Sergey,
The FP-AI-V100m is designed to measure voltage signals from 0 to 100 mV. It is not designed to measure negative voltage. This is specified on page 18 of the user manual. The user manuals is found online at: http://www.ni.com/pdf/manuals/322717c.pdf .
The FP-AI-VxxB modules are designed for bipolar measurements such as -5 to 5 Volts. However, since the smallest range on the bipolar module is +/-5 Volts, that make not give you sufficient resolution given you are trying for +/- 100 mV.
For reading signals that are +/-100 mV, the best modules are the [c]FP-AI-110 and [c]FP-TC-120 which both offer 16 bits of resolution.
According to the specs, the Dual Channel voltage input modules are overvoltage protected to +/-30 Volts. I have never tried this method, but theoretically you could use two of the FP-AI-V100m modules, using one channel per module (the per module isolation is needed for this), and hook both of them to the same signal, but in reverse of each other. This would allow you to use one of the modules to measure the signal when it is postive, and one when it is negative. For the most part, it should be fairly easy to determine which one has the accurate reading at any given time, however, when the signal is very close to zero, it may prove difficult to know which reading is accurate and which reading is just reading noise.
Regards,
Aaron
LabVIEW Champion, CLA, CPI
