Noise filter - justification for better performance?

[explorick]

noise filter diagram

For the diagram below which circuit is effective for noise blocking. What is the justification for better performance.

 

[frankvh]

Re: noise filter

If we assume the noise is of a high frequency, so it's conducted by the capacitor & blocked by the inductor...

Some people will tell you your two circuits are the same. But if it were me, I'd go with the left-hand circuit.

The reason being that I have a philosophy that I want to minimise noise *currents*.

For the left circuit, the noise first sees the inductor, which provides the noise with a high series impedance, which reduces the noise current. Then it sees the cap, which presents a low impedance to ground. So you're going to get a noise current flowing, through the inductor, cap & ground, but it'll be low if the inductor puts up a decent amount of impedance.

For the right circuit, the noise first sees a low-impedance cap to ground. So the noise current flowing through the ground plane will be much higher.

That's my opinion, but I'm really curious to see what others think.

Frank.
https://www.frankvh.com

 

[LvW]

Re: noise filter

For the diagram below which circuit is effective for noise blocking. What is the justification for better performance.

Your diagram shows electrical components. Therefore, the input and output parameters have to be electrical ones and -- "noise" is no electrical parameter.
You must tell us if you speak about noise imposed on a current or on a voltage.

 

[FvM]

noise filter

The effect of a passive filter can't be determined without knowing the source and load impedance. Also the answer to your question can be different.

 

[explorick]

Re: noise filter

The noise input is a voltage input with high frequency noise that needs to be filtered. The load impedance could be assumed high such as a gate input.

 

[LvW]

Re: noise filter

The noise input is a voltage input with high frequency noise that needs to be filtered. The load impedance could be assumed high such as a gate input.

In this case, with reference to the second circuit you can ask yourself what happens when you (1) put a capacitor across a voltage source and when you (2) add an inductance without any current through it (because the load is infinite).
Does such a circuit make sense ?

 

[explorick]

Re: noise filter

Agreed that the second circuit doesnt make sense if the input impedance is high. What would be the case if used as a filter on the power supply line of an IC?

 

[LvW]

Re: noise filter

Agreed that the second circuit doesnt make sense if the input impedance is high. ..........

A voltage source has a low source resistance (ideally zero) !

 

[explorick]

Re: noise filter

I was wrongly typing as input impedance instead of output impedance. What would happen to the mentioned ciruits if they are used on power line?