Re: Noise reduction: Inductor vs. Capacitor

jidan1@xxxxxxxxxxx wrote:
Hi,

It is common to use a bypass capacitor connected in parallel from VCC
to GND to reduce noise on the power line. I have seen also inductors
connected in series to do the same thing. I have seen also both (cap &
inductor) being used at the same circuit to reduce noise on the power
line. In my opinion, the caps are smaller and more cheaper than
inductors, so why use an inductor?

Keep in mind that the voltage across a capacitor has to change, in order for it to deliver current to a load. In the case of a bypass, this voltage can be reduced to an arbitrarily low value by making the capacitor arbitrarily large. But for any practical capacitor, the voltage never gets all the way to zero change.

Let me give you an example of where a combination of capacitor and inductor worked well. I have a magnetometer that requires a fairly large pulse of current (80 mA peak) to activate it, 65000 times a second. I want to phase lock it to a crystal oscillator that runs on the same supply. Any supply bounce caused by the magnetometer pulse tends to influence the crystal oscillator, so that it is not an independent frequency reference.

No matter how large a capacitor I tried as bypass at the magnetometer, I still saw influences at the oscillator when they got near the same frequency. Very large capacitors had increasing series inductance that kept them from ideally lowering the bounce at the edges of the pulse. But once I got up to a 10 uF ceramic at the magnetometer, even if the supply line was long and inductive the +5 supply, the magnetometer had a stable enough supply to work fine. So I picked that value of bypass but added a small inductor in series with the +5 line to that bypass. There is a measurable but harmless bounce at the magnetometer, but the inductor isolates that small bounce from a separate bypass capacitor at the crystal oscillator, so that it now operates essentially independently from the magnetometer. In effect, the inductor in the +5 line forces all the voltage noise generated by the magnetometer pulse to occur on the +5 line (gives that line elasticity), and the ground referenced output has a clean ground line to be compared against (looks more rigid by comparison), while the rest of the +5 system is nice and quiet.
.

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