：xsj19810419如何有效解决仪表放大器内存在的潜在射频整流问题 答：SteveYou can build an RFI filter at the input. There is an applicatio note, AN671. It is also important to keep th

40、e inputs as balanced as possible so that most of the signal is common-mode.You can also use ferrite beads or common-mode chokes (inductors) to stop the signal before it reaches the in-amp. 问：Jeff.Liu现在要放大一5UV的电压信号，能直接用AD8221吗？ 答：JingI think you can"t. Because the offset voltage of AD8

41、221 is large than 5uV. For this kind of application you can use a chop amplifier such as AD8628,AD8230 问：Guozhong我看了一些App. Note了解了如何在single surpply时放大,但是滤波怎幺做<可不可以? 答：Yiming单电源应用时可以做滤波。 问：mxslgybwhat"s meaning "Rail to Rail"? Thank you! 答：Wenshuai轨到轨，也就是输入或输出的范围

42、与供电电压的靠近程度。一般轨到轨输入/输出会跟供电电压很靠近，大约可以到10mV左右。 问：unaffectedwhy not in-amp use differential output? This can make CMRR larger. 答：SteveYes. You can use differential outputs with an in-amp. The AD8221"s data sheet shows how you can make a differential output, Figure 51. We will also have a

43、new in-amp, the AD8222 which can be configured for differential outputs.Most of the applications in the market ask for single ended outputs that is why most of the in-amps have single ended outputs. But, we show in the data sheet and in some articles how to make a differential output. 问：eargle我

44、在设计ECG的电路,请问AD620对RF的干扰在弱信号采集采用图示的电路可能会有多大改善 答：Mariah使用如幻灯片29页图所示电路和0.01微法的电容 ，3 dB的差分信号带宽约为1900Hz。增益为5时，采用1V p-p的应用信号，在10 Hz20 MHz的频率上，测得电路的输入端直流偏移的漂移小于6微伏。在单位增益的情况下，没有直流偏移的漂移。问：ljd166我们选择了AD623，但是如何解决温漂问题，除了选择低温漂的仪放，有没有简单的方法消除温漂 答：Eagle(1)Please check if you can use AD623BR. B grade has

45、lower temperature drift.200 mV Max Input Offset Voltage (AD623A)2 mV/"C Max Input Offset Drift (AD623A)100 mV Max Input Offset Voltage (AD623B)1 mV/"C Max Input Offset Drift (AD623B)(2)Cnsider other parts such as AD8221BR (not pin to pin).25 V max input offset voltage0.3 V/°C max inpu

46、t offset drift.(3)Input offset drift will be different with different gain. (4)Lower down other device"s temperature drift will also help to lower down the system temperature drift. Choose low drift resistor if you have serial input resistors for protection purpose.问：zcw0707如何用运放精确平移直流电位，比如从8V移

47、到2V，但这个8V是变化的，6V是确定的。 答：Yiming请检查一下你的问题。谢谢 2005-12-14 11:13:34问：都市劣人基于成本考虑，用3个通用放大器搭仪用放大器是否合适？ 答：MariahYes, if cost is the most concern. 2005-12-14 11:13:58问：gostray我把微弱信号用AD8304放大后,当与计算机控制板接上后发现其噪声变大,不知是什幺原因?是否换用其它仪表放大器会有好处? 答：JingIt"s very hard to clearify it what&quo

48、t;s wrong with it.Because you just provide us  a little information.  In fact,if you use AD8304 to interface with a photo diode,it"s a good selection, the optical power could be less than -70dbm. I think you should pay attentaion to the bias circuit of AD8304. please call 80

49、0 810 1742 to give us more information. 2005-12-14 11:14:05问：hugoliang仪器放大器与普通运放的区别.谢谢 答：Forest仪表放大器是由三个运放，和一些集成的电阻组成，具有比较高的输入阻抗，和很高的共模抑制比。普通运放要组成仪表放大器的话，由于运放和电阻的不匹配，会有很多性能的损失。 2005-12-14 11:14:29问：peng-kai原先ADI公司的AD620运放都作为了医疗生物电放大处理的典范，现在，ADI公司又推出的取代芯片在这方面的处理上是否增加了更好的或优于以前的芯片的方法或其它呢

