X5R caps in EEG circuits - A bad design practice? - Page 1
Forum section: electronics / projects

author: SolarSunrise
author_rank: Regular Contributor
timestamp: March 02, 2014, 02:57:12 pm
content: Yes I know X5R's are terrible with piezo electric effect, terrible temperature coefficients, and so on. However, on numerous EEG designs (where uV precision is needed), I saw many many X5R's used as filtering caps. For example, the filtering caps used in ADS1299EEG (schematic & BOM -http://www.ti.com/lit/ug/slau443/slau443.pdf) are ALL X5R's and not even a single C0G was present in the BOM.So what's going on? Is Texas Instruments trying to shave few bucks off of their demonstration kit? Or is C0G an overkill for EEG designs... Doesn't make any sense at all...

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 02, 2014, 03:06:56 pm
content: X5R's are OK - just a lower temp range than X7R.Y5Vs are the crap ones.

author: SolarSunrise
author_rank: Regular Contributor
timestamp: March 02, 2014, 03:10:29 pm
content: Quote from: mikeselectricstuff on March 02, 2014, 03:06:56 pmYes, I was aware of that difference. My real question was though, are X5R's adequate for precision applications? I heard so many horror stories of piezoelectric effect and temperature coefficients that I thought C0G's were the only adequate solution to precision circuits...

author: megajocke
author_rank: Regular Contributor
timestamp: March 02, 2014, 03:42:07 pm
content: Whether they are adequate or not would depend on how they are used in the circuit. Most of those caps are for decoupling and input EMI filtering.Also, low signal levels and high precision are not necessarily related. I'd suspect EEG to be quite a low dynamic-range application with quite low precision requirements if the waveforms are just going to be shown on an oscilloscope-like display.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 02, 2014, 11:31:49 pm
content: Yes, bad design practice in general. I never use anything worse than X7R.In that particular design, it looks like just dumb bypassing of the inputs? So all the actual bandpass filtering is done inside the SoC? Doesn't matter. Microphony might matter, but that may also be filtered out too (transients from tapping on the PCB are usually in the sub-10ms range, corresponding to acoustic resonant modes of the PCB).If all the analog front end, filtering and isolation stuff were built in discrete, rather than being integrated, it would absolutely be a problem to use X7R. They would probably want a very accurate filter, and X7R does not offer precise values to achieve that. C0G would be just fine, or good old film capacitors (I believe PPS dielectric is supposed to be excellent, and is available in SMTs).Tim

author: SolarSunrise
author_rank: Regular Contributor
timestamp: March 11, 2014, 05:27:35 pm
content: I heard somewhere that dielectric absorption affects low frequency content. Is that true? A PPS has almost 6 times less than ceramic caps (C0G) therefore are PPS's a better solution for filtering?Sorry for going a little off topic there...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 11, 2014, 09:13:58 pm
content: Yes, dielectric absorption is a low frequency thing (<< 1Hz?). If you imagine, for example, every capacitor in your circuit has a parasitic R+C in parallel with it (R in the megs or gigs, C some small fraction of the main C, say, 1nF or less out of a 1uF cap), you can qualitatively evaluate the frequency response. The main consequence is a small gain/phase shift error at low frequencies (for frequency-domain signals), or poor and history-dependent settling time (for time-domain signals).Tim

author: doktor pyta
author_rank: Frequent Contributor
timestamp: May 14, 2014, 10:10:41 pm
content: Quote from: mikeselectricstuff on March 02, 2014, 03:06:56 pmUsing that Y5V capacitors You can build the worst VCO in the worldEducational example:http://rfscientific.eu/simple-vco