Varying Signal

[james_p_martin]

Hello

I am acquiring analog input voltage from a pressure transducer which is ranging from 2.18 to 9 volts.I am using LV 8.0 and NI DAQ Card 6221.

But I have  following problems...

1). Intially when no pressure is applied at Pressure transducer....it gives me a voltage of 2.18 volts but I am getting a varying signal which varies around 2.18.....which sholud be a constant 2.18...so how can I remove that varying signal from my input signal so that I get a  constatnt signal.

2). As I am getting 2.18V for no applied pressure so I want to show this voltage as 0 volts how can I remove this offset present in my application.

I am using shielded cables for my all analog inputs. Could anybody suggest how can I remove this noise kind of thing from my measurement and the offset present.

I am attaching the jpeg image of the acquired signal.

Thanks & Regards

[Kenny_K]

To get rid of the noise you could place a smal capacitor (~.1 uF) in parallel with your inputs and see if that smooths out the signal some.  It may also be that your pressure transducer is producing the noise.  It may have a certain tolerance at no load (aka no pressure) which may make it vary slightly.

As for the 2.18 V being displayed as 0 volts, why not just subtract 2.18 from all your voltage measurements.  That would move your "voltage" readings from 2.18-9V to 0-8.82 V

Kenny

[Approximately]

James,

That looks pretty normal (random broadband noise) to me, but I have a big noisy Variable Frequency Drive driving my pressure source. I like to use the RMS vi to flatten this kind of reading out. I've played with various filter vis but the RMS seems the best for my purposes.

You could try the capacitor idea as well, I think that would improve your response.

Also, read this

http://zone.ni.com/devzone/conceptd.nsf/webmain/01f147e156a1be15862568650057df15

I think it's one of the most informative papers for wiring that I have ever seen and I based my design rules for my project upon it. It seemed to help a lot.

Also if your pressure source is also a source for noise like mine is, get an inline filter to elecrically isolate/seperate your DAQ platform from your machine's power source. I found that to be absolutely necessary.

Good luck,

Rick