Interpreting acquired data from SignalExpress 3.0

[Boiler]

I recently purchased NI USB-6211 and I'm using SignalExpress 3.0 to acquire and view my data. 

First of all, I don't have much background knowledge about signals, filtering and etc.  

Basically, I'm measuring an analog output (voltage) from a Laser meter that range from 0 to 1 volt.

Before I acquire data using NI USB 6211, I wanted to test it with voltmeter (True RMS Multimeter) to make sure cable connections are all tight.

It was reading (0.253-0.254 V) which was very stable. 

When I run acquired data from SignalExpress 3.0, below is the display I'm seeing.

(I didn't change anything in the setup since I installed. Acquisition mode: continuous, samples to read: 100, Rate(Hz): 1k )   

 

I have also attached the excel exported data for 4 seconds and it's Voltage vs. Time (s) graph. 

My question is, I need to store data such as what voltmeter gives me (0.253-0.254 V) but I don't know how to interpret (or make conversion) what SignalExpress 3.0 shows me.  

The laser meter reading is almost constant either when I read it from the detector itself (it displays reading in mW) or when I measured analog output from the voltmeter. Signal Express 3.0 takes 1000 samples per 1 second, which include all kinds of values between 0 and 0.55 V (as shown above).

I tried taking the average, standard deviation of samples taken, but it doesn't get any close to what I saw from the voltmeter.  

Any information would be very helpful. Maybe I need to make some adjustment in my setup?  Filtering mode or etc to get stable reading? 

Boiler.

Message Edited by Boiler on 07-24-2009 10:20 AM

[DFGray]

It appears you have some sort of oscillation that is almost synchronous with your data collection speed and is being undersampled (or your data could really look like this).  This could be due to a plethora of things.  Here are some generic things to try:

1.  Using your DMM, measure AC volts to see if the variation is present there, as well.  DC volts will just give you an average, usually fairly heavily filtered, depending on the DMM.
2. Take data in SignalExpress at 10kHz instead of 1kHz.  Your data looks like a signal is going in and out of phase with the collection period and this should help sort that out.  You may want to try odd frequencies (e.g. 9.86kHz) to destroy any coherence between acquistion and data.
3. Verify that you have proper signal connection/grounding.  Depending on whether the 6211 is configured for differential or single-ended, you will need different connections to ensure proper signal reading.  If your laser meter has, for example, a 50Ω output, you will need to properly terminate the wire going into the 6211.

Please let us know what the output specifications of the laser meter are so we can help you better.  The manufacturer and model number would also be nice.

[Boiler]

Hello DFGray, 

Thank you for your response.

The laser meter is "Laser Mate Q" from COHERENT.

The manual says "Analog Output

The RCA phono jack connector provides a 0 to 1 volt (nominal) analog

signal (uncalibrated) corresponding with zero to full scale of the meter

movement. It can be used to operate a strip-chart recorder." (please refer to the attached manual for detail). 

So I plugged in RCA cable to the Laser meter then I cut the other end of the cable to have positive and negative strips, which I connected to the

port 17(+) and 18(-).  

Could it be the connection problem? Please verify me if my connection is setup right.

Message Edited by Boiler on 07-27-2009 09:13 AM

Message Edited by Boiler on 07-27-2009 09:13 AM

[DFGray · Accepted solution]

Your connection is your problem.  The two connections you used are for differential inputs.  Given your signal levels, this is a good choice, but you also need bias resistors to ground. To quote the user manual

"It is important to connect the negative lead of a floating source to AI GND (either directly or through a bias resistor). Otherwise the source may float out of the maximum working voltage range of the NI-PGIA and the DAQ device returns erroneous data."

You have two choices.

1. You can continue to use differential mode inputs.  You will need to connect both inputs (17 and 18) to Analog Input Ground (pin 28) using a high value resistor.  This resistor should be about 100X the value of your source impedance.  Unfortunately, Coherent does not give you this value (give them a call or e-mail).  Try about 100kΩ. Make sure the device is set to read differential input on channel 1.
2. You can use referenced, single-ended input.  Connect your positive terminal to pin 17, your negative to pin 28 and configure the device to use referenced, single ended (RSE) input on channel 1.  Your measurements will be more subject to noise (which could be bad, depending on the laser, there could be a lot of electrical noise in the area).

I would try the second one to get moving, but use the first for any serious measurements.  Read through the user manual for the 6211.  It has a very good section on connecting analog inputs.  If you do not have a copy, you can get one here.  For a quick start, look about page 4-23.

Good Luck!