Lucy Lin

How to calculate the oscillator capacitor value for a switched capacitor (MAX7405)?

Discussion created by Lucy Lin on Aug 10, 2016

Hello, everyone!

I am thinking of  using an 8-order LPF switched capacitor: MAX7405 http://www.kynix.com/Detail/407475/MAX7405ESA%2B.html

The cut-off frequency should be set by simply connecting an external oscillator capacitor, Cosc.

I would like to design a 50Hz cut-off frequency but I have doubts on my calculation for the given formula in the datasheet.

max7405 datasheet

Since I want to have a 50Hz fC, I needs the fCLK to be 5000Hz

From the given equation:

fOSC(kHz) = (K * 10^3) / Cosc; Cos in pF , K = 34 for max7405 
5000(k) = (34 * 10^3) / ( C * 1pF)
5000 = 34 / ( C * 1pF)
C = 34 / 5nF
C = 6800M

By using the above equation, I calculated an extreme large capacitor with Mega value which is not reasonable.

Am I calculating it in the right way?

Best regards,

Lucy Lin

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