| 主题: 高性能ADC的变压器耦合前端设计 |
| 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ADI公司的专家就“高性能ADC的变压器耦合前端设计”举行在线座谈。在座谈中，您可就您关心的问题与ADI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“高性能ADC的变压器耦合前端设计”的了解和掌握，而且能够为大家事业的发展带来裨益。 |
[2006-6-14 10:16:54] |
| [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。 |
[2006-6-14 10:35:24] |
| [问：liefen] |
请教一下，一般AD前端信号调理都是使用高精度的OP来做放大和匹配，现在为何使用变压器耦合呢？这2者的优缺点能否比较一下？
谢谢 |
| [答：ZhangJ] |
Of course, some you can use OP or transformer to match and drive the
ADC. For transformer, it can provide good isolation and balance performance
than OP. But somewhat expensive. Thanks! |
[2006-6-14 10:36:22] |
| [问：kittin] |
请问两个33欧姆的阻抗匹配原理是如何设计的？谢谢 |
| [答：Rob] |
The 33ohm resistors are to de-q the ADC"s internal front end parasitics
and dampen the charge injection or kick-back from the ADC"s sampling switch. |
[2006-6-14 10:41:03] |
| [问：blueprince] |
请问：使用ADE7759设计的时候要注意些什么？还有在datasheet的参考连接中，模拟电源和数字电源没有分开，可以给说说吗？谢谢。 |
| [答：ZhangJ] |
For ADE7759, you should pay attentation to the power supply ripple/REF
accuracy and clock jitter. For analog and digital GND issue, in our datasheet
,it is not been seperated. This diagram is just a functional connection
diagram. But you can also refer to our ADE7759 Eval datasheet. In this
datasheet , these two GND has been seperated for better performance. Thanks! |
[2006-6-14 10:41:55] |
| [问：qizhi_liu] |
一般对作为ADC输入耦合变压器有什么要求？ |
| [答：Zhao] |
要保证信号不失真，变压器阻抗比要合适。完成单端到差分的变换时注意给出合适的共模电压。 |
[2006-6-14 10:41:59] |
| [问：ecnanjing
_EBY7E] |
变压器输入会引入脉冲振铃,如何消除? |
| [答：Rob] |
If there is ringing at the transformer. Use a termination resistor in
parallel or series. The value of the resistor and combination may vary
per application. |
[2006-6-14 10:42:15] |
| [问：jwfeng] |
请问对于非缓冲ADC，输入端前加电压跟随器是否可以达到近似于缓冲型ADC的效果 |
| [答：Rob] |
Yes. |
[2006-6-14 10:42:26] |
| [问：cheng37bin] |
是否讲变压器结构及绕制参数 |
| [答：ZhangJ] |
Very littel for these part. We will focus on the external electric performance
and the application issue with ADC. |
[2006-6-14 10:42:41] |
| [问：ecnanjing
_EBY7E] |
如何克服输入阻抗随频率变化而变化,而达到最佳的信号匹配? |
| [答：ZhangJ] |
Generally, the input inpedence will range with frequency. You can refer
to the presentation for the solution. -----Add a L R C network to match
the variation. |
[2006-6-14 10:44:13] |
| [问：wangjade] |
请问，变压器耦合设计的优点有哪些？
谢谢！ |
| [答：Nie] |
它可以比较容易的实现高频的阻抗匹配，而且可以实现单端到差分的转换。 |
[2006-6-14 10:44:34] |
| [问：ecnanjing
_EBY7E] |
开关电容式ADC的输入藕合是否必须用输入变压器吗?特别是在高频段的插入损耗如何补偿? |
| [答：ZhangX] |
1. Use AD8137 or AD8352 differential ADC driver is a good choice if
input frequency is not too high (140MHz). Use 25Ohm serial resistor help
to remove reflection, 100p shut capacitor between +Ain and -Ain help to
balance difference of the capacitor on +Ain and -Ain.
