Selection for DAQ card

[Sacl]

Hi:

 Thanks for the reply.  Now I start using the PCI-6251 card for input&output function, and one of biggest problem is that I need to have a very low noise resolution.  Therefore,

I would like to: Is there any reference telling people how to eliminate noise from environment/wires/etc.  (Besides adding a filter) when measuring using DAQ card?  Do NI sell shielded BNC cable?  

Also, could you please tell me what's the diameter of the screw in CB-68LP card?  Thank you very much.

Jason

[Neal M]

Hello Jason,

Since your PCI-6251has an ADC with 16 bits of resolution and the application you have been talking about seems very sensitive, I would imagine noise in your signal is very undesirable.  From your question about sheilded cabling, you may already know a significant amout about noise reduction, but we do have a very good Developer Zone Article titled Field Wiring and Noise Considerations for Analog Signals which goes over how to reduce noise in your signal.  The highlights of the article are basically that differential measurements, short shielded cables, sampling rate, and filtering can help reduce your noise.  It also goes over grounding and how to wire that up.

To answer your second question, yes NI does sell shielded BNC cables (Part Number 779697-02). There is no product page for it, but they are listed under accessories for most of our devices that use them (e.g. our DSA cards).  As for theCB-68LP,  I grabbed one and measured the screws, and they were 0.34".  The wire opening is 0.2". If you are worried about noise in your signal, then you might want to consider the SCB-68.  This connector block is shielded and would help reduce environmental noise. It also has the same size screws.

Just as a note, if you go to www.ni.com/contact you can contact a technical representative whos job it is to spec out systems and help you choose the best setup for your application.  They are a very good resource.  They also will be able to give you a quote of everything you need (as well as tell you whether we have specific cables/etc).

[Sacl]

Hi:

  I have a question about reversing the sign of input voltage correspond to a variant output voltage value.  So my experimental setup is:

  I applied a constant positive input voltage, say 2V, to a device consists of electrolytes, and measure the output current.  Besides, I have converted this output current to an equivalent voltage value, so in other words, I am measuring an output voltage value correspond to a constant input voltage. 

    Also, my device's equivalent resistance would change with time,  and I plan to  "inverse" the constant voltage input , say 2V to negative input -2V, when measured output voltage drop/increase to some value. 

Could you please tell me how to implement this process using Labview's GUI? Or releveant references so that I will have an idea where to look up for this specific application?   Thank you very much.

Jason

[Chris_D]

I put together a simple example using the DAQ Assistant to output a value based on a certain input value. Hopefully, this will be a good starting point for what you are trying to do. I saved the VI for LabVIEW 8.0 or greater.

[Sacl]

Hi:

  My application is that I need to First generate an analog output(voltage),  and then based on the value of the signal I measured, I would update/change the output voltage.    However, the example above doesn't start generating the output voltage until after DAQ measured the signal.    So with my application, could you please give me some examples?  Thank you very much.

Jason

[Sacl]

Hi:

The attached file is the one I wrote for my applicaiton.  So what my code did is that i generate a voltage using DAQ assistant, at the same time measuring the corresponding value from using another DAQ assistant.  When the value goes has some kind of pulse,i.e., goes over value of 4 and goes back below the value of 4, I Reverse the bias of the output voltage value. 

  My biggest question is that: Why is the indicator at left, named" voltage indicator", attached to the static variable "input voltage control",  cannot be changed correspoinding to the static variable?? 

  Since this is the way I tried in order to change the output according to input, could you please tell me how to change it, so that the input control to the DAQ assistant(generating the voltage) can be changed accordingly?  Thank you very much.

Jason

[Chris_D]

I am a little confused at what you mean when you say, "cannot be changed corresponding to the static variable?" and what exactly you are trying to accomplish. If you are trying to change the variable to be able to input a value to the variable or output a value to the variable you can right-click the variable and select Change To Read or Change To Write.