“放大器+模拟/数字转换器”网上研讨会(简体中文版)-National Semiconductor厂商_在线座谈

精彩问答

主題：放大器+模拟/数字转换器网上研讨会
在线问答：
[主持人：ChinaECNet]	各位听众（网友），上午好！欢迎参加中电网在线座谈。今天，我们有幸邀请到美国国家半导体公司的专家就“放大器+模拟/数字转换器网上研讨会”举行在线座谈。在座谈中，您可就您关心的问题与美国国家半导体公司的专家在线进行直接、实时的对话交流。中电网衷心希望通过大家的共同努力，不仅能够增进各位听众（网友）对“多通道模数转换器设计考虑”的了解和掌握，而且能够为大家事业的发展带来裨益。	[2008-2-27 10:03:05]
[问：wingwind]	差分输入经过差分放大送给AD，如果希望对差分放大的输入或者AD的输入进行保护，限幅二极管直接加在差分信号之间好？还是应该分别加在两个信号端和参考地之间？
[答：Steven Feng]	both , but depend on which protection you want.	[2008-2-27 10:23:21]
[问：logic001]	高速ADC电路PCB设计时有无特殊要求？
[答：Stanley Tsang]	Yes,It"s big question. Many things need to take care. A brief answer are the items : 1.clean Power(prefer to linear , not switching) 2.single ground. 3.very low clock jitter 4.stable voltage reference. 5. isolate digital & analog power...	[2008-2-27 10:28:20]
[问：chinesehuhong]	放大器及模拟/数字转换器之间的搭配,关键在于什么?
[答：Steven Feng]	single /diff input, bandwidth flatness, anti-alising filter ,LSB requirement...	[2008-2-27 10:32:17]
[问：haicheng315]	怎样能保证信号采样的准确性？谢谢
[答：Michelle So]	1. use a clean clock 2. use filter to filter out unwanted signals 3. amplify input signal to ADC"s max dynamic range 4. Amplifier"s bandwidth must be competitble with ADC"s bandwidth. 5. Choose an ADC with DNL/INL < 1.	[2008-2-27 10:33:01]
[问：xiaojunseu]	NS的 WEBENCH网上设计工具是免费使用的吗？它的运放库是否只有NS的？
[答：Steven Feng]	yes, it is free ,for NS simulation.	[2008-2-27 10:33:58]
[问：logic001]	高速ADC设计时对匹配元件参数的要求是否要高一些？
[答：Stanley Tsang]	Yes,it"s better to have higher performance ckt in front of ADC. Otherwise it would downgrade ADC"s performance even you use a good ADC.	[2008-2-27 10:36:39]
[问：birdshyyang]	Layout时有特别需要注意的地方吗？
[答：Robert Shen]	应注意信号间的干扰。避免高频数字信号对模拟小信号的干扰。	[2008-2-27 10:38:31]
[问：psb]	如何选择适用于DC80～150mV的AD芯片
[答：Thomas Chang]	1.For DC 80mV to 150V input range ,we should use AMP. increasing amplitude to ADC full scale . 2 .We suggest used precision amp. aslike NAtional LMP2011 to do this . 3.Secondly you can use Lower power ADC as like National ADC121S101 (12 bit ,1MHz sample rate ,SPI interfact ) to get digital signal.	[2008-2-27 10:38:58]
[问：Zhang Zhongping]	单端输入与差分输入的ADC有何区别？二者的在技术性能优和主要差异，谁更适合工业数据采集系统？
[答：Payne Wang]	在设计中选择差分还是单端,主要是根据测量信号的要求决定的. 一般差分的输入方式在有对某些性能有较高要求的时候可以选择的. 比如在我们资料中讲到的LMH6550驱动ADC12DL065,就是需要对高速信号进行处理时候需要具有较高的CMRR. 再如,如果应用为高精度的微弱信号,例如温度,压力等,传感器部分就可选择差分输出方式,放大和后面的滤波也需要使用差分输入输出,此时需要选择差分输入的ADC与之配合.	[2008-2-27 10:39:53]
[问：wyq98]	对于低频信号(<100Hz)进行ADC转换，前端的滤波电容是否选用较大一些的好？
[答：Stanley Tsang]	Yes, as 100Hz is very low band width signal. So you can use larger capacitor ckt to make low pass filter to filter higher frequency.	[2008-2-27 10:40:31]
[问：dianbiao2006]	在信号采样时怎样避免受到干扰？
