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Old 29th February 2012, 12:24 PM   #1
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Default Choosing the right cap

Hey,

I'm currently designing a CODEC board for the TLC320ad77 from TI. That is my first project of this kind, so I'm having quite a hard time.

This is the datasheet:
http://www.ti.com/lit/ds/slas194/slas194.pdf

At the application section, page 25, there is an example of analog front-end circuit. In the negative feedback of both opamps, there is polarized caps of really low values (10pF). What kind of cap should it be? And is C4 electrolytic?

Thank you for your help!
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Old 29th February 2012, 12:50 PM   #2
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C4 drawing seems electrolytic.
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Old 29th February 2012, 12:52 PM   #3
qusp is offline qusp  Australia
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very small value polarized caps are probably tantalum. but high quality ceramic like np0/c0g type will also do
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Old 29th February 2012, 01:05 PM   #4
oshifis is offline oshifis  Hungary
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The person who created the diagram had only this symbol at hand
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Old 29th February 2012, 01:34 PM   #5
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Thanks for answering.

Quote:
C4 drawing seems electrolytic.
yes, C4 seems electrolytic, but I can't find any electrolytic cap of 1nF

Quote:
very small value polarized caps are probably tantalum.
It seems hard to find so small values with tantalum caps too. Farnell min is 0.1uF.

Quote:
The person who created the diagram had only this symbol at hand.
oshifis, this is exactly what I thougth ^^.
I don't really understand the purpose of having polarized caps here (talking about C2 and C3).
Do you think I can use non-polarized caps?
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Old 29th February 2012, 03:15 PM   #6
qusp is offline qusp  Australia
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tbh i didnt look at the schema, but rather just went by your description of where it was. My guess is oshifis is dead on with that, yes you can use a nonpolar cap, thats why i recommended the quality ceramic in np0/c0g type. for highest performance an RF grade mica, teflon or thin film cap should be used, but good ceramics will do just fine.
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Old 29th February 2012, 03:46 PM   #7
rdf is offline rdf  Canada
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Quote:
Originally Posted by nirv4nh4ck View Post
oshifis, this is exactly what I thougth ^^.
Figure 5-2 shows the correct symbol for a feedback cap. My guess would be student co-op term or hours of work directly adjacent a long weekend.
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Old 29th February 2012, 04:29 PM   #8
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That's NOT the symbol for electrolytic capacitors. It is the US symbol for regular non-polarized capacitor (that might have an outer plate or not).

Click the image to open in full size.

Last edited by SoNic_real_one; 29th February 2012 at 04:33 PM.
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Old 29th February 2012, 05:08 PM   #9
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Oups... this is confusing...
So, there is not a single polarized capacitor in my schematic.
And, in this case, why will c6 symbol be different? Just for fun, I guess.
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Old 29th February 2012, 05:59 PM   #10
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The meaning is that the capacitor with the curved plate MIGHT be or not polarized. For small ones of course they will be non-polarized, but in case that they have a polarity or a metallic case, TI showed the correct mounting of that case (usually the side with larger signal or lower impedance).
For exemple C1 and C20 MIGHT be polarized, with ground terminal (negative) at position #2. You COULD use a non-polarized one if you find one, but would be an increased cost.
The ones that don't have rounded plate, usually means that they NEED to be symetrical and non-polarized. Like a ceramic or mica one, not a rolled one...

Last edited by SoNic_real_one; 29th February 2012 at 06:06 PM.
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