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.
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.
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.
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.
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