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Differential mode operation

Hi Analogkid:

1) I am sorry. I did not show a connection to anything on the bottom side of the bias resistor. I don't know what I can put because I know the config as the graph I attatced. Would you please tell me what else I can put on the bottom side of the bias resistor.

2) Once you answer the question 1. Would you please tell me how the current flows,

3) I have also attached anther graph, I would like to ask Vs(0) means test fixture ground. Gain and 1st Pole(0) and Output buffer (0), RC(0) are refer to AIGND.

4) I read the specifification, it said the input range of AI+ and AIGND is +/- 10V , is that right? In fact, when you have two input AI+ and AI- from my test fixture, as you know my test fixture is orginally a current source to ground, now I put a 100ohm resistor across AI+ and AI-. The input of voltage should not mean two inputs. It should means like a DMM, you have red lead as a input and black lead to the ground. Is this the actual meaning? Could you please verify me?

Here is a link

http://zone.ni.com/devzone/conceptd.nsf/webmain/177A8B29FEDC0F5886256FA90083C0F8

Figure 4 Differential Measurement System

5) I would like to know if the red and black lead cable contain noise because BNC does not due to the coaxial shield in the outter. How about the lead cable? If they contain noise, the method that you handle is to use bias resistors to remove the noise in CB68LP and 6250. The capacitor in Gain and first Pole can remove noise. If you can, would you please give direct answer to me? RSE mode can not remove noise right? only diff mode, right?

 

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6) I would like to ask the setup of differential mode config. Attached is my graph, I am not very sure wether I don't need to put a wire from AI+ to AIGND if my measuremnet don't need a bias resistor from AI+ and AIGND to get the correct result.  I would like to make sure with you I remeber I don't need to put any wire from AI+ to AIGND with no bias resistor, am I correct?

 

7) I think that the result I measure from RSE config and DIfferentiail config are the same. It is just the different method. The result on MAX are the same. am I correct here?

 

 

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Message 12 of 29
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Hi AnalogKid:

I hope you can give me a good answer, especially the result of measurement in Differential mode and RSE mode. I am not quite sure if I am thinking it right? My company urges me. Nowsaday, it is not to get a job. Could you please help me out? Don't worry. I will take care of the small problem.

 

Regards,

John

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Message 13 of 29
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Reborn:
 
1. Bias resistors typically go to AIGND. You use them in pairs as in your pic1.gif image for differential input mode.
 
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.
 
3. I did not draw the schematic, so I do not know exactly what the ground connected to Vs(0) refers to. It is notated with a 0, so I would presume it is the same ground as the other nodes that are marked 0. If you connect your test fixture ground there, then that is what it is.
 
4. If the specification states +/-10V, then that it what it is. You need two connections to make a voltage measurment, just like a DMM. But black lead of a DMM does not always have to connect to ground, it depends on what you are trying to measure. For example you can measure a 9V battery with a DMM- there is NO ground involved here yet you still can read the voltage.
 
5. The noise in your leads depends on the electrical environment, which I  have no idea as to what level it might be. 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. Only differential mode can be used to remove common mode noise.
 
6. Use two bias resistors, see your pic1.gif instructions.
 
7. If you get the same results from the two methods then they are equivalent for your setup and needs.
 
You seem to be stuck on this RSE versus DIFF issue. Just follow the instructions in your manual, wire it up, try it out, and don't let the smoke out. Sounds like you are getting the same results from both. If they both match your DMM reading then pick the one you deem best suited for your needs.
 
Good Luck
 
 
S/N = -6dB?

Message Edited by AnalogKid2DigitalMan on 12-13-2005 10:27 AM

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"It’s the questions that drive us.”
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Message 14 of 29
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Hi AnalogKid:

 

1) I would like to make sure the output voltage of RSE and Diff config. The RSE output from Vout to AIGND is the voltage difference of the input voltage from AI+ to AIGND. The Diff config output voltage from Vout to AIGND is the voltage difference across AI+ and AI-, am I correct?

2) So, the current flow through in Diff config is depends on the bias resistors, as far as my understanding of your previous mail. The Rin (lum sum of the resistors, remeber you mentioned in the previous mail). What is it actually? Could you please give good answer here? Is it the bias resisitor.

