Differential mode operation

Hi:
Could you tell me how the current through AI+ and AI-.There is also one biase resistor from AI- to AIGND.
Attached is the digram that I cut from ni site. The AI- input is also connected to my test fixture ground at the end. I could not see how to current flow. Could you please tell me?
I am in a very urgent. I really a quick reponse.
Regards.
John
Attachments:
Differential Mode Operation.doc ‏87 KB

Hi AnalogKid:
Thanks for your replying. You said current flows through the lump sum parameter RIN nodes (1) and (2) regardless if VS- terminal 0 is connected to ground or not.
Then what is Rin? Rin is the resistor inside the 6250 board or the bias resistors from AI- to ground? Could you please tell me?
Attached is my graph. I take VS as the voltage across 100ohm resistor. Is that correct? I would like to verify with you.
Attachments:
First Picture.JPG ‏33 KB

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How can I change the daq setting to differential mode in my labview program (example)?

I would appreciate if I could get an example in Labview 5.1. I realized that I need to create a cluster but for some reason it would not work

Examples for data acquisition are in the LabVIEW >> Examples >> Daq >> anlogin >> anlogin.llb directory. You will still have to do the following to programmatically set the measurement mode to differential.
You can set your data acquisition with the AI Config.vi in LabVIEW. There is an input for "coupling and input config". A good trick to make sure that you create the cluster correctly is to use your mouse as the wiring tool and right-click over that input. Then, choose Create Control or Create Constant. The cluster will pop up, and you can use your operating tool to choose differential mode for the second pull-down ring in the cluster.
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http://www.ni.com/ask

Problem of shift of groud level in AI channel in Differential mode ?

We are using PXI-6052E card ,
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The ground level shifts by about 1.11 Volt .
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Greetings,
I would first suggest that, if this device is in a system connected to the internet, you run the Online E Series Diagnostic. This utility will test the functionality of your board and perform a self-calibration. The link to this utility is available below:
http://ni.com/support/selftest/
Unless this calibration fixes the problem, I would create a phone support request at the following location to arrange the repair of your board:
http://sine.ni.com/apps/we/niae_asc.main
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I am getting noise clusters in differential mode for ground refeenced source

I am using DAQ board PCI-MIO-16E-1 and 2090 BNC as termination accessory. I want to aquire data from a ground referenced source.I am getting a lots of shot noise which is having a peiodic envope of 55 Hz frequency .These clusters are never removed in all the three modes but the strange thing is that i am having better results in RSE mode than in differential mode for the same source.This looks like a conflicting obervation from what is recomended.What may be the source for these peiodic noise clusters which are unavoidable.
One clue is that we get the same signal if we acquire data between our source ground and earth and same is the case for our computer.This noise is not removed ev
en if we short grounds of PC and our ground reference source.
Attaced is the file having the picture of noise i get.
Attachments:
noise_pic.bmp ‏231 KB

Hello;
Noise is a difficult issue to advise about.
There are some things you can try, but the result will depend on different factors, such as, environment, lenght of cables, equipment on vicinity, etc.
Anyways, you can try to hook up a BIAS resistor in between the ch- and AIGND on your BNC device. That resistor will bleed out the common voltage current and help on noise balancing. Another thing you can try is to hook up a simple RC low pass filter at the analog input. That might help to get rid of that noise.
I'm also attaching an App Note that talks about Noise and some ways to get around it.
Hope this helps.
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Applications Engineer
National Instruments
Attachments:
Field_Wiring_and_Noise_Considerations.zip ‏196 KB

