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suitable capacitance value for Sallen Key low pass filter

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Wildi

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It is more proper to use 10nF capacitor or 1nF capacitor for the low pass filter design?
 

Re: suitable capacitance value for Sallen Key low pass filte

Wildi said:
It is more proper to use 10nF capacitor or 1nF capacitor for the low pass filter design?

It really doesn´t matter; perhaps the consequence on the resistor values is more important.
 

    Wildi

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Re: suitable capacitance value for Sallen Key low pass filte

Mine to explain more about the consequences?

I mean, if i use higher capacitance, resistor value that will be used is smaller?
what consequence does it have?
 

Re: suitable capacitance value for Sallen Key low pass filte

Wildi said:
I mean, if i use higher capacitance, resistor value that will be used is smaller?
what consequence does it have?

Yes, that´s what I mean. If for a small cap the resistor is - let´s say - 6.8 Mohms, then it´s better to use the higher cap value and only 680 kohms.
But it depends on your impedance level and on the frequency range which both I don´t know.
 

    Wildi

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I am browsing through some online shop selling electronic components.
For capacitor, i saw some different voltage rating.
some is 200Vdc, some is 200Vac, some is 200V

any different? for the one with rating 200Vdc, does it means that it only can be used for dc voltage supply?
 

Re: suitable capacitance value for Sallen Key low pass filte

Don´t care about rating when you are going to design an active filter for supply voltages of 10-20 volts. Instead, watch the technologie : ceramic is best and electrolyt is worst.
 

    Wildi

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Re: suitable capacitance value for Sallen Key low pass filte

You forgot to attach the schematic of your filter for Us to see if the caoacitors have AC or DC across them.
I use metalized plastic film capacitors with an accurate 5% tolerance. Large values 330nF and 470nF are 63V rated and smaller values are 100V rated. They work with AC or DC. They are small and inexpensive.

Don't use electrolytic, tantalum or ceramic capacitors for audio frequency filters.
 

    Wildi

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Re: suitable capacitance value for Sallen Key low pass filte

Can you help me to check if this capacitor is suitable for me to use for my filter circuit? thanks.

**broken link removed**

freq range: 100 Hz to 10Mhz, and it is not for audio filter purpose. just a small school assignment on filter.
 

The ceramic capacitor has an accurate 5% tolerance but adds distortion and acts like a microphone. A metalized plstic film capacitor is better. In the Orient, a "green-cap" is a metalized plastic film type. J is 5%.

The lousy old LM324 opamp barely goes to 6kHz and I think your filter is 7.5kHz so the dropoff at higher frequencies might not work properly.
 

    Wildi

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Re: suitable capacitance value for Sallen Key low pass filte

Then is UA741 better?
 

The lousy old 741 opamp is 40 years old this year. It barely goes to 9kHz.

A TL074 or TL084 has a good response up to 100kHz.
 

Re: suitable capacitance value for Sallen Key low pass filte

Wildi,
Tradeoffs involve the effects of stray capacitances vs DC performance. Smaller capacitors require larger value resistors for the same filter characteristics. The higher the resistances are, the higher will be the errors associated with bias currents and offset currents. Smaller capacitors also mean the the effects of stray capacitances will be proportionaly higher.
Regards,
Kral
 

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