 

3) According to you previous mail, it seems the current will not occur if AIGND is the same as AI-. It seems you are telling me that only the bias resistor can cause current to go through from AI+ to AI- and then go to AIGND through bias resistor, then how can the point at AI- current can go back to the test fixture ground, the circuit need a completer path. Would  you please explain to me here? What is your perspective?

4) I heard someone told me that I can use one bias resistor from AI- and AIGND if it give you the right result. The result I use with one bias resistor is the same as RSE mode. This is correct, right? Could you please tell me? She said that if it is not giving you the result, you then use two pair of resistors.

I tried it, I found when I use a pair of bias resistor, the value drop to 0.32V, not 0.6V which is the same as RSE config.

 

 

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

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Would you please give a confirm answer? I really need to solve this problem
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4)  In my previous mail graph,  I would like to ask  Gain and 1st Pole(0) and Output buffer (0), RC(0) are refer to AIGND?

 

 

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The black text was what you wrote. I still have a question.

 

 

5 If the specification states +/-10V, then that it what it is. You need two connections to make a voltage measurment, just like a DMM. But black lead of a DMM does not always have to connect to ground, it depends on what you are trying to measure. For example you can measure a 9V battery with a DMM- there is NO ground involved here yet you still can read the voltage.

 

OK, I know the specification states +/-10V.

The second problem is two inputs to differential configuration. It sounds like I also apply the same concept when doing voltage measurement, just like DMM. Am I correct here? If so, could you please tell me how this differential configuration is like DMM? Would you please tell me?

 

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6) I would like to know if the red and black lead cable contain noise because BNC does not due to the coaxial shield in the outter. How about the lead cable? If they contain noise,  the method that you handle is to use bias resistors to remove the noise in CB68LP and 6250. The capacitor in Gain and first Pole can remove noise. If you can, would you please give direct answer to me? RSE mode can not remove noise right? only diff mode, right?

The noise in your leads depends on the electrical environment, which I  have no idea as to what level it might be. 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. Only differential mode can be used to remove common mode noise.

 

The noise in my leads depends on the electrical environemnt, could you please give me some example here? I think I will understand. By the way, I put red and black lead out of my test fixutre to output current. The red lead and black lead then go to the CB68LP and then go to 6250 board in the computer. What I mean is if the lead cables in this location has noise or not? If so, how can you remove it.

Could you please tell me . The capacitor in Gain and first Pole can remove noise in the graph which I attached in the previous mail.

 

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. But, could you please tell me what you are talking about the high imput impendance front end of the amplifer? I don't understand here. Only differential mode can be used to remove common mode noise.

 

What other things can remove the noise here in this lead cable case in CB68LP, 6250? Would you please tell me?

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😎 I know the concept of differential measurement. The concept is talking about the voltage difference across two points Let say the Vs is the voltage you are going to measure. I am actually measuring the voltage difference across the Vs two points. So, one of your staff told me to take ohm law I= V/R  to get the current value of the resistor across two point. It is using only one wire from AI- to AIGND.

In fact, the concept is like, then I can prove that if I use one wire from AI- to AIGND . I can get the right voltage value and current value using ohm law. So I think it is correct? Do you think so? I tried using two wires from AI- to AIGND and AI+ to AIGND. The voltage I got and then divided by the resistor value , it is not my current value, so one of your staff said that the positive side is only when you don't get the right value of the negative side. Do you think I am right here?

So you think I am right? I don't think I need to put any wire from AI+ to AIGND because I already got the right value. Do you think I am right here too?

The reason I ask is because I could not find these person anymore. That is why I ask you.

By the way, attached is my graph. I would like to ask you that if I add a BNC cable under the test fixture. It will be different ground. One is my fixture ground, another is my BNC NI5112 ground. I used DMM. It did not go beep. I think I am right, right? But if I connected AI+ and AI- not connected to NI5112 board,  to 6250 board first, then I used DMM to measure the voltage across R1 as the attached graph, the current will go through. Is the because there is some current going through to the AIGND so now, the test fixture current can go from test fixture to NI5112, even BNC is a hard ground? I don't know if you understand it. If not, please tell me

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