High value C0G/NP0 SMD ceramics - Page 1
Forum section: electronics / projects

author: sirhaggis
author_rank: Regular Contributor
timestamp: January 03, 2017, 02:21:56 pm
content: I am implementing an audio circuit that uses the TI PCM5121 DAC here:http://www.ti.com/lit/ds/symlink/pcm5121.pdfThe manual is a little confusing and seems inconsistent in places, but on p66 on layout recommendations it specifies the use of '2.2uF COG Ceramics' for a number of the signal caps. This layout is unclear and seems to contradict the typical application on p58, however I'm presuming it is referring to the VNEG and CAPM caps. The problem is that I cannot seem to find any C0G ceramics that have a value this high - the highest I can find on digikey/mouser is 0.47uF. Am I missing something, or is there somewhere else that sells C0G/NP0 ceramics at this capacitance?ThanksLuke

author: gamalot
author_rank: Super Contributor
timestamp: January 03, 2017, 02:48:38 pm
content: The only one place in the datasheet where mentioned NP0/C0G type capacitors is the output filter in page 42, and their value is 2.2nF, not 2.2uF

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 03, 2017, 02:51:03 pm
content: Quote from: gamalot on January 03, 2017, 02:48:38 pm..and if you're looking in Digikey, that's 2200pf - DK don't speak nanofarads!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 03, 2017, 03:21:37 pm
content: And if you DID need uF -- look for stacked PPS film. It's as good as C0G, but actually available. (The price is about as bad, though.) Or, a tantalum or two might not be bad, or just use an old fashioned THT film cap.Tim

author: sirhaggis
author_rank: Regular Contributor
timestamp: January 03, 2017, 03:22:57 pm
content: QuoteActually it does - if you look under the layout diagram on p66 it says "Capacitor values unless noted should be 2.2uF COG Ceramics" (spelled COG not C0G). This statement seems inconsistent with the application as some of the caps that are not 'noted' are not 2.2uF according to the circuit diagram.

author: sirhaggis
author_rank: Regular Contributor
timestamp: January 03, 2017, 03:30:07 pm
content: QuoteFilm caps look like an option, but geez... $ for a single SMD cap!!

author: gamalot
author_rank: Super Contributor
timestamp: January 03, 2017, 03:38:16 pm
content: Quote from: sirhaggis on January 03, 2017, 03:22:57 pmI think that's a typo, they mean 2.2nF C0G ceramics capacitors

author: bktemp
author_rank: Super Contributor
timestamp: January 03, 2017, 03:53:37 pm
content: +1 for the typo.If you use 2.2nF for every capacitor without a designation, it makes sense.Why are so many layout examples found in datasheets impossible to do? Finding 10uF capacitor that fits next to pins with 0.65mm pitch is impossible. Placing both capacitors on AVDD/AGND/VCOM pins like shown in the example requires something like 0201 or even 01005 parts.But it is still better than the recommended crystal oscillator placement found in many Microchip datasheets:You need a crystal with less than 0.5mm length if you want to follow the recommended layout!

author: sirhaggis
author_rank: Regular Contributor
timestamp: January 03, 2017, 04:01:39 pm
content: Yeah you are probably right... TI has some horrible datasheets sometimes! It's like they get the work experience students to put them together.That still leaves the question of what caps to use for the 2.2uFs on VNEG and CAPM. Their non-polarized so I'm presuming ceramic or film caps, but my understanding is that ceramics are not necessarily great for analog? Perhaps film caps are the way to go for these, or are there other (SMD) options are are not ridiculously expensive and will have low distortion characteristics?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 03, 2017, 04:04:19 pm
content: Quote from: sirhaggis on January 03, 2017, 04:01:39 pmThese aren't for analogue - they're for power decoupling and charge pump - cearmics will be just fine.Chips like this usually have an eval board available, so look at what they've used on that if anything's not clearhttp://www.ti.com/tool/pcm5122evm-u

author: bktemp
author_rank: Super Contributor
timestamp: January 03, 2017, 04:06:10 pm
content: C0G/NP0 are fine for analogue signals. They don't show the large voltage or temperature dependency like class 2 or higher ceramic capacitors.

author: sirhaggis
author_rank: Regular Contributor
timestamp: January 03, 2017, 04:23:50 pm
content: QuotePerfect! That's exactly what I needed, it lists its schematic and part numbers. Thanks Mike!

author: Zero999
author_rank: Super Contributor
timestamp: January 03, 2017, 04:41:18 pm
content: Reading through the data sheet, the signal never passes through any large capacitors which are only used for supply decoupling and in a charge pump so aluminium or tantalum capacitors will be fine for the 2.2µF capacitors.