PLEASE do not attempt to acquire signals directly using a NI DAQ
board or any NI signal conditioning products. NI does not make any
hardware that is suitable for direct connection to ANY living
organism. There are two big problems:
1. SAFETY - you
need an isolated amplifier system with appropriate safety
certifications (UL 60601-1 or EN60601-1) which is critical to provide
protection against shocks and/or burns in the case of an electrical
fault. And not just faults originating from the EEG system - also
protection from providing a ground path for electrical faults
originating from some other device that may come into contact with the
subject. If you are trying to record from a frog or something, you may not care so much about safety, but then you have the next problem:
2. PERFORMANCE - surface biosignals (EEG, EMG,
ECG, etc.) require specialized amplifiers to acquire cleanly. The
signals are riding on top of 56/60Hz line frequency interference that
is often orders of magnitude greater than the actual biosignal. The
signals are also riding on a large DC offset generated by the
electrodes and electrolyte used to make a good connection with the
skin, and the source impedance (electrode to skin/body) is high
(5-10Kohms or more) and often badly mismatched. Signal conditioners
for biosignals are designed specially to deal with these issues - they
are usually AC coupled, have adjustable low and high pass filters (and
often a notch filter at the line frequency) to block the offset
potential and prevent noise from being digitized and aliased, have
multiple stages of gain to reduce amplifier blocking (saturation) due
to movement and other transients, and have very high impedance balanced
inputs (high CMRR) to reject the common-mode noise and only amplify the
signal of interest.
Specialized amplifiers are
available from a number of sources and have high-level analog outputs
that are ready to be digitized by an NI DAQ board. Look at companies
like Grass Technologies, AD Instruments, MIndware Technologies, and
others.
Steve
