SCOPE Soft Front Panel

[wang]

Hi DFGray:

I am sorry I fogot to tell you I am  attempting to measure current with the 6250.  I already know 6250 is measured voltage, not current. Yes, I can put a resistor across AI+ and AIGND to measure the voltage across my current meter in my diagram.  I am also setting a +/-10V  range of the 6250 and a small resistor using 460okm. I am measuing in RSE mode with the positive side to the red binding post, the negative side to the black.

In your previous mail, you said "Adjust the 6250 gain and resistor value as necessary to get good readings". How am I suppose to adjust the 6250 gain because I am only using a CB-68LP board, 6250, NI 5112 and a test fixture. Could you tell me?

[wang]

Hi DFGRAY:

I am sorry. This is connected with the above passage.

Then I set everthing up. I put 460kohm across AI+ and AIGND in RSE mode and digital lines to CB-68LP board. I could get my correct value of 0.016V. But, the problem is when I plug in my BNC cable into the test fixture. The 0.016V will go to 0V. Why? I know BNC has a hard ground on the connector. How am suppose to solve this problem with digital I/O line this time?I tried using differential mode with digital line. It did not work. The result was the same thing due to the BNC cable plug in.I would like to tell the real thing happening right now. I hope you could give me some suggestions. Thanks!

[DFGray]

Gain and range refer to about the same thing.  Gain refers to the mulitplier applied to the input voltage before it is converted by the ADC on the analog acquisition board.  You set the gain when you set the max and min input values (you also set the offset).  If you set the input to +/-10V, you are setting the gain.  Note that the range should always be set to the lowest possible value.  If you data does not get higher than +/- 0.5V, then you should set your gain there.  Setting the gain too high results in a significant loss of resolution and a lot of added noise.

Note that the DAQ and DAQmx use the max-min model to set gain and offset.  NI-SCOPE uses the range-offset model.  They are linearly equivalent.

[DFGray]

At a guess, I would say that you have an alternate ground path through your device under test.  The current that would normally flow through the sensing resistor is now flowing directly to the scope ground.  You could try putting your sensing resistor in this path and see if that helps.  You run the risk of disturbing your scope measurement, however, since the ground path is no longer zero impedance.

[nyborn]

Hi DFGray:

After I have recived your previous mail, I already for sure that I have an alternate groud path. So , I tried putting a rsistor of 440ohm across

the BNC cable with positive lead and netgative lead end that I made from the output of my current source 0.000034A and then put another

BNC cable with positive lead and negative lead end to the input of NI 5112. I looked at the test panel in RSE mode with an 100kohm resistor

across AI+ and AIGND. It dd not work. The result still stay in 0V. Am I not doing thing corretly? Would  you pls let me know, Attached is my

digram of the connection to the BNC output. Then I put one more bias resistor with 18kohm for differentila mode. The result is a negative

number. It did not work.

There is one thing I found and need to mentioned to you. When I set everything up in RSE mode. The BNC cable has no resistor connected

to it. Although I got 0V on the test panel. But, after I press the "beep button", there is a voltage appeared of 0.05V between 0.1 and 0V range.

But I used a multimeter to measure the resisor across the 100Kohm, the reading was a negative number, not 0,05V on the test panel.

What do you think about this problem? I also tried change my computer power cord from earth ground into without earth ground (two plug)

It did not work. The result was 0 volts in the test panel. The reason I tried is because when I used DMM, I could read correct value in DMM

and oscilloscope. That was why I tried. How come I put everything in terminal block 68-Lp and 6250. It did not work like DMM.

I hope you can have better idea how to slove this issue

[nyborn]

Hi DFGRAY:

I have not heard form you in two days. I don't know why?

I tried to think what you said in the previous mail. Here is what you said.

"My guess at that point would be you have the 6250 in differential mode.  The 5112 is always referenced single-ended (it has a hard ground) and the ground is pulling your signal down.  Good luck."

"In this case, you can use the 6250 in either RSE or differential mode.  If in RSE, make sure the ground of the 5112 and the ground of the 6250 are the same wire.  Switching them could easily kill your signal (and is easily done on the terminal block)."

As you know, I use CB-68LP. This is a terminal block. So, according to what you said, it will kill my signal.

Now, I don't understand why I add one more digital I/O to measure my test fixture, the voltage on 6250 test panel drop down to negative 0.7V. Why

it that? Do you have any clue. I wish I could you some clues in these last two mails.

[DFGray]

My apologies for the late reply.  I do this for fun when I have the time.  The last few days have been rather busy...

You did not connect the 6250 to the circuit correctly, so you are getting what would be expected.  I have attached a drawing to show you what I think is the correct method.  You move your current sensing resistor from its present location to the ground leg of the circuit to the 5112.  Note that the 5112 must be connected for this to work, since the ground is going through the 5112.  Use differential mode on the 6250 to avoid ground problems with the digital lines.

At a guess, I would say connecting the digital lines added yet another ground path and current is now flowing the wrong way through your sense resistor.  When you attach anything to your system, you need to be very aware of where the grounds are and whether or not the instrument you are using is isolated or requires a ground.  The DMM you used is probably a true isolated differential system that does not require a ground (which is why it worked).  The 6250 has to be carefully wired to achieve the same result (see your manual).  The 5112 is always in RSE mode and always requires a ground for proper operation, unless you know exactly what you are doing.

[nyborn]

Hi DFGray:

I am very glad that I received your mail. This is a very difficult questions. I hope you can help me out.

I followed your guildline that you gave me a diagram. I am not quite sure if I have to put any resistors like 100kohm, 33kohm, 18kohm

in between the AI+ and AI- for 6260.  I put the 100Kohm in R1 that is shown in the diagram. Then, I also put digital I/O into

the test fixture. This time. I got signal back to positive voltage to 1.8V something. But if I replaced my 1kohm resistor into 440 ohm,

the voltage that is shown on the test panel will be netgave value. In fact, is there anything wrong with that? Actually, you told me

to set differential mode in AI+ and AI. I would like to ask the voltage I got from differential mode the same as the voltage I got when

I put R1 into the hard ground? In fact, I put 100kohm resistor across AI+ and AI- in differential mode, I got 1.8V when I had a 100kohm

R1 resistor on the hard ground. Without a hard ground, I will get 0.51V in between AI+ and AI-. I also tried to see the voltage signal

from the BNC cable using SFP. I have attached the picture.If you see the picture, the voltage have a little bit offset from 0V.

Why is that? Is that becasue of I add a resistor R1 on the hard ground.

[DFGray]

Let's start with the easy question. Your voltage offset on the 5112 is probably  due to the resistor in the ground path. I warned you about this earlier. You need to keep this resistor as low as possible (I would use 100Ω or less). The voltage you see in the 6250 will scale with this resistor, so you will need to decrease the max and min voltages on the 6250 to get high resolution data.

AI+ and AI- of the 6250 should be connected directly to the ends of R1 in the diagram. There should be no other resistors used for the 6250 (leave the resistor network for the 5112 as it is). If you get a negative value with a low value resistor, that means you are getting current flowing from the ground to the device under test. Without knowing more about your device under test, I can't say much. One thing you might check - is the ground of your power supply connected directly to the ground of your instruments. If not, you may have a ground loop issue.

[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.