What type of capacitors do I use?

Hello,

I am trying to build the circuit in the picture supplied but I am not sure what type of capacitors I should be using (I am very new to this!). Would anyone be able to send a suggestion my way?

It is intended to be used as a capacitive sensor for an ECG.

Thank you very much in advance!

Circuit.png

Hi Fiona,

I assume when you ask what "type" of capacitor, you are asking if you should use an electrolytic cap. or a ceramic cap (since the value of the capacitors are clearly indicated in your diagram).

Electrolytic capacitors must be polarized in the correct direction, and thus the circuit element for an electrolytic cap will clearly identify the correct orientation of the cap.

In the diagram you posted, there is no indication of what direction the capacitors should be arranged, and thus they are likely ceramic capacitors.

Was this your question?

Dustin

Here I would use a polyester capicitor.

TwoChain:
Hi Fiona,

I assume when you ask what "type" of capacitor, you are asking if you should use an electrolytic cap. or a ceramic cap (since the value of the capacitors are clearly indicated in your diagram).

Electrolytic capacitors must be polarized in the correct direction, and thus the circuit element for an electrolytic cap will clearly identify the correct orientation of the cap.

In the diagram you posted, there is no indication of what direction the capacitors should be arranged, and thus they are likely ceramic capacitors.

Was this your question?

Dustin

Yes that was my question! Sorry for the poor explanation!

And thank you for the response Dustin!

Grumpy_Mike:
Here I would use a polyester capicitor.

http://en.m.wikipedia.org/wiki/Types_of_capacitor

Why would you suggest polyester capacitors Grumpy_Mike?

Thanks for the response!

Why would you suggest polyester capacitors

Much lower dielectric loss than a ceramic.

As a rule of thumb, most (definitely not all!!!) Electrolytic caps start at 1µF. I've seen Electrolytics lower and other non polarized caps (Ceramic, Polyester Film, Mylar, etc) higher than 1µF, so you can't rely on this every single time. The schematic leaves no doubt, though, as there are no polarity markings. That leaves you choosing between the various low value cap styles.
In the link Grumpy Mike posted, there is a chart showing various applications for the three main classes of diodes used in electronics (Ceramic, Film, and Electrolytic). Since the application is as a sensor, precision is probably a priority. Not knowing precisely what you'll be measuring/sensing, it still seems safe to assume that it fits closer to the category of a peak voltage detector than something like hf blocking... This leaves you squarely in the Film category.
Now, that being said, your circuit will probably work with ceramics as well. Ideally, though, a Film cap would be the right fit.
Quick reminder, that 0.22µF Cap is 220nF, and will likely be marked "224" on the body.

These capacitors are being used as coupling between amplfier stages so the properties
of the capacitor are not very critical. However plastic film are always recommended for
low frequency analog signal use due to their high linearity and absence of microphony.

Polyester, polycarbonate, polysulfone, PTFE, polythene, etc etc.

If the caps were part of an analog filter stage the precision of the value might be important
and 1% devices might be appropriate, but that should be marked in the diagram.

As an aside, any caps used in an op amp active filter to set the frequency will also be critical. Mylar are ok
for that application but capacitor technology has come a long way since Mylar caps were introduced so you
can probably find several other types more precise (like

Polyester, polycarbonate, polysulfone, PTFE, polythene, etc etc.