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[nyborn]

Hi DFGray:

I followed what you said. I put 116ohm resistors at R1. The offset voltage has been back to 0V which is good. But I still don't get any

voltage (0 volt) on 6250. I don't understand why. Let me tell you the information abut my device under test. I think it will help.

My device under test has a current source which is 0.000034A that I already measure in DMM and Labview using RSE mode.

The current source is in the test fixture which I have attached my diagram to you last time. The test fixture is a floating source

which I have proved previouly using DIFF mode and NRSE mode. There is also a BNC scope ouptut to 5112. Attached is my diagram

that show if I have correctly setup the hardware. Would you pls help me to verify if I set up correctly? The problem right

now is I got 0 volt on 6250. I suppose to get some voltages and use ohm law to get the value of 0.000034volts. Did I do anything wrong?

Power supply has only two plug without the earth.

[DFGray]

If you did what the drawing says, you made a serious mistake and 0V is what you would expect.  The 6250 should be disconnected from the original location (which should be left open, with no resistor, as long as the 5112 is connected).  The 6250 should then be connected to the new resistor (see attached diagram).  In your diagram, the inputs to the 6250 are connected to the same location, the ground end of the new resistor, giving you zero volts.

[nyborn]

Dear DFGray:

I follow what your diagram said. I don't put any resistor between AI+ and AI-. I put 100ohm resistor in R1 position. I took the positive lead AI+ to the one end of the resistor and the negavtive lead AI- to the AIGND of NI 5112 as your diagram siad. I got 0.004 V which is correct value if I take this value divided by 100ohm. I will get 0.000034A. That is my expected current. Then, I tried 18kohm and 33.8kohm. I also got correct results. But I tried 100kohm for R1, then I got 2.2V which is not the correct result by calculation of ohm law for AI+ and AI- right? Is that because it will be an offset if you put larger resistor in R1. Could you verify what I did please? I am not sure. As you know, I need to make sure in order to keep on going. Otherwise it will be stuck. The AI+ and AI- is must be differential mode, right? there is no need to put bias resistor from AI- to AIGND in this case, why? If you follow the diagram of the configurator of differentila mode in floating source, you will see a bias reistor connedted, why is this case without bias resistor? Is my hardware setting correctly? Could you pls verify me? I hope you can give me a direct answer.

By the way, if the above setup is everything correct. I tried putting digital I/O into the CB-68LP to measure control the button on my test fixture. I used 0,1,2,3,4 for my digital ouput. Attached is my diagram. The problem is when I disconnected the AI+ and AI-, I can use digital I/O to control my test fixture. But when I put back my AI+ and AI-, the digital I/O no 4 could not work, but when i put DGND connected to AI- end of R1. The no4 output works. But it was not working suppose the way it was without connecting AI+ and AI-. It was working the opposite way. Without the connection of AI+ and AI-, the no 4 is working the way as the no 3 check box needed to uncheck. With the connection of AI+ and AI-, the no4 output  is working with the no 3 check box needed to check. I don't understand why?Is it because of the ground.

[nyborn]

Here is also the attached diagram for reference

[DFGray]

First, you should disconnect the AI+ and AI- of the 6250 from the old location of R1.  It is not a real problem, but it could cause you noise and ground loop issues.  AI+ and AI- should only be connected to the new location of R1.

You are getting the expected behavior when changing your resistor to higher values.  Resistors limit current.  The higher your value, the less current (your voltage is fixed).  That is why I told you to use the smallest resistor you could.  Also, make sure the input range on the 6250 is set to the lowest possible setting so you get high resolution answers.  As your resistor starts to approach the value of the internal resistance of your device under test, its effect becomes more obvious.  By changing the resistor and carefully measuring the currents, you can easily find the internal reisistance of the device under test, knowing the supply voltage.

The 6250 can be used in either differential or referenced single ended mode in this setup.  If you use AI+ and AI- (two analog inputs), you need to use differential mode.  In differential mode, you don't need a bias resistor because there is no bias.  One of the terminals is at ground.  To use referenced single ended mode, attach AI+ to the device side of the resistor and analog ground to the 5112 side.

Your problems with the digital inputs is probably ground related.  Look at your circuit carefully and make sure you are not connecting grounds into signal leads and signal leads into grounds.

Finally, you really need to find a local electronics guru who can help you.  This entire problem could have been solved in about 30 minutes had I been with you at your lab bench.  We have now spent over a week.  Failing that, become an electronics expert yourself.  Take a course or find a good book on electronics, read it, and, more importantly, create the circuits.  There is no substitute for actual lab experience.  You may know the facts, but if you can't apply them, in my opinion, they are useless (I am biased, since I was an experimental industrial physicist before joining NI).

Good Luck!

[nyborn]

Dear DFGray:

Thanks for your great help. I tried what you told me, but I still have sth I am not sure if my hardware setup correctly. Could you pls help me to verify.

Attached is my diagram. I took a postive lead of my current source which is an "open" word in the diagram to AI+ of the CB68 board and then took the positive lead to the R1 of the device side. After that, I took the negative lead of my current source to AI- of the CB68 board and then took the negative lead to the R1 of the NI5112 ground. Is this the correct setting?

In your second paragraph of your previous mail, do you mean even though my current source is 0.000034ma using DMM to measure, it will still affect by the resistor changing in R1 because resistor limit current. The higher the reistor, the lower the current. That is why I put 100kohm, the current source become less current, is that right?

In your third paragraph, I tried RSE and DIFF mode. I got the same result. But I don't understand why there is no bias resitor in diff mode because my test fixture is a floating source according the RSE mode. Could you please explain to me? because I am suppose to follow the config of RSE mode according to NI ground tutorial. That means the source is a floating source. But you said I should have no bias resitor in Diff mode. I am confuse here.

In the fourth paragraph,  you said make sure I am not connecting grounds into signal leads and signal lead to ground. Attached is another diagram that shows the I put the wires in the orgianl dragram plus the digital I/O lines to CB68. Is this correct?

[nyborn]

DGND graph for verifying

[nyborn]

Dear DFGRAY:

I am sorry. Attached is a better graph to describe my DGND question.

Is the first picture for DGND hardware setup correctly?

If the hardware setup correct, how come the way it setup with AI+ and AI- connected to CB68-LP will need to turn no3 digital I/O unchecked, but without AI+ and AI- connected from the test fixture to CB-68LP board, the no3 digital I/O is checed?

[DFGray]

AI+ and AI- should not be connected to the location labelled "open". They should only be connected to the ends of R1.

Ohms law states that the current (I) flowing through a device equals the voltage (V) divided by the resistance (R) or I=V/R. So, if you increase the resistance, you will decrease the current.

The purpose of the bias resistor in differential mode is to allow both inputs a path to ground so that charge does not build up on them, destroying the measurement. In your case, the AI- is already at ground, so you don't need the resistor.

In looking at your original configuration, I noticed that the digital signals on the 6250 are configured as outputs. This implies that you are controlling the system with the 6250 and you should disconnect your pushbuttons. If you are trying to monitor the system, the digital signals should be set as inputs.