[答：Thomas Chang]	1.PCB lay out : aavoid other signals couple 2.Use filter in signal path as like low pass filter . 3.Impedance math between source and device .	[2008-2-27 10:41:08]
[问：6666lf]	您好，理论计算得出的电阻电容值在市场上不一定买到，选择参数相近的怕对滤波、放大等效果造成影响，该怎么办？谢谢
[答：Steven Feng]	depend on your system requirement, it should be different accuracy resistor.	[2008-2-27 10:41:12]
[主持人：ChinaECNet]	我们已经进入问答阶段如果听众想重温演讲或内容可以点击下面“回顾演示”重看演讲。	[2008-2-27 10:41:32]
[问：xuxm]	能否解释一下, 什么是超采样？有何好处和应用？
[答：Steven Feng]	samples rate is more than 2 times signal frequency.	[2008-2-27 10:42:04]
[问：timchen]	请问专家，四种不同的抗混叠滤波器的选型的主要依据是什么？例如什么时候要选用椭圆型滤波器？
[答：Michelle So]	in the presentation"s example, 1. calculate expected SNR with this equation: SNR= 6.02 x N + 1.76 (dB) where N is the resolution. 2. Then determine 0.5Fs 3. pick a filter then can hit 0.5Fs at the desire SNR. 4. Please see graph on P.7 of the presentation.	[2008-2-27 10:42:08]
[问：chongyang1999]	布线时是否在AD下面将数字地和模拟地分开?分割地平面如何连接?有什么根据?
[答：Robert Shen]	请参见网上有关混合信号噪声与辐射的控制课程： http://www.national.com/AU/design/0,4706,0_15_,00.html	[2008-2-27 10:42:37]
[问：licki518]	A/D转换器参考电压一直是个问题,都有那些好的解决方案?谢谢
[答：Payne Wang]	参考电压的选择是需要根据实际应用来选择的. 首先需要看你前端模拟电路的结构,选择分压电阻的方式或者使用专门的参考电压芯片. 一般需要考虑前端模拟电路的供电等方面的一致性以及信号随温度的漂移特性. 对于高精度的应用如果需要选择高精度的参考电压芯片可以参考LM4132其温漂仅为10ppm.	[2008-2-27 10:44:45]
[问：wypqq2qq]	有人提出，驱动ADC的运放增益带宽积要求大于100倍采样速率。请问专家，您的建议是多少？
[答：Thomas Chang]	YA! It is depends . It is depend on amp. Gain and ADC ENOB . For lower speed input signal ,100 times ADC sample is used in Amplifier is ok .	[2008-2-27 10:45:24]
[问：PETERZHOU]	ADC中为达到EMC的要求我们对PCB的设计有何要求? 谢谢
[答：Steven Feng]	请参见网上有关混合信号噪声与辐射的控制课程： http://www.national.com/AU/design/0,4706,0_15_,00.html	[2008-2-27 10:47:18]
[问：Zhang Zhongping]	请问在AD选择中应注意哪些参数？
[答：Michelle So]	SNR, SINAD, THD, INL, DNL.	[2008-2-27 10:48:01]
[问：haicheng315]	采样信号的频谱范围是多少？
[答：Robert Shen]	1/2的采样频率以内。	[2008-2-27 10:48:03]
[主持人：ChinaECNet]	我们已经进入问答阶段如果听众想重温演讲或内容可以点击下面“回顾演示”重看演讲。	[2008-2-27 10:48:30]
[主持人：ChinaECNet]	各位观众，现在用户提问很踊跃，专家正在逐一回答。请耐心等待您问题的答案，同一问题请不要多次提交。	[2008-2-27 10:48:40]
[问：PETERZHOU]	NS公司有无16BIT的DAC和ADC产品(500KSPS)? 谢谢
[答：Steven Feng]	it will be released soon.	[2008-2-27 10:49:14]
[问：birdshyyang]	贵公司有哪些型号运算放大器和A/D转换器？？价格？？
[答：Alice Hui]	Please refer to our website for more details about our products. http://www.national.com/CHS/appinfo/adc/ and http://www.national.com/CHS/appinfo/amps/. For the price details, please refer to http://www.national.com/pf/master.html	[2008-2-27 10:50:01]
[问：genglei21]	运算放大器有时候会有温漂的现象，请问这种温漂对ADC的影响如何？
[答：Michelle So]	Temperature drift creates amplifier output error, and that error gets convert to ADC output also. Therefore, it is important to choose amplifer with low drift in application that has a big temperature range. i.e. LMP2011	[2008-2-27 10:50:13]
