I am not very good with electrical components so any help on this circuit would be greatly appreciated! I googled MKT "metallized polyesther flim" capacitors and am not having much luck in finding them. Can I use an alternative to what they have listed for the cap's here? Like a ceramic disc or an electolytic or what?
Ceramic would probably work. You can get "monolythic" or "dipped" ceramics, which are better (mechanically) than "discs". Typically, film capacitors are used because you can get tighter tolerance, but they didn't mention tolerance and I don't think it's critical here.
If you live in the U.S., you can order film capacitors form Jameco (they might be mylar film) and I'm sure Digi-Key has a good selection. If you do live in the U.S. get ahold of both catalogs. Sometimes it's easier to flip-through the printed-pages than to search web pages. Oh... Parts Express is probably good for film capacitors too.
Electrolytics are not what you want. They are polarized and I don't know if you can get them in such low values.
I don't think MKT caps are necessary, rather just plain old ceramic. The cap
on Vdd is simply a standard bypass cap, as used on ALL Vdd busses, and
tolerance is completely unimportant, and people use just about any values
between 0.01 uF and 0.47 uF.
The 10nF caps across the 22k R's are simply in there to reduce high-frequency
noise in the opAmp signals. Since you're really measuring low-frequencies here,
close to DC, tolerance is also non-critical.
The 3-dB low-pass frequency for the opAmps is approx
Ceramic capacitors are non-linear, microphonic and often have very low tolerances (+22/-56% in Z5U dielectrics). Ceramics are used primarily for decoupling or energy-storage, not as active filter capacitors for which plastic film types are superior. Some classes of ceramic aren't too bad, but unless you know what class you have assume the worst.
In this circuit the 10nF caps are supposed to be matched in value to get a particular low-pass filter behaviour - otherwise the response is a slightly more complex low-pass filter behaviour. This may not be critical.
Basically good practice would avoid ceramics (other than class I perhaps) in this kind of instrumentation amplifier circuit except for supply-decoupling. It may not be that critical to you of course, but that's why MKT's are specified for this circuit.
DVDdoug:
Typically, film capacitors are used because you can get tighter tolerance, but they didn't mention tolerance and I don't think it's critical here.
Class 2 and 3 ceramics (X7R/X5R and Y5V/Z5U) have temperature and voltage coefficients. So it isn't a matter of tolerance, but effective capacitance. Class 2 ceramics can change +/- 15% with temperature AND voltage. Class 3 is far worse. While films, such as polypropylene, will only change +/- 3% with temperature (and not voltage).
For hobby electronics projects, the temperature coefficient doesn't usually matter. However, ceramic's voltage coefficient is a concern and often over looked.
MarkT mentions Class 1 ceramics (C0G), which are actually more stable than film. However, C0G dielectrics generally have a very low K and it is hard to find caps with much capacitance. (Ideal for filters and that's about it.)
