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

I would like to use an 8-order LPF switched capacitor: MAX7405 datasheet

The cut-off frequency can 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.

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?

• should be internal clock – jhyap Jul 23 '14 at 7:22
• From the description, it says the fc:fclk = 1:100 – jhyap Jul 23 '14 at 7:24