[问：chinesehuhong]	据我所知，贵公司AD,DA芯片比较少，如有的话能推荐些典型应用芯片吗，我们以前主要用ADI,Ti的．
[答：Steven Feng]	we have Giga Hz ADC, high speed ADC and precision ADC family. please see our website.	[2008-2-27 10:52:13]
[问：wypqq2qq]	请问应如何处理板级噪声对于高精度ADC输入部分噪声的影响？
[答：Stanley Tsang]	As you know, it"s important to prevent noise close to ADC,above all higher resolution like 16bit you mentioned. First, use higher ADC, it"s better to transfer to differential input signal to ADC to prevent noise coupling. 2nd, provide clean linear power source to ADC and not use switching power source to ADC. And as you know, make other noisy signals as far from ADC input as possible.	[2008-2-27 10:53:16]
[问：Zhang Zhongping]	ADC的微分非线性是什么概念，目前的最好的ADC的指标能够达到什么水平？
[答：Thomas Chang]	1.ADC DNL is quantization noise (Dual to process of assigning 0 and 1 to the signal) . 2.Best DNL vale :It is depend on how many bit it is . Normally, under 0.5LSB is good to use the ADC .	[2008-2-27 10:54:26]
[问：psb]	请推荐关于PT100测温的直流小信号测量用高精度AD芯片
[答：Payne Wang]	所使用的AD芯片的精度需要根据你所需要达到的系统精度来决定的. ADC141S626, ADC16061 可以用于你14位和16位选择. 如果12bit足够您的使用那么我们会有一整系列的Pin-to-pin兼容的产品供您选择. 详情请见:http://www.national.com/appinfo/adc/	[2008-2-27 10:55:41]
[问：gumulaoyi]	噪音主要是放大器这块儿比较大吗？
[答：Steven Feng]	ADC: SNR=6.02N+1.76 , you can select the noise parameter from OP AMP datasheet.	[2008-2-27 10:56:24]
[问：zmt]	在ADC输入端加入放大器有时会出现自激和来自外部的干扰,如何避免?
[答：Robert Shen]	要看那类的干扰。通常用合理的布局和布线加滤波补偿来解决。	[2008-2-27 10:56:56]
[问：zongxl]	什么时候需要对电源50Hz进行滤波?如何实现?是加在抗混叠滤波器前吗？
[答：Thomas Chang]	1.Your signal including 50Hz noise ,so you need a filter. 2.Yes ,The filter place infront of anti-alias filter .	[2008-2-27 10:57:00]
[问：timchen]	在精密测量中，要求ADC模拟电源和数字电源隔离供电，请专家讲解如何做到较好的电源隔离？
[答：Steven Feng]	1.use different power supply 2,use inductor to isolate.	[2008-2-27 10:57:47]
[问：yijin]	在24位的AD转换器件中，如何合理地选择其模拟输入驱动运放？请推荐几种NS的运放。谢谢！
[答：Steven Feng]	NS will release 24 bits ADC soon, promote LMP2015 for precision amp.	[2008-2-27 11:01:18]
[问：jida_song]	双极性输入信号，采用运放转换为单极性信号输入到ADC，设计时应关注哪些问题？
[答：Payne Wang]	主要需要注意信号的丢失问题. 如果需要将输入的负信号转换到正信号,此时需要注意你所使用的信号放大器的信号平移.一般对会在输入端适当的加入参考电平信号. 这种状况一般出现在单端输入的情况.	[2008-2-27 11:02:12]
[问：jida_song]	RC滤波器设计在ADC前会出现相位漂移的情况吗？
[答：Michelle So]	It will change depending on temperature and affect phase drift. However, choosing the right value of R & C, with low drift can help ADC performance.	[2008-2-27 11:02:34]
[问：wyq98]	LMH6502自动增益控制后级接ADC，如何根据输出结果计算输入的电压？能否知道某一时刻的放大倍数？
[答：Steven Feng]	yes, you can refer to LMH6502 datasheet, it has the details description.	[2008-2-27 11:02:50]
[问：dingjiong618]	在精密测量中,外部的电源文波对AD转换的结果影响大吗?
