noise aaddition in the measured signal by ni daq 6281

[a1234]

hi,

 I was cheking the board (ni DAQ 6281), i have attached the function generator at the diff mode  at pin 33 + and 66-.Isaw that without giving the I/p to the card I saw  asquare wave of 11to -1 peak waveform with friquency 100 hz,

  Can uhelp me to discover the reasons .

 thanking u in advance.

 with best regardes

 ayukt

[Elijah_K]

ayukt,

Just to confirm: you have a function generator attached to the second analog input in differential mode, and you're receiving a TTL square wave when nothing is beign generated?  11V actually exceedes the range of the analog input, so that shouldn't even be possible.  Are you using a scale at all?  what does the input do when nothing is connected? 

What I might recommend is that you use the analog output to generate an arbitrary waveform and see if the analog input measures it accurately.

If this doesn't work, contact national instruments to seek possible repair of your card.  If it does work you'll need to confirm the output of the function generator.

[a1234]

hi ,

 I Cheked the card with analog output of Daq card (as u told me) , It is working properly in this condition ,when i was using function generator  I found that if function generator is off  from main supply ,DAQ card was not receiving anything garbage , but when i on the function generator , card  again startes generating or I can say displays in labview  erronious values.

   Is it picking noise from fuction generator?

 with best regades

 a1234

[Elijah_K]

It really shouldn't be possible for it to read a voltage over 10V and you'll need to be more specific when you describe the noise.   Could you attach a screen shot and add any other information you can think of?  Have you used any external mechanisms to confirm what the DAQ card is reading?

[johnsold]

It sounds as though you may have a ground reference problem. When you see large signals at or near the ppower line frequency or its harmonics, it suggests a connection problem.

Is your power line frequency 50 Hz? This could lead to a 100 Hz signal on your DAQ inputs. Make sure you have a path for bias currents on the DAQ inputs. I am not familiar with your device but bias currents are sometimes an issue with differential connections.

Lynn

[a1234]

hi ,

I am sending the waveform which I  got with the help of VI logger when there is no output from function generator but connection are at there place and function generator is on  .When i check this o/p with the scope I get nothing.Thos graph  is attached with mail ,

When I use biasing resistence  at the  in put of daq card I get the proper output but it is modulated with a low friquency sine wave ,  I am using sampling rate 600 kHz and signal fri is only 20 k. And when I connect the output as earlier stated conition with biasing resistence  o/p is same a square wave.

with best regaedes

[Matt_A]

Hello ayukt,

You have mentioned that this behavior occurs in LabVIEW and when using VI Logger. Are you using an NI-DAQmx Task to configure your acquisition? If you are, you may want to check the Task for Custom Scaling. This could lead to values that appear out of range. I would recommend that you view the data in a Test Panel in the Measurement & Automation Explorer. This should give you the raw data without any scaling. For more information about using Test Panels you can view this document.

What signal are you generating with the function generator when you see the values you described? Have you tried outputting a DC value of 0V from the function generator and observing the data then? If you output a known value of 0V and still view noise on the line, then it is likely that there is an external device inducing the noise on the line.