0
\$\begingroup\$

I am Using an NET-35B SMPS from Meanwell to power a strain gauge type pressure sensor. I am using 24 bit ADC to obtain the output of the sensor. I am getting Fluctuations and I think that those fluctuations are induced by power supply noise (Ripple= 80mV p-p).
I want to reduce the noise from power supply using a filter.

Questions :
1)How should I design a filter for above-given specs?

2)I have tried adding capacitors(Random)previously and it works, but I want to know how it exactly works and how precise filter can be designed ?

\$\endgroup\$
  • 1
    \$\begingroup\$ The first step would be to determine the frequency of the noise. Without that it is really not possible to apply a design process. \$\endgroup\$ – mkeith Feb 13 '17 at 5:33
  • 1
    \$\begingroup\$ The 2nd step would be to show the circuit diagram. My car's making a strange (but unspecified) noise. \$\endgroup\$ – Andy aka Feb 13 '17 at 8:41
  • \$\begingroup\$ In truth, sometimes I just apply the biggest RC I can get away with. R is limited by the DC error it introduces (due to finite input impedance of op-amp). C is limited by package size. Anything bigger than 10uF is usually not practical in terms of size or cost (for small, high-volume products). The RC time constant must be kept small enough so that the signal of interest gets through the filter. Since I don't know any of the necessary parameters for your system, I will leave it at that. \$\endgroup\$ – mkeith Feb 14 '17 at 4:08
0
\$\begingroup\$

Have you added decoupling capacitors to all the ICs? add 0.1uF on each supply pin of the ICs, sometimes the datasheet of the component specifies the capacitor that you need and make sure the decoupling capacitors are close to the component.

Also a decoupling capacitor of 22uF or 47uF should be added on the output of the power supply, as noted in the datasheet of the NET-35B using 47uF with 0.1uF will make ripple if you are working on a 20Mhz frequency, a decoupling capacitor would filter noises with high frequency, there is really no rule in the capacitance needed most people add 0.1uF or 0.01uF sometimes they go with what is recommended in the component datasheet, if an IC pulls a very big amount of current such as a mosfet driver you would need to use a 10uF Bulk capacitor in parallel with the 0.1uF the reason for this is that a capacitor has a small resistance which is called ESR increasing the current would increase the voltage for a resistor and this isn't good for our goal, adding a capacitor in parallel is like adding also a resistor in parallel 2 resistors in parallel would decrease the resistance and that means will decrease the voltage, also adding a capacitor with a big capacitance would reduce current swing in and it's good for eliminating low frequency ripples.

If the ripples still occurs you would need a ferrite bead to add on the supply line, the ferrite bead eliminates chosen frequency by varying it's resistance according to the frequency, i can't help you much on this one as you would need to know the frequency you're working with and the frequency you need to eliminate. you can see this link from TI for making DC filter and this link for Choosing and Using Ferrite Beads this link also have previous articles about bypass capacitors.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.