Hello sry,
Your post appears to have two questions:
1) Can your USB 6251 read your 50-200uV signal?
2) How can you reduce noise on your low level signal?
Your second question is more straight foward, so I will answer that first. NI has a great website with links to pages about low level measurements and noise. A majority of these are oriented toward our DMMs, which is what we usually reccomend for low level measurements, but many of the links are pertinant for all measurements. You also might want to look at our Field Wiring and Noise Considerations for Analog Signals article. Since your signals are so low, you may already be doing what these suggest. You could also do some signal processing (FFT's, etc) to try and get your signal out.
The questions about your 6251 is not quite so easy to answer. I looked at the specifications, and although the card is 16 bits of resolution, its minimum range accuracy is 52uV and range sensitivity is 6uV. Those values also assume you are doing DC measurements, taking 100 points and averaging them as well:
This means that your % accuracy will not be very good. You could average more points to get rid of more noise, but that can only do so much. All of this is assuming you are trying to read DC values, which appears not to be the case. Measuring AC signals at this level is extremely difficult. For frequency measurements, our DSA systems would be the way to go, but there are some limitations with this as well.
I hope I gave you some of the info you were looking for. If you have any other questions, post back.
Neal M.
Applications Engineering National Instruments www.ni.com/support
