USB-6009 2 channel measurement problem!

[ChesterRush]

hey everybody, happy new year!

I just recently started to use an USB-6009 to connect my instruments with my PC. I am connecting a hot-wire anemometer and a pressure sensor. As long as i only connect one of the measurement instruments i get a good signal in Labview, but as soon as both instruments are attached to the USB-6009 my signals in Labview have crazy peaks. A friend of mine told me it could be because the whole setup was not grounded. i tried some different things and in the end i build an Oscilloscope into my signal way before the USB-6009 and my signals are now nice and smooth. My problem is that i have to give back the Oscilloscope tomorrow and so i need another solution for this problem. Hopefully anyone here knows how to deal with this.

Chester

[johnsold]

Chester,

The advice you were given about grounding may be where you need to start.  Adding the scope probably provided a ground path so that your DAQ system worked.

Without specifics it is impossible to tell what is going on in your system  Please tell us exactly how everything is connected and what settings you are using for the 6009.  The anemometer is probably a line powered instrument. Is its output ground referenced?  What kind of pressure transducer are you using? Are you using any singal conditioning equipment with it?

Lynn

[ChesterRush]

Hi Lynn,

I am using a DISA Type 55M01 for the hot-wire and a MENSOR Series 6100 Digital Pressure Transducer. As i am measuring an analog signal in differential mode I connected the two instruments like the handbook told. The MENSOR into channel AI0 with + and - and the DISA into channel AI1 with + and -.Right now I am not using any signal conditioning equipment.I am sorry but, probably cause of my poor english, i do not understand what you mean with output ground referenced on the DISA. The power plug of the DISA does have a ground if it is that what you mean.

Chester

[johnsold]

Chester,

Your anemometer appears to be quite old.  I could not find any data sheets for it.  Some of the references to it were published in the late 1990s.

The pressure transducer may be where the problem occurs. On page 24 of the CPT6100 Series Operating Instructions it indicates that the analog output should be measured with a device having an input impedance greater than "1 Megaohm."  <Pet Peeve mode = ON> The proper usage is megohm, not Megaohm.  It always makes me wonder about the quality of the device if the manufacturer does not care enough to get the units right. <Pet Peeve mode = OFF> The input impedance of the 6009 is 144 kilohms, or about 1/7 of the minimum requirement.  The output may be loaded down or distorted by the lower input impedance of the 6009.

Lynn

[ChesterRush]

Hi Lynn

thanks for your tip. I checked it and you are right. The problem was that the USB-6009 only got 144kOhm input impedance. I explained my problem to the guys in the electronics lab and they did not quite know what to do about it, so they lend me the oscilloscope again, this time till my measurements are done. Once again thank you very much for your help! Would have been totaly lost without you!

Chester

[johnsold]

Chester,

As an electronics engineer with 40 years of experience, I find your comments about your electronics lab disappointing.  They must not have much measurement or instrumentation experience.

To use the pressure transducer with the USB-6009 you need to have a buffer amplifier between them.  It needs to have an input impedance > 1 megohm, an output impedance < 100 ohms, and unity gain.

Lynn

[SnowMule]

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@ChesterRush wrote:

The problem was that the USB-6009 only got 144kOhm input impedance. I explained my problem to the guys in the electronics lab and they did not quite know what to do about it

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whaaaat? 

http://en.wikipedia.org/wiki/Operational_amplifier_applications#Voltage_follower_.28Unity_Buffer_Amp...

[johnsold]

My thoughts exactly!

Lynn

[ChesterRush]

yeah looks like they don't know their stuff, but in their defense they head of the lab is on holiday and i asked his substitution who might as well be an apprentice, i don't know. Anyways thanks again