How use ferrite bead to reducing noise from charge pump?

[sghosh]

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!

 

[analogLow]

I guess what you trying to do is filter any noise before putting it to the opamp input. Here is a good discussion on the filter (see post#2):
https://www.edaboard.com/threads/219943/

Also, when I build this type of low pass filter, I use the following: ferrite bead (see Farnell part#163-5723) between charge pump output and opamp input, both 10uF ceramic cap and 10nF tantulum cap from opamp input to ground.

Hope that helps,
analogLow

 

[ashokok]

both 10uF ceramic cap and 10nF tantulum cap from opamp input to ground.

if i am right 10uF "tantalum cap" and 10nF "ceramic" cap.


P.Ashok kumar

 

[FvM]

Ferrite beads are specified by an impedance value (Z@100 MHz). The suggested Farnell part (Würth 742 792 091, 1500 ohm/200 mA/0805) is basically a good choice. At low frequencies (< 50 MHz), it's equivalent to a 1.2 µH choke. LC filters have a resonance frequency, with 10 µF you get about 45 kHz, but ferrite bead series resistance and additionally capacitor ESR are causing a sufficient low Q in this case.

The filter will reduce the MAX232 140 kHz switching frequency ripple by about factor 10 and the probably more problematic spikes to effectively nothing. You'll notice however, that a lower impedance ferrite bead or lower C won't work in this case. Ferrite beads are sensitive to DC current saturation by the way, you won't be able to use the filter up to the rated 200 mA current with full performance.

P.S.: You should also pay attention to the "noise" caused by the MAX232 at the 5V node. I experienced an eval board, where the MAX232 caused occasional failure of an USB PLL. So sufficient bypassing and possibly filtering may be reasonable.

 

[analogLow]

if i am right 10uF "tantalum cap" and 10nF "ceramic" cap.


P.Ashok kumar

yes correct ... I meant 10uF "tantalum cap" and 10nF "ceramic" cap