Can I use Ceramic caps as signal processing filter? Wouldn't it degrade & induce noise to the signal? If not, I'll definitely check them out! Thanks!
Yes, ceramic capacitors should work for your application as a high pass filter. But why do you need it at all? I would filter any slow changes or DC just in software, much easier.
How does your circuit look like? Amplifying microvolts is not easy. There is an open source project for an EEG amplifier, this is the input stage:
http://openeeg.sourceforge.net/doc/modeeg/modEEGamp-v1.0.png
It uses instrumentation amplifiers ( http://en.wikipedia.org/wiki/Instrumentation_amplifier ) and a low pass filters for the inputs, which is more important, because the cable is a very good antenna. And of course, you should always use batteries to operate your device and optocouplers, if you connect it to a PC.
That is an X7R dielectric part and it will have a capacitance versus voltage curve that makes it somewhere between undesirable and unusable in a filter application depending on topology. It will also not have good temperature stability, but that might be less important in an indoor environment.
The capacitance vs. voltage curve doesn't matter for this application. See for example this X7R description, page 14:
http://www.ece.ucdavis.edu/vcl/asap/asap_v1/docs/X7R_C.pdf
Capacitance will change max. 10% and this is for full voltage rating. My example was rated for 16V, but it is only used for microvolts (there is a simple voltage follower at the side of the active electrodes). I would expect that it is very stable and would be good for a high pass filter.
Temperature is maybe just 5%, for room temperature in winter and summer. But it would be not important anyway for an EEG filter, if your cutoff frequency is 0.7 Hz or 0.6 Hz.
But the filter will introduce some noise. There is a nice calculator for the thermal resistor noise:
http://www.daycounter.com/Calculators/Thermal-Noise-Calculator.phtml
At 20°C temperature and 10 Hz bandwidth, a 1 megohm resistor will create 0.4 uV noise. Might be in the range of the EEG signal.
I can't locate a C versus V plot on the data sheet you linked to, but the plot below is one I made for a couple of 1uF 1206 X7R capacitors, one rated at 25V and one at 50V. Data taken with an HP4192A impedance meter and internal bias voltage generator. The bias cycles so as to yield the classic "butterfly" C versus V response.
In some limited cases, where one does not care about distortion, linearity, accuracy and low Q / high D for the capacitor, an X7R dielectric might be suitable. It might well work with zero bias voltage and a microvolt signal level, but I do not wish to leave the impression that an X7R capacitor makes sense for filters except in extremely low performance applications.
I would draw your attention to a couple of points in the plot. First, note that there is significant capacitance change with applied voltage even about the 0 volt point, although it is somewhat flatter there than other parts of the C versus V curve.
Second, note the hysteresis exhibited in the plot, due at least in part to stored charge. Not a good thing for a filter that will be working at low frequencies, to say the least.
In addition, X7R capacitors are well known to exhibit piezoelectric effect where applied voltage causes a change in mechanical dimensions and, of more importance in a filter application, where board flexing will cause an induced voltage in the capacitor. And, the board flexing may be something as trivial as nearby fan coupling or even air pressure changes from sound waves at some distance. This will also be a problem at the microvolt level.