how to avoid floating values rather than zero when atempt by a pulse

[Ortenzi]

Hello,

I'm using the 9401 with a module 9223 to reading and measuring time and amplitude based signals. My problem is: The value of sensors without any interference presents variation from -0,003 V up to +0,004 V and these variations aren't constant. This mean that may be -0,0008; -0,0015 for negative values and +0,0022; +0,0018; +0,0031 for positive values. So I can't read the Zero value (+/- 0,0075 mV) that could be acceptable tolerance. I'm using a piezoelectric sensor for acoustic emission measurement without filter and preamp connected. it has a response frequency of 20 kHz up to 600 kHz and maximum amplitude of -65 dB. Thanks for any help.

B. Regards 

[Yamaeda]

With noise you'll usually need to do averaging or filtering to get decent measurements. How about the average of a 10 slot queue of the latest measurements? That or the simpler alpha-filter (x% of the latest measurement + 1-x% of the previous value, 20-40% is often a good starting point)

/Y

[Ortenzi]

I didn't understand very well your solution because I'm newer in all NI solutions. Generally I use a VISHAY data system rather than NI. If I'm right, you mean that a filter can adjusts it. Actually the sensor is plugged in a isolated surface without noise, so the theoretical amplitude value might be Zero or +/-0,0001mV, but the soft receives (in self test mode) a floatation wave and may be due to impedance or ground interference. I don't know. I tried to ground the cable but without good result neither with a 1000 OHM capacitor. The signal is similar to a oscilloscope without calibration. May the NI-9223 isn't calibrated too??

[Yamaeda]

Why your measurement is floating i really cant answer. With such small measurements it could be antenna effects in cables or a number of different reasons. A way to reduce the effects is to average a number of samples. Many multi meters use this approach (or has the ability).

/Y

[MarcoMar]

Hi!

NI 9223 is a 16bit device and its maximum accuracy is 0.003V. If you read fluctuations below 0.003V it is a 100% expected behavior.
If you need more accuracy, you have to choose a 24bit device. 

[Coq_Rouge]

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@Ortenzi wrote:

Hello,

I'm using the 9401 with a module 9223 to reading and measuring time and amplitude based signals. My problem is: The value of sensors without any interference presents variation from -0,003 V up to +0,004 V and these variations aren't constant. This mean that may be -0,0008; -0,0015 for negative values and +0,0022; +0,0018; +0,0031 for positive values. So I can't read the Zero value (+/- 0,0075 mV) that could be acceptable tolerance. I'm using a piezoelectric sensor for acoustic emission measurement without filter and preamp connected. it has a response frequency of 20 kHz up to 600 kHz and maximum amplitude of -65 dB. Thanks for any help.

B. Regards 

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Then dealing with ADC equipment. The least significant bit(s) will always flicker some. So you will never read a stable zero value. Even if you tie a input channel to ground and do some measuring. The next thing is the ADC resolution. For your device that is (20 volt)/(2^16) equal to about 0.3 mV. So your " can't read the Zero value (+/- 0,0075 mV) that could be acceptable tolerance" Do not make any sense in your setting. Then it is the term Absolute Accuracy you can read more about it here http://www.ni.com/advisor/accuracy/acc_support.htm for your device this number is 3mV. Another thing to point out. Is that you say. You do not use any preamp. I guess the output from sensor is quite low. So you should use an amplifier. And hence make use of full ADC range. To conclude I would say you have some good tools in your toolbox. But since you are not using them correct. Your output would be more or less gobbledygook 

[Ortenzi]

Thanks for all. It is a good lesson to learn. You must only buy a product that you can proof it's good for you.  

[Ortenzi]

Si Marco, lo so. I think I sent you the sensor to test with your instruments in Italy prior to return to Brazil. Actually, I couldn't buy this module because it isn't good for my purposes. By anyway I'll resell it here and I'll buy another kind of hardware. tks.