[答：Thomas Chang]	Yes,.It is a big issue in Vref . It causes LSB value wrose . => ENOB wrose ,software design margin less	[2008-2-27 11:02:55]
[问：zongxl]	如何提高A/D转换的有效位数，尤其对高频信号的转换？
[答：Stanley Tsang]	First, you need to select good performance ADC. Then things to notice like : 1.Stable & precise voltage reference to ADC. Make the signal amplitude close and not over to full scale of ADC Vin. 2.For high frequency, if you know the bandwidth, then it"s better to add proper band-pass filter to filter unwanted noise. 3.Recommend to use differential input to ADC. 4. Use very low jitter clock as ADC clock source (e.g. NS LMK3000 series)	[2008-2-27 11:03:20]
[问：zhangjiakouzf]	模数转换器的速率是多少
[答：Steven Feng]	it has different sample rate for different ADC.	[2008-2-27 11:03:31]
[问：lxd1982]	为了得到好的电压增益线性或dB线性，在运算应用时要注意什么？
[答：Thomas Chang]	1.Choose right linear gain parts 2.Frequency v.s. Gain curve 3.Analog or digital signal to adjust the gain range	[2008-2-27 11:06:06]
[问：xuxm]	如何降低差分放大器输出噪声对模数转换器的影响？
[答：Payne Wang]	首先我们需要分析在放大器输出端的噪声的来源,来自于运算放大器的电源还是输入信号,或者是运放本身?处理的方法有所不同.我们考虑的一般是尽量处理噪声源. 当然在OPAMP与ADC之间的接口部分需要考虑使用滤波器.例如在我们演讲资料中的LMH6550+ADC12DL065的实例,加入了RC的滤波器,同时可以起到滤波的作用,还可以起到防止信号折叠和ADC输入的开关电容引起的不稳定问题.	[2008-2-27 11:07:01]
[问：genglei21]	怎样解决ADC中的电磁干扰问题？
[答：Robert Shen]	电磁干扰有两方面，一个是被别人干扰，另一个是干扰别人。对于前者，只能采取屏蔽，和滤波的办法。对于后者，则可以用各种方法解决，如，PCB布线，终端匹配，LVDS技术等。	[2008-2-27 11:07:18]
[问：lxd1982]	什么是pipe line ADC？它有何特点? NS有那些pipe line ADC？
[答：Stanley Tsang]	It"s big question, basicly it"s to use lower bit of ADC (e.g. 2~3 bit)in several stage to achieve total resolution. (e.g. 6 stage of 2bit ADC to achieve 12 bit resolution). One advantage is it consume lower power consumption compared to Flash ADC. Many newer high speed ADCs use Pipeline architecture now. For example, ADC12DL080, ADC11DS105, ADC14V155, ADC12V170...	[2008-2-27 11:08:24]
[问：genglei21]	请问专家,您对放大器的温度补偿方法有何见解?
