Noise Reduction

[AshaG]

Hello,

I have a SCXI 1102 which uses SCXI 1303 as its terminal block. My aim is to measure the voltage of a signal with a frequency of around 2000 Hz. As a newbie, I connected the thermocouple given along with the 'Getting Started' pack to channel 0 of the terminal block and found the signal to have a lot of noise. I understood that the pullup and bias resistors are part of SCXI -1303 circuitry and I do not need to connect them for noise reduction.
I connected a 9V battery to the terminals connected to channel 0 and got weird waveforms. If I use a low-pass filter after the signal acquisition.the waveform shows a dc of around 8 V.
As my aim is to measure a signal with frequent amplitude shifts, it is important to filter the noise to a certain extent.
What should I do to minimize  the noise in the signal?


Message Edited by AshaG on 02-26-2008 11:59 AM

[hoffiz]

Hi,

It will help us a lot if you can attach a print screen of how are you reading the voltage; so are you using LabVIEW, Signal Express, Test Panels or what IDE?

I have also a couple of question for you, what voltage do you read if you "short" the same pins of the thermocouple? Where are you connecting you signals on the SCXI-1303, 1303 Specifications, page 10? How are you connecting the 9 Volts battery? Do you have a connector block? it will help us isolating the source of noise from the chassis by connecting your thermocouple directly to the DAQ card, which DAQ card do you have? Can you do a power spectrum measurement and localize in which range of frequencies is the noise concentrate?

Some helpful resources are: Field Wiring and Noise Considerations for Analog Signals and Reducing Thermocouple Noise to Improve Measurements.

Waiting for your answers

[AshaG]

I am using LabView to read in the voltage read by the SCXI-1102.The block diagram has a DAQ assistant configured to measure +/- 10 V. When the battery is not connected, the output voltage should read 5V. As you can see in the front panel, the voltage is pretty noisy. Is this solely because of the cables connected to the SCXI. I tried using a shielded cable and connected its ground to the earth lug(There are 10 steps in the online manual(SCXI 1303 installation guide) while the manual which I got along with the pack has just 9 and does not have the step pertaining to connecting the earth lug). But still the noise remains.

what voltage do you read if you "short" the same pins of the thermocouple? Do you want me to short the channel and measure the voltage?

Where are you connecting you signals on the SCXI-1303, 1303 Specifications, page 10? I had connected the cables to Channel 0(CH0+ and CH0-)

How are you connecting the 9 Volts battery? I had connected the cables to the channel  and then took the  output connections through the strain opening. The output connections were then used to measure the voltage across the battery by holding it against its output terminals. I connected it to Channel 10 this time.

Do you have a connector block? No

which DAQ card do you have? I use NI SCXI-1600 USB.

Can you do a power spectrum measurement and localize in which range of frequencies is the noise concentrate? The power spectrum has a peak at 0 Hz and it is pretty uniform for other frequencies.

Could you tell me what I am doing wrong or what I need to do more?

Thanks for the links too.
The first link mentions about measuring floating voltages and battery is one.Do I need those connections as in the link as the SCXI -1303 is configured for measuring floating voltages as well?
Waiting to hear from you.
Asha



Message Edited by AshaG on 02-28-2008 11:38 AM

Message Edited by AshaG on 02-28-2008 11:41 AM

[hoffiz]

Hi

For the floating measurment connections you don’t need the bias resistor since those connection are already built in on the SCXI-1303 module, just make sure they are connected in the right place, SCXI-1303 page 6.

Yes if you short the channel you will be measuring the noise floor of your system, and this will give us a better insight of the source of the noise.

Also after more researching I have been able to find something that might helps us with this issue, and is the Accuracy Calculator I get this values: Total System Accuracy and +/- 9.2401 mV. You have a range of 0.01 Volts from minimum to maximum as you can see it should be expected. One suggestion would be connecting you thermocouple and decreasing the input range, this will result in a higher resolution.

Let me know if this helps

[AshaG]

Hi,
I forgot to ask a question last time.My colleague remembers speaking to the tech support and the person at the support team asking him to use shunting for the instrument, as the instrument has a high sampling rate and is therefore prone to noise. Do you know what that means?

[hoffiz]

Hi,

The shunt resistor is built in the 1303, as I showed you before. I don’t think saying that a instrument is noisy because it has a high sampling rate is accurate so we would need to know the context in which the Application Engineer was trying to explain this manner.

Some reference I would like to point you to are: Data Acquisition: How Should I Troubleshoot Noisy Readings? 

I hope it helps

[AshaG]

Hi,
 The voltage I expect at the output is 5V. But I get a voltage roughly 30 mV more. That is past the accuracy of the instrument

[AshaG]

Hi.
I will look at the links and your suggestions to find the problem. I will let you know my results.
Thanks,
Asha