Differential mode of PXI

I'm using PXI-4461 based on Visual Basic programming. My goal is to generate
differential mode signal with 0.5v offset using the PXI-4461 and VB.
The followings are details, but I'm attaching a ppt file for better understanding of my question.
How can I shift up the negative sinewave by +1v?
The following is the VB code which is the same as the code in the attached ppt file.
Public Sub test(ByVal numSamples As Long, ByRef writeData() As Double, dataRate As Double)
     If chanDAQ.taskHandle <> 0 Then
        DAQmxErrChkMod DAQmxTaskControl(chanDAQ.taskHandle, DAQmx_Val_Task_Unreserve)
        DAQmxErrChkMod DAQmxStopTask(chanDAQ.taskHandle)
        DAQmxErrChkMod DAQmxCfgSampClkTiming(chanDAQ.taskHandle, "", dataRate, DAQmx_Val_Rising, DAQmx_Val_AcquisitionType_ContSamps, numSamples)
        DAQmxErrChkMod DAQmxWriteAnalogF64(chanDAQ.taskHandle, numSamples, False, 10#, DAQmx_Val_GroupByChannel, writeData(0), sampsPerChanWritten, ByVal 0&)
        DAQmxErrChkMod DAQmxStartTask(chanDAQ.taskHandle)
        chanDAQ.taskStarted = True
    End If
End Sub
<Description to explain the same problem descripbed in the attached ppt file.>
PXI-4461 is connected to an oscilloscope with BNC cable. 4461 uses the differencial mode (NOT pseudo-differential mode)
due to some reason. The positive signal from 4461 (i.e. the center core of BNC) is connected
to channel 1 of the oscilloscope, and the negative signal (i.e. the shield of BNC) is connected to
channel 2.
Then A sinewave which has the range from 0 to 2v and 1v offset, was generated in VB program.
Then it was applied to an oscilloscope. The channel 1 of oscilloscope showed a sinewave which has
the range from 0 to 1v with 0.5v offset, and the channel 2 showed a sine which has the range
from -1v to 0v with -0.5v offset.
For my application, the channel 1 is ok, but channel 2 should be shift up by 1v, basically the channel 2
should be a sine which has the range from 0v to +1v and +0.5v offset, and also its phase should lead
over 180 degree.
Message Edited by talking on 05-03-2010 09:27 PM
Attachments:
question1.ppt ‏153 KB

Thank you very very much, Adam.
Let me answer to your questions first.
>it might be best to get the signal working in Measurement and Automation Explorer.
Yes, I have just tried to use MAX. But results are the same as one that I got using my VB code.
But it was good suggestion. I think using task in MAX is powerful.
>I imagine you are splitting the BNC to a 2 wire pair, and then putting that 2 wire pair into another BNC cable for each channel. Is this true?
Yes, Correct. I followed the direction from the following document.
www.ni.com/pdf/manuals/373088a.pdf (page 2-13, 3-3 and 3-5)
Only one BNC cable was used to make connection between a signle channel of PXI-4461 and two channels of oscilloscope.
One side of the BNC was connected to a singla channel of 4461, and then the other side of the BNC was connected to two channels of oscilloscope. As shown in page 3-3 of the following document, "+" of BNC (which means positive signal of differential mode) was connected channel 1 prober of the oscilloscipe, and "-" of BNC was connected to the channel 2 prober of oscilloscope.
Then the ground of these two probers are shared to the ground of the 4461 as shown in page 3-5.
Is this connection of the ground correct? Please let me know anythin wrong in my setup.
> Is there a reason that you have to split the BNC output to 2 channels?
Yes, I need the specific reason of it. I'm working on ADC test.
My first goal is to generate the differential mode sinewave signal from PXI-4461.
Then the second goal is to connect this differential signal to my differetial input ADC.
The ADC has a specification that both of positive and negative signals have 0.5v offset, and their max
voltage is 1v, and min voltage is 0v, as I showed in the ppt fiile.
So I need to first both of positivie and negative signal from 4461 on the scope. Then I can apply this signal to my ADC.
The oscilloscope is not a product dedicated for differential mode signal. This is regular one (agilent DSO5034A).
> From my recollection a scope should expect a positive signal, and a ground or negative signal to the BNC, which the analog output will provide.
It sounds like my scope (agilent DSO5034A) might measure wrong value for offset. Isn't there any way to measure each sigle signal of differential analog signal of 4461, using my scope?
> are you using AC or DC coupling on the o-scope?
Yes, I was using DC coupling on the scope.
> ensure that the signal is zero'd out correctly on the vertical scale.
Yes, I carefully observed each signal using "cursor" from the screen of my scope.
I'm for sure, the positive signal offset was +0.5v, and the negative offset was -0.5v as shown in my ppt file.
Thanks.
talking

Voir variable en temps réel en mode operator

Bonsoir,
J'aimerai savoir comment récupérer la valeur des variables à chaque instant des étapes de mes séquences de tests lorsque je suis en mode operator et en mode pas à pas.
D'avance je vous remercie,
Cordialement,
Nelson

Duplicate: http://forums.ni.com/ni/board/message?board.id=4170&message.id=32316
Aurélien J.
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PCI-1409 in differential mode

Hello all,
I'm using PCI-1409 for non-standard analogue video acquisition.
My video image has a noise band which moves from below to top in the frame.
I'm wondering my PCI-1409 is not working in the differential mode.
How can I set this to differential mode?
Any response will be deeply appreciated.
Regards

CRSmania,
If you take a look at the PXI/PCI 1409 user manual, on page 3-5 it says
VIDEO0±: VIDEO0± supports RSE connection only. To operate in RSE mode, you must
connect VIDEO0– to DGND. When you use VIDEO0+ or VIDEO0–, you must
disconnect the BNC connector.
VIDEO<3..1>±: VIDEO<3..1>± allows for a DIFF or RSE connection to video channels
1, 2, and 3. To operate in RSE mode, connect VIDEO<3..1>– to DGND.
Use VIDEO 1,2 and/or 3 if you want to set PCI-1409 to be a differential mode.
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Differential mode

