"zero" biased polarized coupling caps

May 19, 2006 8 Replies
A

I've have some cascaded op-amp stages, each providing +20dB, and worst case DC offset is +-30mV at the output of each stage.



I want to AC couple each amplifier stage with polarized capacitors. I cannot find any documentation recommending the use of polarized capacitors for continuous reversed bias, even when the bias is very small (< 50mV).



However there is the following PDF from kyocera,

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basically saying tantalums should never be designed in circuits where a continuous reverse bias exists and since they fail to present a definition of revers biased I find it hard to believe a microvolt is going to do harm them. So whats the truth about using tantalum's in such applications ?


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M

What size of capacitors do you need? I.e. do you really need polarized caps?

Meindert

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T

I've graphed the I-V characteristic of electrolytics and it looks pretty flat up to 25% of the rated voltage, reverse. It improves with time when voltage is applied.

I don't see anything wrong with it.

Tim

-- Deep Fryer: a very philosophical monk. Website:

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P

What kind of signal ? Audio ?

Graham

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S

Hmm. AVX (nee Aerovox) is owned by Kyocera (Kyoto Ceramics) eh? Did not know that.

I've used aluminum electrolytics in such applications with no problems, in volume applications over many years. Tantalums- I have a bad translation from Japanese that seems to say (not what it says, but it doesn't make sense the way they wrote it) that for "short periods" (not defined) 10% of rated voltage or 0.5V max is okay. Which fits with AVX pdf.

I'd guess (which is worth what you paid for it) that -50 or -100mV should be okay over temperature and time, and -30mV should be quite safe.

Best regards, Spehro Pefhany

"it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
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A

They are 10uF into inverting amplifiers with Rin=2.2k, Rf=22k (gain -10) with a 7Hz lower cutoff. Freq of interest is 100Hz~100kHz. Its for an instrument to measure AC signal levlel at a single frequency. The excessively low cutoff frequency to minimize phase error. DC coupling is out of the question at +60dB. Would you suggest X7R/X5R ceramics ?

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S

Maybe. Microphonics might an issue. Can you increase the resistances by 100:1 and use some nice 0.1uF film caps, or will bias currents or something else screw things up?

Best regards, Spehro Pefhany

"it\'s the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
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M

Indeed. Although Spehro's reply got me thinking. I didn't know ceramics suffered from microphonics.

Meindert

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B

They don't suffer from it, ceramics ENJOY being microphonic!

Okay, sorry. But I really must try making some "electronic drums" using nothing but ceramic capacitors. Too bad they're so brittle.

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