Multifunction DAQ

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Differential mode operation

Hi AnalogKid:

I have not heard from you. What is happening? Could you please email me back.

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Message 21 of 29
Maybe you wore him out.
John Weeks

WaveMetrics, Inc.
Phone (503) 620-3001
Fax (503) 620-6754
Message 22 of 29

Hi AnalogKid and John:


In your previous mail, you said "

2. Since you show Terminal 0 at some ground connection, the current flow through the resistor will be: (Vs(0)-Vaignd)/10,000. If Vs(0)=Vaignd, then no current flows. "


Are you talking about the current flowing through 10K bias resistor here.

If Vs(0) = Vaignd, that mean there is no bias resistor, what do you mean by no current flow? How about the Rin Lum sum resistor. Lum sum resistor can cause voltage drop. Could you please explain why there is no current flow? I don't understand.


You said you need to connection to make a measurement. AI+ and AI- is like a DMM red input lead and black input lead. That mean AI- input is actually the return path of the AI+, am I correct by your meaning to understand?

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Message 23 of 29

As you know, DMM is actually a parallel circuit that measure the voltage Vs. There is only small amount of current going through the galvometer. How about here I don't understand why you said there is no current going through Rin Lum Sum resistor. Could you please explain? There is also a ground AIGND in differential mode configuartion. You said "Bias resistors are not used to reduce noise, they are used to drain away residual charge that build up on the high input impedance front end of the amplifier so multichannel scanning yields accurate results." The high input impendance front end of the amplifer is the bias resistor 10K to 100K ohm resistor, am my understanding correct? I need to verify with you here.



You said "

7. If you get the same results from the two methods then they are equivalent for your setup and needs."


Did you mean if you set up a resistor out from your current source in the test fixture and you use DMM to measure the voltage that is 0.6V Then you put AI+ and AIGND for RSE to measure it on the MAX which 0.6V and then you measure it using the method of AI- to AIGND with 10K bias resistor. You got 0.6V.

Since you first get 0.6V without setup any configuration, you can use this value to judge your measuremnt value, am I correct? That is very important to me here. You college said I can use only use AI- adn AIGND as long as the result is correct.
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Message 24 of 29

Then, there is no wire connected between AI+ and AIGND, I can not see how the current can go through the AI+ to the bias resistor and then to AIGND then go to another bias resistor to AIGND to form a complete path. Could you please tell me? 


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Message 25 of 29

Hi AnalogKid:


I just call Mr. Weeks. His collegue said he will be back on Thursday. I am in an urgent. Would you please solve my question if you can regrading the result I got from differential mode method. This is my main important question. I would like to see if I got my result correctly using DMM to measure without connecting to NI boards and then measuring it using dfferential method. Do you think this is correct?



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Message 26 of 29

Hi Analogkid:

I wish you could reply to me.

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Message 27 of 29

Hi AnalogKid:


I don't know what is happening with you. Could you please write me back? If you can not answer, it is OK. Nobody knows everthing. But if you can answer some to me . At least, I can solve some questions.

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Message 28 of 29

Hi AnalogKid:


I have not received your email. I don't know what is going on with you. I think if I don't receive your email, it is OK. But I would like to ask someone.

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Message 29 of 29