unable to read accurate input values with NI-DAQ 6008

[GoviRe]

Hello,

I got new NIUSB-6008 DAQ and started taking some measurements. But I don’t know why it is not measuring the accurate input voltage and current .I observed the DAQ measured values as shown in the attached image when tried to 1) read input voltage of around 0.294V and a current 28mv.Settings of DAQ are also shown in the image with the simple VI used.

Can someone explain me why there are peaks with the measured signals and why NI-USB DAQ is unable to measure the accurate values.

Thanks,

[Dennis_Knutson]

What is the purpose of the collector express VI? I don't use express functions and I would recommend you get rid of it and switch to continuous samples. There are also several examples that come with LabVIEW and do not use the DAQ Assistant.

[GoviRe]

Hi  Dennis_Knutson,

Now I am measuring voltage signal with out DAQ Assistant as in the attached vi.

I am trying to read the voltage supplied from voltage source using 1 DAQ channel of NI-USB 6008 where 

1) Termianl2(AI0) is wired to the point on breadboard where the positive supply of volatge source is connected and

2)Termianl1(GND) is wired to the ground point on breadboard to which the negative end of the votlage source is connected.

But now also I am unable to measure the correct voltage.I am getting some strange reading as shown in the attached images when input supply voltage was 0.5V,1v,2V.

I noticed that the even if I set the value 'RSE' for 'input terminal configuration' for 'DAQmx create channel' it showing as 'default' as shown in the attached iamges.Is this the problem or is there any problem with NI USB 6008 DAQ.

Can someone please help me in figuring out the problem.

 Thanks.

[GoviRe]

And 2V measurement is also attched here.

[johnsold]

It appears that you are seeing the results of the limited resolution of the USB-6008 and the internal noise. According to the specifications system noise on the +/-1 V range is 0.5 mV rms and 5 mV rms on the +/-20 V range. The peak voltages will be several times higher than the rms voltages with the exact range depending on the statistics of the noise sources.

You may also be seeing the effect of the unusual input corcuit configuration of the device. It is the equivalent of 144000 ohm terminated at 1.4 V.

What is the source impedance of your signal?

Lynn

[_Bryan]

One easy thing to try when having trouble is to measure a AA (~1.5v) or 9v battery voltage, then compare the reading to a multimeter.  I do not have LabVIEW on this machine, so I was not able to look at your latest code.

[stefan57]

Hi Lynn,

 what does "The peak voltages will be several times higher than the rms voltages with the exact range depending on the statistics of the noise sources."can you explain a bit more abotu this.

Thanks.

[stefan57]

Thanks.

[GoviRe]

Hi,
Can someone suggest a solution for this.
when the sensor output was sampled at sample rate of 100hz with terminal configuration as 'Let DAQ choose" using 6016DAQunit is as shown in figure(DAQ_6016) of the attachment and when the same sensor output is sampled at sample rate of 100hz with DAQ-6008 unit with 1)Terminal configuration as 'Let DAQ choose" the output is as shown in figure(DAQ_6008_LetDAQchoose) attachment 2)Terminal configuration as 'RSE" the output is as shown in figure(DAQ_6008_RSE) attachment.

Is there any way to make sampling better using DAQ-6008 (as it is very handy to use and I will use it only) with Terminal configuration setting as 'RSE".
Thanks.

[Dennis_Knutson]

Since there are different wiring configurations for rse and differential, why on earth would you do something random? Pick the best configuration based on your signal and explicitly program out.