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How use ferrite bead to reducing noise from charge pump?

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sghosh

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Old senior design project and in schematic I see he uses ferrite bead and capacitor after charge pump voltage output befor connecting to opamp input

Circuit very simple:

1. charge pump uses MAX232 to get +- 12V output from 0-5V input
2. +- 12V output rails from MAX232 having ferrite bead and capacitor in line
3. output of above connected to Vdd/Vss of dual rail opamp

Any way of knowing how much ripple reduced from output of charge pump because of using ferrite bead and capacitor in line?

Any measurement to select value of ferrite bead and capacitor?

I also interested as I see ferrite bead and capacitor in line on Arduino board also but they give no value in schematic!
 
Hi,

These things vary in the inductance they introduce and of course the capacitor value will change things a lot too depending on its value. The best you can do probably is to try to find out the manufacturers specs for the bead in question and take it from there.

You could try to measure the ripple before and after the bead. The wire lead connecting the circuit already has some inductance, the bead just increases the inductance of the wire, so you would test the same way you test an inductor and capacitor filter. It may be a small inductance however so it may only reduce the much higher harmonics and not actually affect the ripple itself too much unless the switching frequency is high. The reason for using them is often just to reduce RFI not to really help the ripple that much.
 
As MrAl noted, ferrite beads have a low inductance that only affects the higher RF frequencies, generally above 10MHz, so it will likely have little effect on ripple. They are primarily used to suppress high frequency harmonic EMI that is generated by fast switching edge speeds in digital circuits and switching power supplies.
 
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