[答：Steven Feng]	you can select NS precision amp LMP familiy , it has very low TCVos..., you can visit our website.	[2008-2-27 11:09:03]
[问：fwangning]	放大器的输入端如何和传感器进行接口？如何降低对传感器信号的影响？
[答：Steven Feng]	depend on different sensor and application, you need selcet different amp and circuit, such as ,transimpedance amp, current sensor...	[2008-2-27 11:10:57]
[问：fwangning]	请专家谈谈，AD转换器在布线时，为了提高信噪比，需要注意那些？谢谢！
[答：Michelle So]	Separate analog plane and digital plane. make sure differential traces has the same length. use recommended capacitor values to filter out power supply noise. avoid signal coupling on the board. Vref should place close to ADC. See datasheet recommendation.	[2008-2-27 11:11:46]
[问：zhangjiakouzf]	相应的信号处理范围是多少
[答：Steven Feng]	please describe your question details.	[2008-2-27 11:12:15]
[问：tianaiv]	输入信号幅度变化对AD转换会带来那些影响？
[答：Thomas Chang]	Good question. 1.It is input frequency v.s. ADC sample rate issue . 2.From today presentation we know input frequncy and Nyquist frequncy relation , 3.For more accuracy digitalize your input signal , you can choose more 10 times input signal frequency .	[2008-2-27 11:14:31]
[问：izguo]	差动放大器主要特点是什么？
[答：Steven Feng]	differential input or single input , differential output, easy to connect with differential ADC, and high CMRR.	[2008-2-27 11:14:50]
[问：yangxw]	NS是否提供50Hz工频干扰信号的陷波方案？
[答：Robert Shen]	没有现成的方案。	[2008-2-27 11:15:03]
[问：tianaiv]	ADC的输入带宽是如何定义的？它和那些因素有关？如何测量？
[答：Stanley Tsang]	The input full power bandwidth of ADC is defined in similar way as Amplifier. Define the maximun input frequency in -3dB amplitude drop from full scale. It"s independent to ADC sample rate and not related to SNR or THD. It"s related to ADC design inside. The way to test it is to feed different input frequency from low to increase higher and higher, amplitude close to full scale of ADC full scale. And see the amplitude on digital output and find -3dB drop frequency.	[2008-2-27 11:15:10]
[问：xiaojunseu]	放大器输出所接的电容一般要比模数转换器的采样电容大多少？
[答：Thomas Chang]	Normally , it is 10 times cap. value of ADC"s cap.	[2008-2-27 11:15:39]
[问：lxd1982]	运算放大器的放大线性度和那些因素有关？应用时要注意什么？
[答：Payne Wang]	在设计时主要需要考虑运放本身的线性特性. 同时还需要考虑实用电路的结构和选用的外部原件.	[2008-2-27 11:16:44]
[问：long2217]	放大器的带宽与模拟/数字转换器的输出错误有什么关系？
[答：Michelle So]	first calculate ADC"s 0.5LSB value (0.5LSB = no error). For 12bit, 0.5LSB = 0.0156 Then figure out Amplifier"s BW with its GBW spec on datasheet. Then determine the maximum input frequency of the amplifier for zero error in the ADC. Therefore, larger GBW of the Amplifier allows higher input frequency without error for ADC. Example: LMP2011: GBW = 3 MHz At a Gain of 20 - BW = 150 kHz For 12 Bit accuracy Max Signal Frequency is 0.0156(150 kHz) = 2.34 kHz	[2008-2-27 11:16:44]
[问：birdshyyang]	放大器+A/D转换器有哪些封装？都是无铅的吗？？
[答：Steven Feng]	all NS product are NoPb now, they have different package for customer"requirement.	[2008-2-27 11:18:25]
[问：njt36]	放大器目前的應用範圍？
[答：Steven Feng]	almost all application , you can see OP amp .	[2008-2-27 11:19:31]
[问：zmt]	对传感器来的弱信号进行放大时，如何降低电源引起的噪声？
[答：Payne Wang]	对于运放的电源一般推荐使用LDO进行供电,例如LP38691,LP38692等可以提供稳定的电压对运放进行供电.另外需要选择具有高PSRR的运放,LMP2011/2/4可以达到130dB,LMP2231/2/4可以达到100dB.这样可以提高运放电路的免疫力. 另外,PCB的布线,和地平面的噪声也需要注意防止电路间的相互干扰.	[2008-2-27 11:19:48]
[主持人：ChinaECNet]	各位观众，现在用户提问很踊跃，专家正在逐一回答。请耐心等待您问题的答案，同一问题请不要多次提交。	[2008-2-27 11:20:35]
[问：2000_zxb]	对于像7805这样的电源芯片供电的AD芯片，其电源端一般需要加多大的电容才能保证采样的稳定
[答：Thomas Chang]	1.You can study LM7805 datasheet firstly. 2.Normally ADC current requiement is not big ,and LM7805 should be ok . 3.The LM7805 need input and output cap to provide stable power supply to ADC . 4.How many cap"s value is suitable for ADC ? It is depend on ADC sample rate and ADC supply current . You can follow this idea to calcualte the exactly cap"s value .	[2008-2-27 11:21:56]
[问：Yangwh]	高稳定性和低漂移最高精度能够达到多大？谢谢！
[答：Steven Feng]	you can get these datasheets of amplifier and ADC from Website, it has details description for different product.	[2008-2-27 11:23:45]
[问：long2217]	ad转换的位数与精度和电压基准源直接应该如何协调以但到最佳效果？
[答：Robert Shen]	要看具体情况。通常基准电压源的误差不要引起ADC输错误为准。	[2008-2-27 11:23:52]
[问：izguo]	放大器主要有哪几种？谢谢
[答：Steven Feng]	different criterion, such as: current feedback , voltage feedback. or high speed, precision, low power. ....	[2008-2-27 11:25:14]
[问：wangjuanfang]	数模、模数转换器、比较器的原理是什么？谢谢
[答：Michelle So]	typically it"s based on comparing with different reference voltage. For example, an 8-bit comparator would have 2^8=256 reference voltages points. Newer architectures use capacitance comparasion instead of voltage comparasion. For example, SAR ADCs use capacitance architecture. Therefore capacitive load is important for system design.	[2008-2-27 11:25:55]
[问：yijin]	请问专家，我们用在ADC前级的放大器的线性度要求较高，NS那种产品性能较好？是选用dB线性好还是增益线性好？
[答：Thomas Chang]	National provide LMH6502 /03/04/05 VGA t o customer choice . You should decide what parameter you need . Example : Gain range , Gain bandwidth ,... DB gain benefit :low gain range got more resolution	[2008-2-27 11:27:37]
[问：shenxm1]	如何测量INL 和DNL ?