Hi:
I read the tutorial. http://zone.ni.com/devzone/conceptd.nsf/webmain/177A8B29FEDC0F5886256FA90083C0F8.
for differential mode.
I am measuring a DC voltage and my test fixture is a very low impendance. So I should only use AI- to AIGND to setup differential mode.
The question I need to ask is I don't need to put any wire from AI+ to AIGND. Is this correct?
The second question is if I then add a BNC cable from my test fixture to 5112 board. the voltage starting changing from 758ohm to 18kohm resistor. The voltage seems starting going down. I don't understand why it is happening like this. Could you explain to me? Attached is my graph.
The third question is I use DMM to test the current from AI- to AIGND, AI- to the negative node of 100ohm and AI+ to the postive node 0f 100ohm. I got 0mA, 0mA and 0mA. I also got no voltage across 10kohm resistor. Could you please tell me why?
If so, how does it remove common mode voltage and noise in this config? I don't understand. Could you pls explain to me?
I would appreicate if you can reply to me promply. I am in a very urgent.

Attachments:
Graph17.JPG ‏32 KB

RSE and differential mode

Hi
I am using a E series card with Labview 7.1. i am trying to read voltage from a voltage source.I am using a conditioning box that is connected to a BNC 2090 and from here it is feed in to NI card in PC.i checked the volts via volt meter. it gives me 0.97 volt now when i try to acquire this voltage using labview i have two options either i can select differential mode or SRE mode with SRE mode it gives me 0.97 but with differential mode it keep on increasing until it reached to 4.9 the maximum range of voltage.I cannot understand this can any body please help me in understanding it.

The difference in voltage between the 2 input modes should not occur if everything is correctly connected. When an input floats up to the maximum level, this often indicates there is an open circuit.
To check your setup, I suggest you initially try using a MAX test panel to measure channels under differential mode.You need to refer to the manual of the 2090 on how to connect up for using differential mode. The setup requires switches S1 to S8 be set to diff mode and then a connection made to only the first 8 BNC connectors.The inner and outer parts of the BNC, form the 2 input connections to the DAQ card.
SimonS

SRe and differential mode

Hi
I am using a E series card with Labview 7.1. i am trying to read voltage from a voltage source.I am using a conditioning box that is connected to a BNC 2090 and from here it is feed in to NI card in PC.i checked the volts via volt meter. it gives me 0.97 volt now when i try to acquire this voltage using labview i have two options either i can select differential mode or SRE mode with SRE mode it gives me 0.97 but with differential mode it keep on increasing until it reached to 4.9 the maximum range of voltage.I cannot understand this can any body please help me in understanding it.

The difference in voltage between the 2 input modes should not occur if everything is correctly connected. When an input floats up to the maximum level, this often indicates there is an open circuit.
To check your setup, I suggest you initially try using a MAX test panel to measure channels under differential mode.You need to refer to the manual of the 2090 on how to connect up for using differential mode. The setup requires switches S1 to S8 be set to diff mode and then a connection made to only the first 8 BNC connectors.The inner and outer parts of the BNC, form the 2 input connections to the DAQ card.
SimonS

Differential Mode in DAQ M series board

Hello all!
I have a doubt concerning the Differential Mode in DAQ M series board. In the board's manual is saiing that the voltage limit is -10 to 10V, but somebody knows if this limit is always referenced to ground, or in differential mode is comparing with the +8 channel (ex. for channel 0 - voltage diference between ch0 and ch8).
Thanks in advance, best regards.
Paulo Carmo

Paulo,
differential mode means that you measure the difference between two input channels. The channels used for this on E- and M-series boards is n and n+8. The difference between the channels might go up to +- 10V. So there is no ground reference in this type of measurement.
However, the device has a limited capacity against overvoltage. So if the offset of either of the channels succees a certain value in regard to the ground of the measurement system (the gnd of your PC), you could possibly damage your hardware.
Therefore it is recommend to use "BIAS-resistors" in such cases (or if you simply don't know the offset).
You can find a very good image on possible ) recommended connections in your DAQ-help. Open the DAQmx Help and navigate to DAQmx Help >> Measurement Fundamentals >> Signals >> Analog >> Connecting Analog Input Signals >> Measurement System Types. Just activate that entry (should have a "book" icon) and in the right side, you should see a something like a reference poster. I suggest you to print it out....
hope this helps,
Norbert
[EDIT]: corrected a typo
Message Edited by Norbert B on 02-26-2009 08:03 AM
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