2. Very high IF under sampling still use Transformer. Use S parameter
to get equal R/L/C and match it. Refer to today"s online seminar material. |
[2006-6-14 10:44:38] |
| [问：ysuhanfei] |
如何在速度和精度上做平衡 |
| [答：Rob] |
This depends on the application. Not sure what this question relates
to? |
[2006-6-14 10:45:15] |
| [问：superne
mocn] |
桥式传感器的供电和运放、AD的供电为同一组，这样对整个系统有没有影响？ |
| [答：Zhao] |
在这类低速精密应用场合，我们推荐模拟和数字分开。如同一组供电不可避免，ADC的有效分辨率可能会下降。 |
[2006-6-14 10:47:55] |
| [问：borry] |
ADI公司现在有没有采样率2G以上的ADC采集芯片提供？ |
| [答：Nie] |
No. |
[2006-6-14 10:47:59] |
| [问：lulu2890] |
如何保证ADC的精度?对于单电源供电,零点的确定和量程都与电源电压有关,如果电源电压波动势必导致转换的误差,特别对小信号的采集电路中如何解决? |
| [答：ZhangJ] |
For ADC accuracy, it depends on it"s INL/DNL/REF/CLOCK/PSRR etc. For
single power supply, I think the accuracy releated much to the REF voltage
while the power supply. If power supply change, the output will change
too. But the change is very little. You can refer to PSRR parameter in
our datasheet to calculate this change. |
[2006-6-14 10:52:31] |
| [问：qizhi_liu] |
请问一般的可以用做ADC前的耦合变压器的频率范围，谢谢！ |
| [答：Zhao] |
一般在几KHz到1GHz。 |
[2006-6-14 10:53:15] |
| [问：robertkliu] |
请介绍一下使用变压耦合后的隔离能达到什么指标？ |
| [答：Rob] |
A transformer has basically no isolation. For example, when the charge
injection or kick-back from an ADC is produced by the sampling switch
you can see this effect through the transformer on the primary side. |
[2006-6-14 10:53:57] |
| [问：Sumoon] |
请问能推荐一下较高精度桥式传感器AD转换器吗？ |
| [答：Nie] |
Generally speaking, sigma-delta ADCs have the highest precision and
accuracy. AD779x are the best parts for this kind of application. |
[2006-6-14 10:55:35] |
| [问：robertkliu] |
使用变压器后的ADC采样速率能达到多少？ |
| [答：Zhao] |
变压器能够耦合的频率范围一般在KHz到1GHz。更高的频率范围可以用巴伦。ADC的采样速率取决于ADC本身。 |
[2006-6-14 10:55:59] |
| [问：qing525] |
高速或高频ADC在PCB板上布局时需要注意什么？ |
| [答：Rob] |
Smart partitioning. Meaning, keep analog components on the analog side
or domain and digital components on the digital side or domain. Do NOT
mix the two, even if on different layers of a pcb. This creates a chance
for coupling back onto the analog input where it is most sensitive. |
[2006-6-14 10:56:05] |
| [问：lulu2890] |
在16位的AD转换器件中，如何合理地选择其模拟输入驱动运放,该运放放在什么位置较为合适? |
| [答：ZhangJ] |
For 16 bit ADC, I recommend you selecting AD8007,AD8009 or AD8139 to
match and drive the ADC. If you are using high speed ADC with IF sampling,
you can also use a transformer to interface with your ADC. The bandwidth
should be 10 times more than ADC and the SNR should also be better than
ADC. |
[2006-6-14 10:56:24] |
| [主持人：ChinaECNet] |
在此回答问题的专家是ADI公司的：Nie Haixia、Zhao Yiming、Zhang Jing、Zhang
Xinming、Song Haoran、Rob Reeder、Salina Downing和Catherine Wang。 |
[2006-6-14 10:57:42] |
| [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。 |
[2006-6-14 10:57:53] |
| [问：shipyan1] |
输入阻抗的匹配除了变压器外,还有什么方法? |
| [答：Rob] |
If you understand the transformer and its parasitics, then you can match
your input impedance best. Making the input as "REAL" as possible will
give you the best match and the most bandwidth on the front-end design
to the ADC. |
[2006-6-14 10:58:00] |
| [问：jwfeng] |
请问缓冲型AD的输入阻抗最小值一般为多少？ |
| [答：Rob] |
1-2kohm differential. |
[2006-6-14 10:58:15] |
| [问：drking] |
请问，目前转换速率最快的ADC可达到多少？能否介绍几个型号？ |
| [答：Nie] |
It depends on the resolution, power consumption requirement and etc.
What is your application? |
[2006-6-14 10:59:09] |
| [问：kittin] |
如果变压器匹配不是很好，对差分信号的损耗有多大？我试验过不同变压器配比，好像影响不大？ |
| [答：ZhangJ] |
It depend on your signal frequency and the performance of the transformer.
If it is mismatched that will result in impendance mismatch issue . For
high frequency, this mismatch will result in ring and reflect. In the
end , it will result in the loss of accuracy for ADC. Thanks! |
[2006-6-14 11:00:53] |
| [问：sealailai] |
您好，请问我采用贵公司AD6645芯片能否在70MHz中频上完成对40MHz带宽信号的采样？若能完成，A/D电路前设计时需注意什么问题及达到什么样的指标？A/D采样后采用AD6636进行数字下变频，请问AD6636通道能否合并，最大处理信号带宽为多少？谢谢！ |
| [答：ZhangX] |
1. 根据采样定律，采样时钟必须是信号带宽2倍以上。 用AD6645的105M版本应该可以，但80M版本肯定不行。问题是AD6645需要进口许可证。（14位限5M内，12位限50M内）。建议考虑AD6654。AD6654是AD6645和AD6636数字下变频器的集成芯片，无需进口许可证。AD6654最高92.16M。另外一个考虑是AD80141（140M
11位ADC). 请直接与CAST联系，或email：eagle.zhang@analog.com.
2. AD6645 + AD6636 = AD6654, 可用4通道并用处理，ADI提供AD6654在WiMAX的设置。也提供AD6633＋AD9779数字上变频的设置，或直接用AD9957DDS数字调制器。
3.例如对4载波WCDMA（5M x 4），可用70M IF 61.44M采样时钟。对20M BW，可以用4ch并联工作。更高带宽，与滤波带外抑制度有关。不够时可在AD6654后再加一FPGA。
4.宽带数据可考虑AD80141与FPGA。
5. AD8348 + AD9862 + AD8349 +ADF4154基带方案限制较少。 |
[2006-6-14 11:01:35] |
| [问：srqiang] |
能否介绍一下用于耦合的变压器？ |
| [答：Zhao] |
Mini-circuits公司，Macomm公司都有相应的变压器。
如ADT1-1WT |
[2006-6-14 11:01:55] |
| [问：chenbaoyuan] |
采用开关电源的电路，如何消除或减小开关电源的开关纹波对ADC精度的影响？ |
| [答：Nie] |
You can add ferrite bead and adequate decoupling to your power supply.