[答：Michelle So]	compare Input and output signals of the ADC and see what is the maximum LSB error between the two signals.	[2008-2-27 11:29:17]
[问：cyberxx]	设计中最需要注意的问题是什么
[答：Robert Shen]	看具体要求。通常是信号带宽，采样频率，ADC精度等。	[2008-2-27 11:30:03]
[问：yijin]	ADC中如采用放大器，设计时应该注意什么呢，如何保证尽量避免噪声的引入？
[答：Thomas Chang]	1.precision : low noise amp. choise to reduce ADC input noise 2.Bandwidth : Amp could not cut loop gain in signal path 3.filter : Add filter in front of ADC	[2008-2-27 11:30:26]
[问：sszzjj]	专家好：在A/D转换中，如何更好转换高频信号？谢谢
[答：Stanley Tsang]	1.Very low jitter clock source. Clock jitter requirement is directly related to input freqnency and ADC resolution. 2.In High freqnency input, if possible filter unwanted frequency -- suppose you know the bandwidth of high frequency signal. 3.stable/precise reference voltage on ADC. 4.Amplitude as closer as possible to full scale of ADC....	[2008-2-27 11:31:29]
[问：wangjuanfang]	什么是缓冲放大器？谢谢
[答：Steven Feng]	it means buffer,for impedance match , driver...	[2008-2-27 11:32:16]
[问：lvkush0903]	AD转换器采样的线性不好，该如何处理？
[答：Michelle So]	Choose another ADC with better linearity specs. The digitization happens internally and external circuit cannot help the process.	[2008-2-27 11:33:28]
[问：pgt661004]	在放大倍数一定的情况下，电阻的值的大小有无严格规定，阻值的大与小有关系吗？
[答：Steven Feng]	yes, you need check the datasheet.	[2008-2-27 11:34:06]
[问：wgqabc]	我要用到6级运放，是用一个4运放（lm324）和2运放（lm358），还是用3个2运放（lm358）？
[答：Steven Feng]	both can be used, depend on your application.	[2008-2-27 11:35:38]
[问：wgqabc]	请问：放大器芯片在电路中接那种地??接数字地还是模拟地??
[答：Michelle So]	Analog ground.	[2008-2-27 11:36:47]
[问：2000_zxb]	放大器输出到ad芯片之间是否需要串联电阻，若是则电阻如何选择，会对信号产生多大的影响
[答：Payne Wang]	这需要根据处理的信号的特性和使用的ADC的的输入方式有关系.一般推荐在OP和ADC之间加入简单的RC网络:串联的电阻+并联的电容. R的值需要根据加入的RC电路所起的作用有关.如果作为滤波或抗混叠,则需要根据滤波器的截至频率和使用的C的值综合进行选择.如果作为ADC输入稳定考虑,R一般选择50~100ohm	[2008-2-27 11:37:00]
[问：BaoYuanzhi]	请问应该从哪些方面看来考虑模数转换器的输入阻抗匹配？
[答：Steven Feng]	Please see input impedance and input capacitance specs on datasheet. These values might present as loading for the previous block.	[2008-2-27 11:38:28]
[问：lvkush0903]	应如何选择输入放大器?带宽,转换速率或建立时间?
