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RC Power filters and adding impedance to power nets

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Junior Member level 2
Jul 24, 2015
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I need to decouple two circuit blocks.
The spec for the low noise block is 10mvpp ripple max.

Adding 3 100uF ceramic capacitors allows me to achieve my 10mvpp spec
However this will be a costly solution.

I have been advised an RC filter in the power may be effective and simulation with 10R/100uF does look like it will bring me into my 10mvpp spec.

However I thought it was bad practice to add impedance to a power rail - as I am adding a 10R resistor
(If I reduced R, would have to increase C therefore defeating the object of using RC as cost saving measure)
I am struggling to find much info anywhere about this.

Could anyone advice if it should be acceptable to add 10R series resistor to power rail?
Is it bad practice?
Could it possibly cause EMC problems?

Would really appreciate any advice

It depends on your load regulation spec. If you need dynamic load regulation noise to be < 1% then the series R must be <1% of the change in load R.
For effective filtering the ripple is the Step current * Source ESR including cap parallel.

Ultra low ESR Caps can be expensive depending on ripple current and voltage.

What are your specs?

e.g. **broken link removed**
$2.65 3300µF 80Vdc 65 mOhm 2.6A Ripple current

The only drawback to adding resistance in supply rails is the voltage drop it introduces. If you can live with that, there is no problem. For sensitive ADC applications, for example where consumption is only a few mA, you will often see resistors of up to 100 Ohms in the supply lines to reduce conducted noise but if your application involves high current you need to consider both the voltage drop and the heat dissipated in the resistor. Don't forget the ESR of capacitors tends to increase as they age so if you only use capacitors, allow a generous safety margin to make it future proof.

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Fantastic Ive found an ADC app note showing 10R in series with power supply, this has removed my worries.
I want to add the RC filter before a 3V3 reg so the resistor voltage drop shouldn't be an issue, my circuit will see constant 3V3 hopefully.


- - - Updated - - -

Hi SunnySkyguy

I have drawn a quick sketch which includes my specs, please see below.

I know that three 100uF ceramic caps is enough to achieve my 10mvpp ripple spec on the 3V3
I am interested in using a single RC stage to try and reduce costs.
Quick simulation shows 10R/100uF should get me to 10mvpp
Was worried about R in power lines but Brian has reassured me about that.
I intend to prototype the RC and measure.
Would appreciate your thoughts


3V3 Decoupling.jpg

You didn't mention the interference frequency.

Series R filter is always an option, within the natural limits of acceptable voltage drop.

My standard filter means for switched power supply interferences are ferrite beads in the several 100 to 1kohm (ferrite bead "Z" specifies the impedance at 100 MHz) range, they have still useful reactive impedance in the 10 kHz range. For lower interference frequencies also cored filter chokes. Resonances and possible ringing with pulsed load should be considered, however.


Yes sorry, the spikes of noise I am trying to filter are in the range 7KHz - 15KHz


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