However, we still recommend you to use the linear power supply for high
precision circuit. |
[2006-6-14 11:03:47] |
| [问：shipyan1] |
阻抗匹配的程度有没有量化的概念?即百分比多高才算匹配? |
| [答：ZhangJ] |
The impendance match is very important for high frequency circuit. If
mismatched, it will result in ringing and reflecting issue. The calculation
is difficult. But you can use the reflecting ratio and reflecting loss
to calculate the noise caused be mismatch. I think 1%--3% should be better. |
[2006-6-14 11:03:51] |
| [问：jwfeng] |
请问变压器耦合与光耦合相比哪个更好些？ |
| [答：ZhangJ] |
different kinds of solution. For high frequency, transformer is better.It
can also provide impendance match. |
[2006-6-14 11:05:21] |
| [问：ysuhanfei] |
如何解决高精度adc的时飘和稳定性 |
| [答：Zhao] |
需要高精度ADC，基准，良好的PCB Layout，高质量的线性电源。 |
[2006-6-14 11:05:22] |
| [问：superne
mocn] |
现在ADI的ΔΣ型AD的最高速率可以达到多少？ |
| [答：Nie] |
several Mega Herz. |
[2006-6-14 11:05:51] |
| [问：kittin] |
演讲中是50Ohm的参考设计，可否给出一个75Ohm的参考设计？谢谢 |
| [答：ZhangJ] |
Of cause.But I can"t give you the circuit this time. Because
I can"t draw circuit here. You can refer to our presentation to calculate
the 75ohm . Or you can call 800 810 1742 after the session. |
[2006-6-14 11:07:18] |
| [问：smx64098] |
专家您好：请问怎样解决变压器（互感器）的非线性问题？我用的是ADS8364,多路同时采集电流与电压，怎样解决由于互感器带来的电流与电压的角差等问题？怎样进行补偿比较合理（适）？
谢谢！ |
| [答：ZhangX] |
对低速电路，如无隔离需要，可以不用变压器，直接用运放驱动ADC即可。互感器匹配电阻大小很重要。
ADS8364是250kSPS 6ch ADC， 与AD7656类似。ADS8364需要2级运放设置中点电压，AD7656可以直接输入+/-10V
or +/-5V 双极性电压，一级运放缓冲即可，一般用OP2177ARZ。 |
[2006-6-14 11:07:25] |
| [问：qing525] |
高速ADC中一般都包括一个S/H电路，是不是这个电路对ADC性能的影响很大？怎么提高其性能？比如说减小开关的电荷注入效应等等。 |
| [答：Zhao] |
内部的S/H决定了ADC的模拟带宽。如果这部分性能不够好，会严重恶化ADC的SNR，尤其是在采样速度较高的应用中。对用户来说，S/H无法更改，您需要做的就是做好模拟前端的匹配，可能的情况下可以用两级变压器，如该讲座中提到的。 |
[2006-6-14 11:08:53] |
| [问：REABOO] |
今天的讲座是关于变压器偶合前端设计,这里的变压器是不是泛指ADC前端的电平调理电路? |
| [答：ZhangX] |
This is for high speed ADC buffer and driver issue. For example, for
70M IF digitize, you can use AD8370 DGA or ADL5330 VGA to control gain,
use AD8352 driver ADC (ADL5330 and AD8352 are differential). Or use normal
RF amplifier gain, then use transformer get from single end to differential. |
[2006-6-14 11:09:23] |
| [问：Sumoon] |
在模拟/数字电源分开设计中，模拟地和数字地之间用那种方式连接较好？ |
| [答：ZhangJ] |
you can use star connection form. But you has better lay out one layer
AGND and one DGND if there is lots of GND nosie in your system. For high
frequency, this will be more critical. |
[2006-6-14 11:10:27] |
| [问：pjb0451] |
在高频数据采集中一般都采用变压器缩小信号吗？用互感器采样不可以吗？ |
| [答：ZhangX] |
For low speed sampling (below 250kSPS), use AD8137 differential driver
or OP2177 OP amp drive ADC is OK. Here we discuss high speed ADC sampling.
(10M f in or above). |
[2006-6-14 11:11:05] |
| [问：kfggx] |
单端输入和双端输入ADC各有什么有缺点？ 怎样选择？ |
| [答：ZhangJ] |
Single ended ADC is simple .But the differential ended input ADC is
better for CMRR performance . If the frequency is high and you want to
get more CMRR/SNR ,you should select differential input ADC. Thanks! |
[2006-6-14 11:12:56] |
| [问：Xihuwuyu] |
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：Zhao] |
ADI提供高性能的差分放大器如AD8138，AD8139完全能够胜任16bit高速ADC的驱动要求。噪声来源于电源，地噪声，电阻热噪声。您可以采用滤波器来滤掉带外的噪声（差模和共模噪声）。 |
[2006-6-14 11:12:59] |
| [问：mayongtao] |
如果一个ADC的采样频率是10Mhz，那么不论ad是并行输出，还是串行输出，输出的数字信号频率都是10Mhz么？ |
| [答：Zhao] |
对。 |
[2006-6-14 11:13:45] |
| [问：zhangwq] |
如何调整差分信号所产生的交越失真? |
| [答：ZhangX] |
It"s very important to get differential amplitute balance and phase
balance. For normal differential amplifier, the match resitors should
be matched. So it"s easier to use some differential dirve such as AD8352
which use only one resitor to set gain.
