Precision of PCI 6040

[Fahac]

Hi!

 I am using a PCI 6040 E connected to a BNC 2010 to measure 3 different voltages to measure the output voltages of a quadrant photodiode with about 22 kHz as a sampling rate. Light inserted on QPD comes from a scattered light from an optically trapped particle, therefore it should change with a high frequency, but, I have just found that the measured signal changes just in some steps: 

1.49414062
1.49414062
1.49414062
1.49902344
1.49902344
1.49414062
1.49902344
1.49902344
1.49902344
1.49902344
1.49902344
1.49902344
1.49902344
1.49414062
1.49902344
1.49414062

I mean some constant numbers exactly have been repeated, which is my surprise. I also have seen that resolution of my PCI should be about 12 bits so I should have 0.0024 volt resolution. Is there any idea?

Thank you,

Faha

[crossrulz]

Actually, your resolution should be 4.88mV based on a +-10V scale (20V peak to peak).  So you are in the noise of a single bit.

[Fahac]

Yes! I have just seen that by changing the minimum and maximum in data aquering, I have the real voltages. I didnt know it that I can change the resolution by Min and Max voltage and I worried I can not use this PCI. So it is solved!

Thank you,

Faha