# LabVIEW

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## How to stabilize Voltage input

Hello All,

I am new to Labview. I am trying my first project that consist in measuring pressure from a transducer. The output of my transducer is 4- 20 mA and I am using the USB 6229 card.

I am using a 250 ohm precision resistor so the signal converts the 4-20 mA current into 1- 5 V.  The problem is that the voltage values that I acquire oscillate a lot: For example for an expected 1 Volt input (equivalent to zero pressure), I read in labview an oscillating signal from 0.095 V and 1.05V. It represents in my sensor an unacceptable variation (about 20 psi). However, when I measure the voltage in the pins from the DAQ module using my multimeter I read a steady signal of 0.995 V for zero pressure.

I appreciate any help with this

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## Re: How to stabilize Voltage input

Have you looked at this "variation"?  Might it have a frequency of 60 Hz (in the US) or 50 Hz (most of the rest of the world)?  Do you have a circuit diagram of how you are using the 250 ohm resistor?  [Why are you using a "precision" resistor?  Are you willing to trust its value?  Were you planning to calibrate your setup?].

Am I correct that you are not an Electrical Engineering student?  [Sorry about asking so many questions and not giving "the answer" -- it is far better for you to think about what you are doing and figure it out for yourself, maybe doing a little reading on the side if necessary ...].

Bob Schor

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## Re: How to stabilize Voltage input

If you have a noisy signal, it's common to take multiple samples and average (or low pass) the values.

/Y

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Message 3 of 17
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## Re: How to stabilize Voltage input

How fast does your multimeter display? What is the smallest division in the range you're measuring? Is it possible that it's already low-pass filtering the signal to give a more steady readout (since when using a multimeter, usually an easy to read display is more valuable than a fast update rate (and indeed a fast update rate makes reading much harder!)).

For a physical source of variation I'd look to an oscilloscope or Fourier Transform of the input to see if you can either visually or numerically pick out a dominant frequency (as Bob Schor said, it might be related to your power supplies).

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## Re: How to stabilize Voltage input

@cbutcher wrote:

(as Bob Schor said, it might be related to your power supplies).

Or, as my (formerly 3-year-old) daughter used to say, on submerging her ears in the tub and listening to herself talk, "Impedance mismatch!".

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## Re: How to stabilize Voltage input

Hello Bob,

No, I am not an electrical engineer. I am mechanical engineer, I have always build my setups using meter but this time I wanted to try something new and challenge my self-using Labiew.  When I refer to a precision resistor, I am referring to a resistor with tight tolerance, I think is 0.1%.

I am planning to calibrate my setup using a linear interpolation,  using the voltage at zero pressure (theoretically 1 V) )and the voltage at span pressure (Theoretically 5 V).  My plan was to calibrate the 0 pressure and span pressure of my sensor using the actual Voltage Values. However I am not having a stable value, but an oscillating value.

Regards,

Jose

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## Re: How to stabilize Voltage input

It make sense, How fo I do that?

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## Re: How to stabilize Voltage input

Are you looking at the data from a simulated device and not your actual USB device?  Simulated devices output an oscillating signal.

aputman
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## Re: How to stabilize Voltage input

Also consider a "running average", in general sample at a fast rate and then take the average of a predetermined amount of measurements.

Every power analyzer I have ever used could do this with user selectable "averaging depths" for exactly this reason.

========================
=== Engineer Ambiguously ===
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Message 9 of 17
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## Re: How to stabilize Voltage input

Hi, I am facing the same problem right now that you were facing almost 4 years ago. I was wondering can you tell me how you overcame that problem. I am also a mechanical engineer like you and I am not too good in this stuff. Your help will be appreciated.

Kind regards,

Shah

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