对差分对管的交越失真，偏置电流非常重要。 |
[2006-6-14 11:14:51] |
| [问：sayman] |
请问ad8504ar和intersil的el5401可以实现相同的功能吗？能否替换？ |
| [答：Zhao] |
EL5401，有这个型号么？如有，您可以咨询ADI中国技术支持中心8008101742。 |
[2006-6-14 11:15:57] |
| [问：zhiliang16] |
是不是在用OP做放大匹配的时候，仍然会存在对unbuffer和buffer需要有不同的设计？？ |
| [答：ZhangX] |
Yes. Unbuffered ADC need carefully match, and need add in serial resistors
to prevent potional unstable. For buffered ADC, it"s much easier. |
[2006-6-14 11:16:09] |
| [问：dyuanan] |
请问能否较精确的检测交流减速电机从正常运转到堵转停止(时间较短暂)之间工作电流的模拟数据?请专家指点采取什么检测方案比较妥当可行.谢谢! |
| [答：ZhangJ] |
Of couse you can. You can use our difference AMP such as AD8212/3,AD8205/6/10,ADM4073
to fulfill the function. Thanks! |
[2006-6-14 11:17:40] |
| [问：lizhaoyin] |
在哪里能获得免费器件资料下载？ |
| [答：Nie] |
Please go to ADI website www.analog.com |
[2006-6-14 11:18:21] |
| [问：lulu2890] |
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：ZhangX] |
Use differential drive such as AD8137 or AD8352 is a good way to avoid
common noise from ground plane. Use transformer if input frequency is
too high (>100MHz).
Carefully match differential trace on PCB board is also very important.
(For example, 6mil thickness from top to GND layer, in order to match
to 50 Ohm, use 8mil trace width, 8mil distance between 2 differential
trace, 24mil distance from other trace.
Good GND plane, good power decoupler, good component displacement will
always be very helpful to mimimize noise. |
[2006-6-14 11:19:37] |
| [问：ysuhanfei] |
想用一个24位的adc做电桥测量，速度10k左右，请介绍一套方案，原来用的adc做完后，精度只有17位，有时候还不到17位，是不是必须得用4层以上得板子布线 |
| [答：ZhangJ] |
you can use AD7799 to realise the function. |
[2006-6-14 11:21:03] |
| [问：shu_chen] |
使用差分ADC驱动放大器应该比使用变压器噪声性能差，对这个问题，我们应该怎么选择？ |
| [答：Rob] |
This is correct. An amplifier will have more noise than a transformer.
To answer your question it is really application dependent. I would use
and amplifier if I need lots of gain. I would also use an amplifier if
noise or SNR is not a key design parameter. However, if you need lot of
bandwidth for sampling very high IF frequencies then I would use a transformer.
These are only some of the tradeoffs. |
[2006-6-14 11:21:42] |
| [问：ecnanjing
_EBY7E] |
使用变压器可以提高输入特性,但体积较大,如何平衡、取舍？ |
| [答：Zhao] |
变压器为无源器件，不会引入噪声。如果您的PCB不允许大体积的变压器，你可以考虑使用差分放大器作高速ADC的驱动。AD8138可以驱动16bit的ADC。 |
[2006-6-14 11:22:16] |
| [问：luckyfairy] |
变压器所引起的相移如何处理? |
| [答：Rob] |
Find a better transformer or use two transformer or baluns as suggested
in the seminar material. |
[2006-6-14 11:22:35] |
| [问：blueprince] |
请问使用AD7656的时候要注意些什么？在我的设计中，CONVSTA,CONVSTB,CONVSTC可以连接在一起控制吗？也就是说一个CONVST信号控制六路？谢谢 |
| [答：ZhangX] |
Yes, YOu can connect CONVSTA, CONCSTB and CONVSTC together. The most
importance thing in AD7656 application is AVCC decoupler. Another is GND
layer. For example, if you use 3pcs OP2177ARZ as input buffer, you will
has +/-12V power supply for OP2177, also +/-12V power supply for AD7656.
Then It"s very difficult for Power layer because there are also 5V AVCC
and 5V DVCC. So, fist of all, use 4 layer PCB, TOP, GND layer, Power Layer,
Bottom. GND layer cut into 2 pcs, AGND and DGND, it"s connect together
under AD7656 or some where near AD7656 (please make sure the power supply"s
GND is not connect together). Then, use every 0.1uF decoupler capacitor
very near to each AVCC pin, use 10uF decouple capacitor on each AVCC pin.
