High pass filter capacitor

cobretti · 10th January 2012, 07:42 PM

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

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.

DF96 · 10th January 2012, 09:09 PM

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.

cobretti · 10th January 2012, 11:18 PM

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.

DF96 · 10th January 2012, 11:26 PM

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.

cobretti · 10th January 2012, 11:30 PM

OK, many thanks.

10th January 2012, 07:42 PM	#1
cobretti diyAudio Member Join Date: Feb 2011 Location: Taxachusetts	High pass filter capacitor I am building a small amplifier with first stage like this: 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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10th January 2012, 09:09 PM	#2
DF96 diyAudio Member Join Date: May 2007	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.

10th January 2012, 11:18 PM	#3
cobretti diyAudio Member Join Date: Feb 2011 Location: Taxachusetts	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.

10th January 2012, 11:26 PM	#4
DF96 diyAudio Member Join Date: May 2007	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.

10th January 2012, 11:30 PM	#5
cobretti diyAudio Member Join Date: Feb 2011 Location: Taxachusetts	OK, many thanks.

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