Will the decoupling capacitor cause the oscillator?

[davidwong]

decoupling capacitor oscillation

Since decoupling capacitor can be considered to be RLC.
Will the decoupling capacitor across the supply cause the oscillation?

 

[szekit]

i am guessing if u choose too small of a size for the decoupling cap, it might oscillate.

 

[lambtron]

Will the decoupling capacitor across the supply cause the oscillation?

When you say "decoupling cap across the supply" do you mean a cap across the regulator's output?

If so, what you are talking about is really an output filter cap rather than a "decoupling" cap. In this case, the cap will in fact help to stabilize the supply output, and many supplies require such capacitance to prevent output oscillations.

 

[davidwong]

Will the decoupling capacitor across the supply cause the oscillation?

When you say "decoupling cap across the supply" do you mean a cap across the regulator's output?

If so, what you are talking about is really an output filter cap rather than a "decoupling" cap. In this case, the cap will in fact help to stabilize the supply output, and many supplies require such capacitance to prevent output oscillations.

no, I do not mean the cap across the regulator output.
Just a decoupling capacitor across the vdd and gnd pins.
I think it should not cause any oscillation as i can not see any negative resistance across the normal VDD and GND.
Am I right ??

 

[lambtron]

Capacitance across a regulator's vdd/gnd will not cause oscillations (unless there is a more serious, unrelated problem with circuit). If fact, such capacitance is essential for stable and reliable operation of most integrated regulator devices.

 

[sachinagg77]

The decoupling capacitors alone might not lead to oscillations. While simulating to check for the oscillation problem, please include the parasitics [Capacitance, inductance, resistance] associated with the Input/Output Pins. These parasitics togther with the decoupling capacitors can lead to oscillations. Simulations are necessary to make sure that the circuit is stable.

 

[davidwong]

The decoupling capacitors alone might not lead to oscillations. While simulating to check for the oscillation problem, please include the parasitics [Capacitance, inductance, resistance] associated with the Input/Output Pins. These parasitics togther with the decoupling capacitors can lead to oscillations. Simulations are necessary to make sure that the circuit is stable.

It seem that it is impractical to simulate the osc by tran simulation if the design is large. e.g adding decoupling to MCU.
any simple and quick method ?
Also I still do not understand why it can cause the oscillation after including parasitics.
Where is the feedback to generate the osc.?

 

[Soben]

Since decoupling capacitor can be considered to be RLC.
Will the decoupling capacitor across the supply cause the oscillation?

I dont know for what application u refer too, but poor decoupling can cause oscillation. If your DC paths are not decoupled properly, they are active, that causes usually low frequency feedback which can end up to instability.

 

[davidwong]

Since decoupling capacitor can be considered to be RLC.
Will the decoupling capacitor across the supply cause the oscillation?

I dont know for what application u refer too, but poor decoupling can cause oscillation. If your DC paths are not decoupled properly, they are active, that causes usually low frequency feedback which can end up to instability.

I just add some on chip decoupling capacitor for the 8-bit MCU.
but I just wonder whether or not it will have any side effect.
as far as I know, the side effect are:
1) some leakage due to the MOS cap
2) the power noise may coupling from the decap to analog circuit if adding improperly.

 

[davidwong]

Since decoupling capacitor can be considered to be RLC.
Will the decoupling capacitor across the supply cause the oscillation?

I dont know for what application u refer too, but poor decoupling can cause oscillation. If your DC paths are not decoupled properly, they are active, that causes usually low frequency feedback which can end up to instability.

I just add some on chip decoupling capacitor for the 8-bit MCU.
but I just wonder whether or not it will have any side effect.
as far as I know, the side effect are:
1) some leakage due to the MOS cap
2) the power noise may coupling from the decap to analog circuit if adding improperly.

Added after 2 minutes:

Since decoupling capacitor can be considered to be RLC.
Will the decoupling capacitor across the supply cause the oscillation?

I dont know for what application u refer too, but poor decoupling can cause oscillation. If your DC paths are not decoupled properly, they are active, that causes usually low frequency feedback which can end up to instability.

I just add some on chip decoupling capacitor for the 8-bit MCU.
but I just wonder whether or not it will have any side effect.
as far as I know, the side effect are:
1) some leakage due to the MOS cap
2) the power noise may coupling from the decap to analog circuit if adding improperly.

 

[ipsc]

Since decoupling capacitor can be considered to be RLC.
Will the decoupling capacitor across the supply cause the oscillation?

An RLC circuit can oscillate alone only theoretically. As you yourself told it needs a "-ve resistor" to oscillate.