In order to connect total 8 AVCC pin, you need use a whole AVCC area under
AD7656 and near AD7656. Then, you need lay +12V and -12V trace (>1mm
width) on the bottom layer. You can still use +12V and -12V in power layer
for the area of OP2177 place. DVCC layer under DSP or FPGA layer. It"s
difficulty to make clear, please contact with me (eagle.zhang@analog.com)
directly. |
[2006-6-14 11:27:20] |
| [问：wyh0009] |
那一款ADC可以达到10M的采样频率 |
| [答：Nie] |
There are a lot of ADCs which can achieve 10MHz sample rate. Please
go to ADI website and have a search according to other requirement. |
[2006-6-14 11:27:30] |
| [问：yingll] |
高速ADC数据输出锁存设计中有什么需要注意的地方？如果将输出端口的数据分路输出同时会带来时钟和数据的复杂关系，如何综合考虑这两者的关系？ |
| [答：ZhangX] |
It"s very importance to understand the data clock phase may need be
later than data, so the clock line normally is longer than data bus. As
you are not sure, please measure on the Eval board before layout.If there
is 2 ch ADC (such as AD9216 or AD9238), you can use 2 data clock. For
LVDS ADC (such as AD9222, AD9219, AD9228, AD9229), only one data clock
for all 8 or 4 ch data stream. It"s very important to use clock block
in FPGA for the data clock. |
[2006-6-14 11:30:54] |
| [问：shen_jerry] |
输入变压器的EMI如何考虑和进行设计? |
| [答：Rob] |
If concerned about EMI effecting the transformers performance, some
manufactures sell high frequency transformers with a metal shield. |
[2006-6-14 11:31:17] |
| [问：luckyfairy] |
就我所知,开关电容输入的阻抗随频率而变化,在这种清情况下,如何设计输入匹配变压器? |
| [答：Rob] |
Match the circuit or front-end at the center frequency of interest around
the total bandwidth required in the application. |
[2006-6-14 11:32:30] |
| [问：shandong] |
ADI有没有输入匹配变压器的计算机设计模拟软件? |
| [答：Nie] |
Not yet. |
[2006-6-14 11:33:50] |
| [问：hongyibu] |
变压器的性能所用的材料有关,对于AD9283的输入变压器,最佳的变压器材料是那种? |
| [答：Rob] |
I don"t know if I can answer this question directly. I don"t deal with
transformer materials themselves as I am not a transformer designer. Manufactures
don"t usually specific these materials either as they are propietary. |
[2006-6-14 11:33:57] |
| [问：windman] |
Can I employ a low noise figure RF amplifer(say
SGA4586) to act as a buffer before ADC in case of input signal frequency
beyond 100MHz in undersampling applications. |
| [答：ZhangJ] |
I am not familiar with SGA4586 . I can"t say yes or no. But for 100MHz
application, you can use a balun or AD8139/8. Thanks! |
[2006-6-14 11:34:19] |
| [问：bloodbird] |
请问AD公司是否有生物医疗专用的AD转换器件:如果有的话，请问它相对其它AD器件，有何独特之处？谢谢 |
| [答：ZhangX] |
For Ultrasoun,d you can use AD8332 VGA + AD9216 dual 10bit ADC. Or use
AD8335 quad VGA + AD9229 quad LVDS ADC. Or use 4pcs AD8332 + 2 pcs AD8335
+ 1pcs AD9222 (8ch LVDS ADC).
For Patient, you can use ADuC7024 or ADuC7027 Microconverter (12bit ADC
is integrated into ARM7 core, on chip Verf, on chip 62k flash). Or you
can use AD7928 12bit serial ADC or AD7398 12bit parallel ADC. You
can use AD7865 16bit serial ADC. If you need muitchallel simulatenous
sampling, then please consider AD7656 (16bit 6ch, AD7657 and AD7658. |
[2006-6-14 11:34:38] |
| [问：xmulrm] |
AD9218是开关电容型ADC，对它的设计有否特殊的要求 |
| [答：Zhao] |
请详细阅读本次讲座的内容，里面的例子就是AD9238开关电容ADC |
[2006-6-14 11:35:20] |
| [问：hongyibu] |
阻抗变换不可避免带来增益上的变化,请问如何处理这一关系?是否要增加另外一级来设定增益? |
| [答：Rob] |
I don"t understand this question?
Gain is not always introduced with impedance matching. No extra gain stage
is necessary. |
[2006-6-14 11:35:27] |
| [问：xuliangyu] |
这里所介绍的输入变压器,是否也适合delta-sigma转换器的输入端? |
| [答：Nie] |
The transformer here is for high speed sampling circuit. It is not the
one for sigma-delta application. |
[2006-6-14 11:36:32] |
| [问：shen_jerry] |
输入匹配变压器对线圈的绕制工艺有何特别的要求?变压器的材料呢? |
| [答：Rob] |
If I understand the question correctly, the answer is no. |
[2006-6-14 11:37:08] |
| [问：hongyibu] |
滤波器和变压器所带来的噪音,特别在高频时如何处理? |
| [答：Zhao] |
变压器为无源器件，不会引入噪声。您可以用差模和共模滤波器来滤除带外的噪声，对于滤波器本身带内电阻热噪声无法去除。 |
[2006-6-14 11:38:37] |
| [问：artin] |
请问如果要构建一个采集(0~200MHz)全频段信号的采样系统,请问前端如何设计?是把运放和耦合变压器组合使用吗? |
| [答：ZhangJ] |
You can use a balun/transformer with a buffered ADC to fulfill it .
