Virtual Impedance Analyzer

[rutgers]

Hi

I am new at labview and i wanted to find out if i could conduct a frequency sweep  with an excitation voltage to create a sort of impedance analyzer. I have a BNC 2120 connected to a 6251 daq card. I plan to connect the circuit and measure Impedance using the I V Method. I assume that if I can supply and input voltage with a range of fequencies, take the output voltage measurement across a low value resistor connect in series to the DUT( Device Under Test), and calculate the impedance for each of the frequency values, I can get the impedance spectra. It would be a great help if someone could tell me whether my reasoning is sound and how to go about writing the program.
Thanks for the help in advance.

I have attached an image of the method i am trying to use

Abhishek
Graduate Student
Rutgers


Message Edited by rutgers on 12-10-2007 11:59 AM

[johnsold]

Abhishek,

Yes, that method will work. Often it is better to put the resistor in the return line rather than on the high side. Then the measurement common point is the junction of the resistor and the impedance being measured. This does require a floating source.

Many low cost DAQ devices do not sample all channels simultaneously. If the device you use is one of these, you may need to compensate for the time delay/phase shift between the voltage and current channels.

Lynn

[rutgers]

HI John

Thanks for the help. I will try connecting the circuit as you mentioned. However, I also want to know how to generate the input signal using labview , ie using different frequecies. I haven't coded in labview before. I have seen the examples under Harware input and output>daqmx>analog generation>voltage......and i feel i can change them to my needs but i can't figure out how to.


Regards
Abhishek

[johnsold]

Abhishek,

First, spend a few hours working through the tutorials available to help LabVIEW beginners. The time spent will be saved many times over compared to struggling to learn everything while trying to put together a non-trivial program.

Second, look at the examples and design patterns which ship with LV. Many of the pieces of things you will need to do are already there.

Third, do not expect someone on the Forum to do your project for you. Start working on the parts you understand and then post specific questions about the parts you do not understand.

Fourth. Design before you build or code. Start with a block diagram of the system. Put in numerical specifications such as minimum and maximum frequencies, magnitude of excitation signals, range of impedances to be measured, sampling rates and resolutions available for the DAQ device(s) to be used. Also get non-numerical specifications for things like what parameters will the user be allowed to change? How will the results be displayed or saved? You will need all this information to plan a system that works well.

Fifth. Then start writing LV code.

Lynn

[EBL]

Abhishek,

You may want to sift through this previous discussion:

http://forums.ni.com/ni/board/message?board.id=250&thread.id=32510

I posted instructions for making an impedance analyzer with the 4461 Dynamic Signal Acquisition card. LabVIEW code and documentation for the fixture are contained within the thread.

The architecture most likely will not carry over completely to the 6251, but perhaps you can get some ideas from the material.

Cheers,
Ed

[rutgers]

Thanks for the help EBL and John. I will work on the ideas and come back if I have any problems.

I have one problem. The vi's that have been posted in the other forum are for labview 8.2 and i have labview 8. Is there anyway you can downgrade to labview 8 and reattach them here.

Regards
Abhishek

[jaced]

Hi,

I have saved these vi's back for you. I hope this helps!! 

[EBL]

Thanks, JaceD - you saved me the trouble. I was hoping to get a chance today to save those back.

Abhishek, I hope the VIs help you out. They work just great on the 4461, but they may not translate well to your board. No doubt you'll run into issues if and when you try to convert them, but it never hurts to try.

Ed

[Brian_Beal]

I did this several years ago using a 2-channel scope card and a function generator.  In comparing readings to those taken with an HP4194a, the shape of the sweep was the same, its just that the absolute impedance values were shifted.