[答：Robert Shen]	主要是根据信号带宽，精度要求（低噪，低漂移），来选择放大器。输入端的共模电压范围也是一个很重要的指标。	[2008-2-27 11:38:33]
[问：psb]	AD(内部带放大器PGA)之前必须得加RC滤波吗？直流100MV的信号，R ，C的最佳值是多少？谢谢
[答：Payne Wang]	一般加RC是出于滤波和稳定ADC输入级考虑的.滤波需要根据信号要求决定.稳定ADC输入级,需要看ADC的输入级是Buffer,Switching Capacitor 还是其他模块.处于稳定功能的设计是针对Switching Capacitor的ADC而言的.	[2008-2-27 11:39:52]
[问：lvkush0903]	影响ADC频率进一步提高的主要障碍在什么地方？
[答：Thomas Chang]	1.Jitter is major concern . 2.High frequncy Noice couple is a big issue . 3.Now National have released ADC08D3000 ,sample rate is 3 GHz .	[2008-2-27 11:41:44]
[问：pgt661004]	比方说：放大倍数k=r1/r2，r1=100k，r2=10k和r1=10k，r2=1k，虽然k相同，但电阻不同，效果有什么不同呢？
[答：Robert Shen]	电阻小，意味着电流大，同样的电压下功耗就大。电阻大，相对噪声也大。要看设计要求来定。	[2008-2-27 11:42:10]
[问：zhiyu03]	滤波器的哪些特性对模数转换器和放大器有影响？
[答：Michelle So]	transition band must be sharp enough to cut off all unwanted signals. Please see P.7 of presentation. If using passive compenents to form the filter. Resistor has temperature drift and will affect phase margin. Need to select low drift components.	[2008-2-27 11:42:22]
[问：zhxl]	目前A/D转换器实现高分辨率高精度AD转换是用什么方法，目前能达到多高的精度？
[答：Thomas Chang]	Sigma Delta , 24bits	[2008-2-27 11:42:32]
[问：wgqabc]	请问怎么样用集成运放或别的什么元件搭一个具有门限的放大器，即门限放大器？
[答：Steven Feng]	you can selcet an variable gain amplifier, such LMH6505.	[2008-2-27 11:42:41]
[问：shenxm1]	请问NS的A/D转换器在过压保护方面有什么特点？
[答：Stanley Tsang]	Notrmally it"s suggest to have input signal not to over full scale of ADC. While if over full scale, in NS ADC, we have diode protection ckt inside ADC. One example you can refer is ADC12V170 data sheet page 3 pin definition. Beside, the data output would still keep with full scale data in this situation. While one key thing is still can"t be over "Absolute Maximun range" defined inside each ADC"s spec.	[2008-2-27 11:43:07]
[问：wuyu0509]	考虑到成本，使用LM324A对小信号进行放大，由于使用单电源供电，无法对输入失调电压进行调零，因而造成采样误差，请专家提供建议，谢谢！
[答：Steven Feng]	you can select new precision amp ,they have better Vos,TCVos..., such LMP family ,you can check in our website.	[2008-2-27 11:44:17]
[主持人：ChinaECNet]	所有问题均已提交给美国国家半导体公司的专家。座谈期间未回答的问题，美国国家半导体公司专家也会逐一回答，并在中电网上公布，请大家注意收看。	[2008-2-27 11:44:58]
[主持人：ChinaECNet]	由于时间关系，本次中电网“在线座谈”马上就要结束了。虽然各位听众（网友）已与美国国家半导体公司的专家讨论了许多问题，但是还有许多提问没有来得及进行交流。本次在线座谈结束后，中电网将请美国国家半导体公司的专家继续答复所有的来自各位听众（网友）的提问，然后整理上载到中电网网站上，以便大家查阅。	[2008-2-27 11:45:10]
[问：shenxm1]	现在的GSM手机的零中频或者近零中频收发器中，各级放大器的增益一般有什么要求？比如低噪声放大器（LNA）、一级放大器、功率放大器（PA）以及信号在进入基带之前的增益大概在什么范围？
[答：Payne Wang]	GSM的Transiver涉及到的Amplifier与普通运放原理类似,但要求和特性有所不同.LNA需要低噪需要在频谱上给GSM信号留有足够的SFDR也就是需要有足够低的噪底,这样可以提供主要的增益来源.PA主要考虑是否具有足够的线性度和驱动能力.	[2008-2-27 11:45:35]
[问：pgt661004]	如何确认AD在具体系统应用中能达到的有效位数?