Try,AD9480,AD9410,AD9430. But for high resolution ,you will encounter
export license issue. Thanks! |
[2006-6-14 11:39:49] |
| [问：xmulrm] |
ADC建议中模拟地与数字地分开，但是在系统中若仅使用双面板，采取什么样的措施能进行较好的隔离呢 |
| [答：ZhangJ] |
You can use a star form connection for AGND and DGND. Then you can connect
these two GND in one point.For dual layer PCB, you can seperate these
two GND into different area and connect them together in one point. Thanks! |
[2006-6-14 11:41:51] |
| [问：54liugang] |
请介绍ADC系统整体性能和ADC的单独性能之间的关系.根据专家的经验,您认为如何处理这种关系? |
| [答：ZhangX] |
The sampling system"s performance will be mailly decided by ADC, but
noise on input, mismatch, sampling clock jitter, power noise, GND noise
will get it worse. Please pay attention to clock jitter if you f input
is 70M. You need very good clock. Norally a cheaper Crystal has clock
at 10ps. But for 100M input frequency, an ideal ADC with 1ps clock jitter
sampling clock can only get 10bit ENOB. Please note DDS type clock such
as AD9852 has clock jitter about 40pS. ADF436-8 can get clock at about
2.5pS RMS. ADF4001 can build a clock at 1.1ps clock jetter. AD9540 can
build a clock at 1ps clock jetter. AD9510 clock distributor has clock
jetter of 0.3ps. |
[2006-6-14 11:42:29] |
| [问：ecnanjing
_EBY7E] |
ADC的输入电平是有严格规定的，而在高频应用时输入信号往往只有mV甚至uV级，前置电路要将信号放大至v级，中间会引入很多失真，噪音；请专家指教。 |
| [答：Zhao] |
前端射频uV级的信号需要用接收机的前端电路，如LNA放大，带通滤波，变频到中频，然后滤波放大，将有用信号选出后，才能进入ADC。 |
[2006-6-14 11:42:49] |
| [问：xuliangyu] |
加入输入变压器后,如何设计高分辨ADC的PCB?或者说设计时要注意哪些要点? |
| [答：ZhangJ] |
The impendance match is key. For high frequency, you can use strip line
design scheme and reserve ADNG/DGND layer. |
[2006-6-14 11:43:36] |
| [问：smx64098] |
信号调理变压器只是起隔离作用吗？如果采用升压方式，是否放大器就可以不用了？另外变压器的线性度能做 的很高吗？如果不是很好对A/D转换一定有影响吧？ 谢谢 |
| [答：ZhangX] |
Transformer can get higher voltage, but not bigger power. So, if voltage
ger higher, the equilant resistor is bigger, still need driver. A RF transfor
has good linear abover 1MHz. If you need lower, please use AD8137 or AD8352
differential driver. |
[2006-6-14 11:44:28] |
| [问：z9200ne] |
在调试时,主要使用的仪表有那些?能否介绍它的主要性能? |
| [答：ZhangX] |
Please consult instrument manufacture. |
[2006-6-14 11:45:06] |
| [问：xuliangyu] |
如果ADC输入需要和运算放大器接口,是否还需要这种匹配的输入变压器?如果需要,应加在什么地方? |
| [答：Zhao] |
本次讲座给出了ADC驱动的一种方式，另外还有其他的方式，如用高速放大器来搭建差分驱动电路，或者直接采用差分放大器，如AD8138,AD8139来驱动。如果用放大器驱动，可以不用变压器。 |
[2006-6-14 11:46:05] |
| [问：shandong] |
变压器匹配和通常所采用的并联终端匹配或串联终端匹配相比,有何优缺点? |
| [答：ZhangX] |
It"s depends on your structure and the type of the transformer. For
voltage source, it has equillant resitor. For current source, if you use
a parallel resitor, then it conver to voltage source with an inner resistor
of this resistor. |
[2006-6-14 11:47:10] |
| [问：cheng37bin] |
高频变压器能在高带宽范围作adc变换？ |
| [答：Zhao] |
高频变压器的带宽一般都在kHz~1GHz。变压器只是作为ADC的驱动。 |
[2006-6-14 11:47:30] |
| [问：ZSQ521] |
您好:我现在用的一款AD2483是隔离的485通讯 工作电源是隔离的 而参考地必须隔离吗?
应用这款芯片有什么注意的吗? 谢谢!! |
| [答：Nie] |
Yes, the power supply and ground should be both isolated. |
[2006-6-14 11:47:57] |
| [问：lacus] |
如果Sigma-Delta ADC的输入端没有缓冲器,输入端如何处理?可以采用输入变压器进行阻抗匹配? |
| [答：Nie] |
For sigma-delta ADC is generally low frequency application. You just
need to add a buffer in front of the ADC to do impedance matching. |
[2006-6-14 11:48:57] |
| [问：smx64098] |
变压器所带来的相移怎样处理？尤其是对相位要求比较严格的场合。因为每个变压器的相位是不完全一直的，如果对于多路需要同步采样的话，那么相移不同所造成的误差是无法容忍的，所以对于相移要求比较苛刻的情况下，变压器方式合适吗？ |
| [答：ZhangX] |
It"s very difficult to deal with phase. Need use adjustable L or adjustable
CAP to adjust one by one.
If you refer to IF or RF, you can use vector modulcater such as AD8340
and AD8341 to adjust phase (provide you can use AD8302 or other method
to measured the phase). |
[2006-6-14 11:49:22] |
| [问：tillerman] |
如何消除传感器噪音和非线性对测量的影响? 用输入变压器能达到吗? |
| [答：Nie] |
The nonlinearity of your sensor can not be removed by transformer. You
should do calibration and compensation in your system.
