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Old 10th January 2012, 07:42 PM   #1
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Default High pass filter capacitor

I am building a small amplifier with first stage like this:

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and have a question about the input cap. Is it important to have an input cap for this high pass filter with low leakage current? I want to use a cap 220-330uF with 100ohm resistor in series. The input voltage will be between 5-35VDC. The amp will measure DC noise.
Thanks.
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Old 10th January 2012, 09:09 PM   #2
DF96 is offline DF96  England
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Any leakage current through C will develop a DC voltage across R2. If your aim is to measure AC noise on a DC supply, why not use a smaller cap and larger value resistors? Leakage through most non-electrolytic caps is negligibly small. You could add a back to back pair of diodes at the inverting input to protect against problems.
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Old 10th January 2012, 11:18 PM   #3
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OK, I didn't realize that. So lets see, metalized polyester like WIMA 4.7uF + 5k6 resistor would probably do about the same fc.
I don't know anything about back to back diodes and their function, and how to connect them either. Can you please enlighten me on this? Many thanks.
I am trying to build LNMP from Tangentsoft.
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Old 10th January 2012, 11:26 PM   #4
DF96 is offline DF96  England
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The idea is to protect the opamp input from capacitor failure. Some opamps have them built in, so provided the input resistor is large enough to limit the fault current no extras are needed.

You might be able to go to smaller caps and bigger resistors. It depends on the trade-off between resistor noise and opamp noise, and how low in frequency you wish to go.
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Old 10th January 2012, 11:30 PM   #5
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OK, many thanks.
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