[答：Stanley Tsang]	One way is to provide clean sine wave signal in different frequency (e.g. the frequency inside data sheet) from signal generator, then you can do FFT analysis from data output. For example our Wavevision software tool, then it will show the real ENOB from different input frequencies.	[2008-2-27 11:46:10]
[问：zhiyu03]	请问放大器输入电平过高，对输出会有什么影响？
[答：Steven Feng]	you need see the CMVR parameter in the datasheet.	[2008-2-27 11:47:44]
[问：pgt661004]	请问：正向放大器和反向放大器除过放大倍数的计算不一样外，还有其他不同吗？在不计较正反向的前提下，选哪种好对信号质量一点。
[答：Payne Wang]	在输入阻抗和共模特性方面都不一样.对于比例放大电路,做什么样的选择需要考虑输入输出信号的关系.最简单的方法就是根据要求的电路的一次函数特性来定.电路的结构,是否需要信号平移以及电路中使用的元件的取值都可以计算出来.在我们国半的以往研讨会的资料中都有涉及到.	[2008-2-27 11:50:48]
[问：liguang169]	设计上如何兼顾平衡A/D转换精度和速度？
[答：Stanley Tsang]	This is also related to ADC"s internal design. And one key thing is to make sure the clock is clean and very low jitter - we will discuss this in next Webninar. And 2nd, to have good linear power source. 3rd, for higher speed and resolution, LVDS output is better like ADC12V170 or ADC14DS105... Beside,also notice if the bandwidth(FPBW) of ADC you use is enough.	[2008-2-27 11:52:30]
[问：haicheng315]	我如果要实现4～13V输入，+5V，3.6V输出的话，输出电流最大300ma左右，我该用控制器好还是转换器好？请问专家有没有推荐的芯片？
[答：Steven Feng]	you can achieve by amp .V-I and high output current, such as LMH6601.	[2008-2-27 11:54:06]
[问：wuyu0509]	大多数书中都是举例设计两级的放大器,请问加了共模反馈（CMFB）有什么变化?
[答：Thomas Chang]	1.Because of feedback resistors, the output common mode will influence the input common mode. 2.Signal-source common mode also will influence the amplifier input common mode.	[2008-2-27 11:54:30]
[主持人：ChinaECNet]	在此，中电网特别感谢给予本次中电网在线座谈巨大支持的美国国家半导体公司，特别感谢专门在线回答各位听众（网友）提问的美国国家半导体公司的各位专家们，特别感谢各位听众（网友）积极热情的参与。	[2008-2-27 11:55:20]
[主持人：ChinaECNet]	祝大家事业有成、生活愉快！欢迎多提宝贵意见，欢迎关注中电网，下次再见。	[2008-2-27 11:55:28]
[问：haicheng315]	在A/D采样系统中输入信号的纹波与突发脉冲如何处理?
[答：Robert Shen]	可以采用滤波的办法处理。	[2008-2-27 11:56:17]
[问：zhxl]	采样、转换速度比较高的高精度A/D转换目前达到多高的精度？采用什么方法？（∑Δ法太慢）
[答：Stanley Tsang]	If not think about Sigma-Delta ADC, so far our 14bit ADC ADC141S626 is the solution. And our 16bit SAR ADC will be available in near future. (SAR ADC is much faster than Sigma Delta). If your speed requirement is higher like 20Msps at least, then you can select ADC14L020/040, it"s pipeline based ADC.	[2008-2-27 11:57:05]
[问：jianbocq]	VDSL系统中ADC精度对DMT系统有哪些影响影响？
[答：Thomas Chang]	High resolution means high dynamic range . High dynamic range means lower noise effect . So ,For VDSL system ,you should care sample rate firstly , because VDSL signal is around 50MHz before my idea.	[2008-2-27 12:05:11]
[问：haicheng315]	如何补偿A/D采样系统的迟延?
[答：Steven Feng]	For ADC, different architechure will have different latency. Like SAR or Pipeline... Normally this latency information would be available inside data sheet. If you want to do compensation, it would be needed to be done inside FPGA.	[2008-2-27 12:05:30]

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