For noise, in band noise cannot be removed. For out of band noise, you
can use a filter. |
[2006-6-14 11:50:19] |
| [问：lacus] |
对于ADC来说，采用的不同电源（开关电源，线性电源，电池）对其精度有影响吗? |
| [答：Nie] |
For high performance ADC, we recommend to use either the battery or
the linear power supply for they tend to be cleaner. |
[2006-6-14 11:51:11] |
| [问：tillerman] |
ADC的输入端如何对布电容和输入变压器进行补偿或消除? |
| [答：ZhangJ] |
I don"t understand the question well. But if you refer to the impendance.
I think you can refer to our presentation to calaulate the impendance.
The C shold be combine with L and R for form a total input impedance.
Thanks! |
[2006-6-14 11:51:32] |
| [问：Zhang Zhongping] |
若被采集信号是pA级的话，请问是否需要放大和采用输入变压器?如何保证精度? |
| [答：Nie] |
For pA current sensing, you can consider ADI"s JFET input stage amplifier.
They have very low input bias current. For example, AD549. |
[2006-6-14 11:52:03] |
| [问：z9200ne] |
单端变压器和双端变压器在输入匹配上有何差别?在应用上有何不同的特性? |
| [答：Zhao] |
匹配的原则是一样的，就是共轭匹配。差分信号具有更好的性能，比如能够抑制偶次谐波。增大信号的范围，等等。 |
[2006-6-14 11:53:37] |
| [问：shu_chen] |
请问能不能把双巴仑和单巴仑的选择进行一个总结？ |
| [答：ZhangX] |
简单来说，单巴仑会有从输入端到输出端的寄生耦合且正负端幅度不同，双巴仑可以避免但增加成本。可考虑用AD8317，AD8319， AD8351，
AD8352 差分驱动器。对时钟路的变压器，可以用差分时钟分配器，如AD9510，AD9511, AD9512, AD9513. |
[2006-6-14 11:53:41] |
| [问：faquir] |
输入变压器的动态范围如何考虑? |
| [答：Rob] |
This depends on the performance you require for your application. This
is hard to answer directly. Generally a better balanced transformer will
give better dynamic range performance, particularly at high frequencies.
At lower frequencies, baseband, the dynamic performance limitation is
shown by the ADC"s limitation not the transformer. |
[2006-6-14 11:55:03] |
| [问：dinghou
gan0312] |
怎样取得联系点
致
顺颂商祺 |
| [答：Nie] |
You can contact ADI 800-810-1742 or send email to china.support@analog.com
to get technical support. |
[2006-6-14 11:55:11] |
| [问：Zhang Zhongping] |
如何选择器件，使放大器和ADC相匹配，使信噪比最高？ |
| [答：ZhangJ] |
This is a big question and I am afraid that I can"t describe it more
detailed . For AMP selection, you should pay attention to the GBP should
be 10 times than ADC and the SNR should be better than ADC. The offset
of the AMP should be small the 1LSB and others ....... |
[2006-6-14 11:55:41] |
| [问：faquir] |
演示中主要介绍了输入变压器的次级设计,初级和传感器输出的匹配设计如何考虑? |
| [答：ZhangX] |
It depends on your sensor type. If hign internal impedance sensor, need
use hign input amplifier (very low bias). |
[2006-6-14 11:55:43] |
| [问：shu_chen] |
如果我们把变压器的输入端两极反过来用，会有什么变化和影响？ |
| [答：Nie] |
It depends on the product. Please refer to the transformer datasheet
for details. |
[2006-6-14 11:56:20] |
| [问：cheng37bin] |
如想用20位以上adc做皮电或脑电前端怎样处理较好 |
| [答：ZhangX] |
You will need a good instrumental amplifier such as AD8221 to remove
common voltage. You can use AD7738 24bit multichannel ADC. AD7760 20bit
2.5M ADC is good but need export license. |
[2006-6-14 11:58:05] |
| [问：zhangwq] |
是否有WM8731互换芯片? |
| [答：Nie] |
Please call 800-810-1742 to get the replacement. |
[2006-6-14 11:58:53] |
| [主持人：ChinaECNet] |
所有问题均已提交给ADI公司的专家。座谈期间未回答的问题，ADI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。 |
[2006-6-14 12:03:11] |
| [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ADI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ADI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。 |
[2006-6-14 12:03:29] |
| [问：Zhang Zhongping] |
输入变压器是否会对取样保持电路有影响?取样保持电路应如何进行计算? |
| [答：Zhao] |
输入变压器提供对输入的匹配，达到最大功率传输，减小驻波比。S/H电路在高速ADC内集成。无需计算。 |
[2006-6-14 12:05:12] |
| [问：lacus] |
如果我要实现＋20dBm到-74dBm的信号采样，在前端信号的调理部分应当采用什么样的方式来实现自动的动态范围调整呢？ |
| [答：ZhangX] |
I do not know your work frequency.
It"s better to use an LOG amplifer, such AD8304. Or use log dector such
as AD8313 / AD8314/ AD8317/AD8318/AD8319, or use VGA (such ad ADL5330,
AD8368). You can use AD8364 or AD8302 + ADL5330 to build a
AGC loop to make stable output, then it"s easy for ADC. |
[2006-6-14 12:05:20] |
| [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ADI公司，特别感谢专门在线回答各位听众（网友）提问的ADI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。 |
[2006-6-14 12:07:49] |
| [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。 |
[2006-6-14 12:07:58] |
| 非在线问答: |
| [问：] |
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：] |
Use differential drive such as AD8137 or AD8352 is a good
way to avoid common noise from ground plane. Use transformer if input
frequency is too high (100MHz).
Carefully match differential trace on PCB board is also very important.
