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Old 14th April 2008, 01:54 PM   #1
Pulse-R is offline Pulse-R  Australia
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Default SMD capacitor - how to tell the value?

I have a circuit here which has some ceramic SMD caps in a MFB filter stage.
According to the circuit diagrams, the value should be 270pf, and 3n9. I know these are incorrect, as the resistor values are different, and the caps must have been changed to match the new r values, but when I measure them in-circuit, I get wrong values also (of course).

I guess what I'm asking is, can the values be determined without pulling them off the board, and risk damaging them, or should I just re-design the filter stage to remove them and use MKP's or something instead...

hmmm

maybe a silly question, but I thought I'd ask anyway
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Old 14th April 2008, 05:04 PM   #2
kevinkr is offline kevinkr  United States
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You've pretty much figured it out.. Ceramic SMD caps are almost never marked, and in circuit measurements may result in misleading results. Sometimes if you are very careful removing them you can measure them once they have cooled down. (In some cases removing other identifiable components first will allow you to make an unambiguous measurement without removal.) You'll want to replace those ceramics with something a little more audio friendly anyway. Watch out when you redesign that MFB (multiple feedback - is it really? ) filter, they aren't quite as intuitive as the usual vcvs based filter.
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Old 15th April 2008, 10:57 AM   #3
rossl is offline rossl  United States
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I would just take those out and replace the 270p with a mica cap of whatever you calculate your new value to be, then replace the 3n9 with a high quality film cap.

Polypropylene cap may be hard to do because the cap will be much larger than the SMT pads, but it will be worth it, sonically speaking, to get those cheap ceramics out of there.
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Old 15th April 2008, 12:32 PM   #4
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The 270pf will almost cetainly be a C0G type of dielectric. These are superior to mica caps, (at audio frequencies), as they do not have the high dielectric absorbtion that mica has.

The 3900pf is likely to be an X7R dielectric. This is worth replacing with a better type.

Avoid Y5V high-K dielectric ceramic capacitors like the plague! They have a VERY non-linear change in capacitance with applied voltage. You can get horrendous low-frequency distortion with these type when used to pass audio.
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Old 18th April 2008, 02:19 AM   #5
rossl is offline rossl  United States
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The 220p mica cap will have very little "soakage" detectable at audio frequencies.

"soakage" is an imporatant spec for an audio coupling cap. C0G ceramics have little "soakage". This is true.

The cap in question is most likely used for compensation and not coupling.

mica caps have less dielectric absorption than a lot of ceramics.

Dielectric absorption is frequency dependant.

Anyway, the 220p is a HF compensation cap. Not meant for audible frequencies. It is for keeping the op-amp happy.

Surface mount caps have low lead inductance and are well suited for bypass and compensation purposes.

I use a low pF mica to keep the op-amps happy.
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Old 18th April 2008, 05:42 AM   #6
Pulse-R is offline Pulse-R  Australia
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Hi Rossl,

This is an MFB filter stage - no coupling cap for RF, it is 2nd pole of filter in feedback loop of opamp.

I think it will be easier to just replace this stage with a simpler filter.

Thanks all.
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