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myDAQ inaccurate reading of analog signal

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Hi,

 

I'm usign the myDAQ in sobstitution of a USB6341 which is busy on another setup. The proble is that with the USB6341 I log two analog input without any noise problem, while with the myDAQ I get what you can see in the attached image.

Now, that (the white line) should be a flat line but often I get all these spikes of ouliers which clearly come from the passage through the myDAQ.

 

Logging two different voltage signals on the two analog inputs of the my DAQ I can see the same spikes going down in the same moments for both signal, which is another confirmation that the noise is added by the myDAQ.

 

Also, trying different myDAQs (we own many at the university) doesn't solve the problem.

 

Any idea on how solve this problem?

 

Thanks a lot.

 

Giacomo

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Hey kurt4916,

 

All the graphs seem to be scaled values.  What is the raw voltage spike level you're seeing?

 

Thanks!

 

-Sam K

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Hi,

 

sorry for the incomplete information.

Now you can find in attachment the raw signals in V. In Fig1 you can see the two signals aquired at the same time and showing the same ~0.5V drop every now and then. Both signals are aquired with a differential connection (+/-), signal1 is connected to port 0 and signal2 to port 1.

The DAQassistant is set:

- from 0 to 10V for signal1

- from 0 to 3V for signal2

 

Measuring the signals with a digital multimeter returns the expected constant values of 5.03V for signal1 and 2.07V for signal2.

 

In Fig2 you can see signal1 still acquired from port 0, while signal2 is acquired from the builtin dmm of myDAQ. Signal1 is still showing the drops of about 0.5V while signal2 is giving a nice constant value as it should be.

This appears to be a general problem for all myDAQ devices since we tried using several different.

 

Questions:

- why the analog input is so disturbed?

- what is causing the voltage drops in the measurements simultaneously in both analog inputs?

- what can be done to fix it?

 

Thanks!

 

Giacomo

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Hello Giacomo,

There might be some other difference between your myDAQ set-up and the X Series (USB-634x)  set-up.

 

First we  will need to find out how your measurement set-up exactly looks like and what exactly is the issue.

 

Some starter-questions:

- Do you have a picture of how your physical signal is connected to the myDAQ?

- Can you share the VI that you are using?

- In which kind of environment are you working?

- Which kind of cables are you using?

- Are you measuring a floating or referenced signal source?

- Are you measuring from a laptop?

  If yes, do you see a difference when you unplug the power cable of your laptop?

- How does the same measured signal look like with the USB-634x and how does your set-up look like in this case?

  What type of measurement are you doing with the USB-634x (RFSE, NRFSE, Differential)?

 

Thanks in advance for your feedback!

Kind Regards,
Thierry C - CLA, CTA - Senior R&D Engineer (Former Support Engineer) - National Instruments
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Hi,

 

thanks for the interest!

Now I'll try to answer to all the questions:

 

- picture1 shows how the phisical channels are connected

- the VI I'm using is in attachment

- I've tested the VI with LV2010 on XP and also with LV2013 on Win7

- cables are cupper cables of 0.75mm^2 with maximum length of 1m

- I'm pretty sure both signals are referenced (signal 1 comes from a dissolved oxygen transmitter, and signal 2 from a gasO2 transmitter, both devices are grounded)

- yes I'm measuring from a laptop but disconnecting the power cable do not influences the signals

- the same signals, connected in the same way on a USB-634x, give a nice steady value (signal1 is constant on 5,1V and signal2 is constant on 2,1V).

 

Thanks again for you kind help!

 

Cheers,

 

Giacomo

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Hello Giacomo,

 

I think one question was unclear from my side.

With environment I rather mean the actual environment (not related to the pc).

 

Is it in both cases (myDAQ and USB-634x) measured in the same room/location?

Are there large motors (or something else that is high power) in your environment?

Are you using the same cables for the USB-634x set-up?

 

Thanks in advance!

Kind Regards,
Thierry C - CLA, CTA - Senior R&D Engineer (Former Support Engineer) - National Instruments
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Hi Thierry,

 

sorry, now I got it.

The environment is exactly the same in both cases, even the cables and all the surroundings.

 

That's the reason why I was thinking that the reason was some cheapest solution in the components of the myDAQ with respect to the more expensive USB-634x.

I thought maybe the USB-634x has already some built in signal optimization/processing. Does it?

 

And also, why acquiring the same signals from the myDAQ dmm we can receive the expected nicely constant signals?

 

Thanks!

 

Cheers,

 

Giacomo

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Solution
Accepted by topic author kurt4916

Hello Giacomo,

 

Sorry for the late reply.

 

Now that you mention the myDAQ DMM.

Please note that the DMM uses different circuitry (Figure 2, page 5):
http://www.ni.com/pdf/manuals/373060e.pdf

 

How does the signal look like with the myDAQ Scope Soft Front Panel?

Although the USB-6341 has different (and better) specifications seems to rather be a noise and/or floating signal issue.
Can you 100% certainly confirm that you don't see this issue with the USB-6341 when you do a differential measurement?

Have you already tried the method explained in Figure 7 on page 13?
Do you see a difference while doing this?

Kind Regards,
Thierry C - CLA, CTA - Senior R&D Engineer (Former Support Engineer) - National Instruments
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Hi Thierry,

 

sorry for the late reply and thanks a lot for the nice feedback!

 

First of all, one colleague of mine tried the method explained in Figure 7 on page 13 of the manual and this actually solves the problem! Sorry if I didn't notice before.

 

I haven't tried looking at the signal with the myDAQ Scope Soft Front Panel, but I can 100% surely tell that I don't see this issue with the USB-6341 when we do a differential measurement.

 

Thanks a lot for the kind help.

 

Best regards,

 

Giacomo

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Hello Giacomo,

 

1) Can you show me (via a pictures or diagram) how you physically connect your cables in both cases?

I am especially interested in which inputs are connected with which wires.

 

2) Are you using the SCB-68A Connector Block with the USB-6341?

    Are you directly wiring with the myDAQ? (this should show from your picture/diagram?

 

3) Can you share with me the VI you used to do the USB-6341 measurement?

    Can you share with me the VI you used for the myDAQ measurement?

 

4) Did you use explicit bias resistors in any of both cases?

Kind Regards,
Thierry C - CLA, CTA - Senior R&D Engineer (Former Support Engineer) - National Instruments
If someone helped you, let them know. Mark as solved and/or give a kudo. 😉
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