(For example, 6mil thickness from top to GND layer, in order to match
to 50 Ohm, use 8mil trace width, 8mil distance between 2 differential
trace, 24mil distance from other trace.
Good GND plane, good power decoupler, good component displacement will
always be very helpful to mimimize noise. |
| [问：] |
变压器设计中，有没有软件来帮助设计呢？谢谢 |
| [答：] |
No. |
| [问：] |
单端输入和双端输入ADC各有什么有缺点？ 怎样选择？ |
| [答：] |
Single ended ADC is simple .But the differential ended input
ADC is better for CMRR performance . If the frequency is high and you
want to get more CMRR/SNR ,you should select differential input ADC. Thanks! |
| [问：] |
高速ADC中一般都包括一个S/H电路，是不是这个电路对ADC性能的影响很大？怎么提高其性能？比如说减小开关的电荷注入效应等等。 |
| [答：] |
内部的S/H决定了ADC的模拟带宽。如果这部分性能不够好，会严重恶化ADC的SNR，尤其是在采样速度较高的应用中。对用户来说，S/H无法更改，您需要做的就是做好模拟前端的匹配，可能的情况下可以用两级变压器，如该讲座中提到的。 |
| [问：] |
高速或高频ADC在PCB板上布局时需要注意什么？ |
| [答：] |
Smart partitioning. Meaning, keep analog components on the
analog side or domain and digital components on the digital side or domain.
Do NOT mix the two, even if on different layers of a pcb. This creates
a chance for coupling back onto the analog input where it is most sensitive. |
| [问：] |
请问如何如果输入信号电压范围变动很大(几mV到几V),请问前端如何设计自动增益调整电路 |
| [答：] |
It depends on the signal frequency. We have many VGA parts.
AD8367, for example, can implement AGC circuit. |
| [问：] |
请问使用AD7656的时候要注意些什么？在我的设计中，CONVSTA,CONVSTB,CONVSTC可以连接在一起控制吗？也就是说一个CONVST信号控制六路？谢谢 |
| [答：] |
Yes,you can connect CONVSTA, CONCSTB and CONVSTC together.
The most importance thing in AD7656 application is AVCC decoupler. Another
is GND layer. For example, if you use 3pcs OP2177ARZ as input buffer,
you will has +/-12V power supply for OP2177, also +/-12V power supply
for AD7656. Then It"s very difficult for Power layer because there
are also 5V AVCC and 5V DVCC. So, fist of all, use 4 layer PCB, TOP, GND
layer, Power Layer, Bottom. GND layer cut into 2 pcs, AGND and DGND, it"s
connect together under AD7656 or some where near AD7656 (please make sure
the power supply"s GND is not connect together). Then, use every
0.1uF decoupler capacitor very near to each AVCC pin, use 10uF decouple
capacitor on each AVCC pin. In order to connect total 8 AVCC pin, you
need use a whole AVCC area under AD7656 and near AD7656. Then, you need
lay +12V and -12V trace (>1mm width) on the bottom layer. You can still
use +12V and -12V in power layer for the area of OP2177 place. DVCC layer
under DSP or FPGA layer. It"s difficulty to make clear, please contact
with me (eagle.zhang@analog.com) directly. |
| [问：] |
在演讲中R和L的值从最初的422Ohm和220nH,变化为最后的620Ohm和120nH，是如何不断调整的，谢谢！ |
| [答：] |
It is done by experiment. Try different L and R and calculate
the SNR, SFDR and THD. |
| [问：] |
如何保证ADC的精度?对于单电源供电,零点的确定和量程都与电源电压有关,如果电源电压波动势必导致转换的误差,特别对小信号的采集电路中如何解决? |
| [答：] |
For ADC accuracy, it depends on its INL/DNL/REF/CLOCK/PSRR
etc. For single power supply, I think the accuracy releated much to the
REF voltage while the power supply. If power supply change, the output
will change too. But the change is very little. You can refer to PSRR
parameter in our datasheet to calculate this change. |
| [问：] |
如何对输入变压的振荡尖峰进行抑制?效果如何? |
| [答：] |
If there is ringing at the transformer. Use a termination
resistor in parallel or series. The
value of the resistor and combination may vary per application. |
| [问：] |
输入阻抗的匹配除了变压器外,还有什么方法? |
| [答：] |
If you understand the transformer and its parasitics, then
you can match your input impedance best. Making the input as REAL as possible
will give you the best match and the most bandwidth on the front-end design
to the ADC. |
| [问：] |
由于频率特性的影响,输入阻抗的匹配要达到多大的精度? |
| [答：] |
The impendance match is very important for high frequency
circuit. If mismatched, it will
result in ringing and reflecting issue. The calculation is difficult.
But you can use the
reflecting ratio and reflecting loss to calculate the noise caused be
mismatch. I think 1%--
3% should be better. |
| [问：] |
在16位的AD转换器件中，如何合理地选择其模拟输入驱动运放,该运放放在什么位置较为合适? |
| [答：] |
For 16 bit ADC, I recommend you selecting AD8007,AD8009
or AD8139 to match and drive the ADC. If you are using high speed ADC
with IF sampling, you can also use a transformer to interface with your
ADC. The bandwidth should be 10 times more than ADC and the SNR should
also be better than ADC. |
| [问：] |
怎样才能得到更详细的资料 |
| [答：] |
Please call 800-810-1742 ask more material. |
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