50、，请列举！ 答：ForestAD8221 就是比较好的新的仪表放大器，他的输入阻抗，共模抑制比都有很大的提高。 2005-12-14 11:15:47问：T4T4T4仪表放大器的输入级在PCB布局时应要注意什幺问题,才能有良好的噪音性能? 答：SteveYes. You should be aware of what size resistors you use at the inputs. Use wide traces at the supply pins and use decouple caps. Sometimes noise from the powe

51、r supplies can cause spurs to show up at the in-amp. 2005-12-14 11:16:30问：jacklee2008如何从仪放的输入电压电流噪声的指标来计算出要测量的微弱信号的最小值? 与普通运放有区别吗? 答：MariahWe recommend you to read the reference manual "Designers_Guide_To_In-Amps_2edition". There should be very detailed calculation in it. The boo

52、k can be downloaded from the website. 2005-12-14 11:17:04问：hyjlm能否给出一个仪表用的长时间积分电路?选用什幺放大器?AD的高输入崭波自稳零器件有什幺型号? 答：EagleIf you need long time in intergration, please choose low bias amplifiers such as AD8610. Also choose low leakage capacitor.AD8551/52/54, AD8628 are good chope auto zero ampl

53、ifiers. AD8555 can adjust gain. 2005-12-14 11:17:41问：myrolite仪表放大器的输入端是否需要进行保护?如何进行? 答：JingSometimes you should add a input protection if your input common mode voltage could be larger than the IN-AMP common mode voltage. You could add shortkey diode or ues resistor divider or use voltage

54、bias circuit to do that. 2005-12-14 11:17:52问：zhangxkADI是否提供应用设计中的技术解决方案？ 答：MariahWhat kind of the application? You can contact the local office to know more information of the solution related to your application. 2005-12-14 11:17:57问：wz623医疗应用中的所谓“右腿驱动”电路如何设计，就是从INAMP的RG处引出到人体参考电极的提

55、高CMRR的电路。 答：MariahYou can find the righ leg driver circuit in the datasheet of AD620 and AD8221. 2005-12-14 11:18:25问：waterbird放大器的接地处理是相当复杂的，对于高精密仪表放大器，是否对电源地平面的铺设有特别设计要求？ 答：JingOf course , you should pay attentation to the granding issue. You can use a ground layer to terminate the

56、AGND and DGND . somethimes you should also use a guard ring to protect the input signal of the AMP. It"s a big topic for your question and I can"t interprete it very clear here.Please call 800 810 1742 for more information. 2005-12-14 11:22:03问：ljd166我们选用了AD623在16BIT的AD采样，是否ADI可以推荐一些详

57、细的PCB LAYOUT的资料给我们，或者推荐一些相应的书籍，资料？ 答：MariahAD623 is not very good for 16 bits system in terms of the noise performance. We recommend you to use AD8221. 2005-12-14 11:22:30问：singer533目前的仪用放大器最高精度可达多少? 答：MariahWhat accuracy spec do you mean? Offset? Noise? 2005-12-14 11:23:18问：mxsl

58、gybHow can I make the differential input to single output? 答：SteveTo make a differential input to a single ended output, use an in-amp, such as the AD8221, AD623, AD620. It takes a differential input and outputs a single ended output. 2005-12-14 11:23:57问：wangqf2422AD745作为（水听器）前置放大器使用为降低功耗

59、，采用-5V+5V供电对于放大信号有什幺影响？输入阻抗过高比如大于500兆欧姆，可以幺？如何消除前置放大电路产生的噪声？ 答：Yiming最低工作电压为+/-4.8V，所以可以工作在+/-5V.这样输入处处信号的范围变小。AD745的差分如入阻抗为10Gohm，远大于0.5Gohm。请用滤波的方法滤除噪声。 2005-12-14 11:24:38问：adlion如何解决仪表放大器在线路应用中的抗干扰问题？ 答：Jingyou can use guard ring to protect the input signal of the AMP. You can also refer to our presentation to realise the RFI filter (page 29). Of course, you should also pay attentation to you power supply. 2005-12-14 11:24:48问：tjqytg溶解氧和ad620放大器应当如何连接,连接时应当注意什幺问