Labview signal processing for millivolt input different from multi-meter reading

[KarenM]

Hi,

I am using Labview 8.5.1 and the DAQ USB6210 to read millivolt values, and am having trouble getting it to read accurately when compared to two conventional multimeter/volt-meter devices. I'm wondering what I need to do to make it accurate.  The input (and voltage reference) comes from 2 skin electrodes which goes to an isolation amplifier and then to the data acquisition device. 5MOhm resistors are connected from each input line to ground (to help common-mode noise). As an example, in terms of resultant voltage, I get a reading of 23 mV using a digital multimeter,and 35 - 40 mV when using a voltage (multifunctions as a pH) meter. Using Labview and a low pass filter, I get 19-20 mV. I know this information is a little rough, but I'm wondering whether anyone may know whether there is some signal processing I need to add?  Right now I just have a low pass filter for any processing. Any help would be appreciated. Thanks, 

[BCho]

Hey Karen,

There are a couple things to mention here. First, it seems that you have three different device measuring three different voltages. That being said, how do you know which one is correct? You have one device reading 20 mV, one reading 23, and one readying 35-40mV? This seems strange and hard to come to the conclusion that one device is accurate and another is not. In terms of how you are reading your signal, are you using a differential or RSE connection? Also, check out the Voltage Measurements: How To guide for a little more info regarding what pull down resistor values you might want to use. In terms of the low pass filter, is this a physical filter or a part of your LabVIEW program. If you take a step out of your LabVIEW program and read the voltage in Measurement & Automation Explorer's test panels, what voltage do you get there. 

Also, it might be a better idea to post in the Multifunction DAQ forum instead of the LabVIEW forum as your troubles seems to be more on the acquiring data and less the LabVIEW environment

[liviacbb]

hi!

I saw your answer above and i think that you may help me. I need to control  a servomotor which rotates a galvo mirror. An analog input of +10V makes the mirror rotate 20°, that's the maximum possible. I need to send 500 different inputs so that i will be able to capture the spectrum while the mirror scans the sample. it means that a need 500 positions for the mirror. (i wanna send 20mV, then 40mV, 60mV, 80mV until 10V) I will do it using labview; i've already tried using the audio output. i've sent a sine of 1khz, then a diode and a capacitor to create a DC signal. By varying the amplitude of the sine sent by labview, i tried to control the DC level. the thing is, i dont know if that system is capable of sending such a small and precising voltage like 20mV.

what do you think about it? the audio output is good solution or i should use serial port? hope you can give me some advice!

[ycui7]

In your case, you want to output a analog signal with high amplitude accuracy. I highly doubt the audio output is accurate enough for your application. I just suggest you buy an analog output device. You can easily setup output voltage on that directly. Without the 1khz audio requency stuff. And the output signal is DC, very suitable for your application.

[ycui7]

The 20mV different is very common between different devices. Even a five thousand dollars data acquistion card has the problem. It depends on many stuff, grounding, shielding, etc. 

[liviacbb]

hi! thank you so much for answering me! what kind of device you're talking about? can you give me an example of a simple ne not very hard to find?

also, i didnt get what you said in the last part. did you mean that a step of 20mV is very hard to have it? thanks