Adding capacitors to opamp negative, positive and output

[wwfieee]

adding caps to opamps

is it wise to add capacitors to opamp negative, positive and output terminals? What would the effect be if let say I add values around 47uF to 200uF to these nodes?

Basically I'm using them as voltage buffers and these caps are to bypass signal noise from rippling back to my inputs and reference nodes.

 

[IanP]

Re: adding caps to opamps

Adding capacitor to an opamp's output gives you next to nothing..

You can use R/C low-pass filters at opamp's inputs and for best results a capacitor between opamp's output and NEG-input (<100nF). This one works like integrator ..

 

[jdhar]

adding caps to opamps

I thought caps can prevent oscillation in certain cases...

 

[wwfieee]

Re: adding caps to opamps

so putting capacitors won't have any bad effects DC wise? the noise frequency i'm dealing with is probably around 160Mhz to 600Mhz. although the 600Mhz noise i doubt is from the circuit. i suspect it was inherent from the scope and probe.

 

[IanP]

Re: adding caps to opamps

In your first post you mentioned capacitors such as 47µF and 200µF. That gives impression that we are talking about kHz frequecies..
In your last post you indicated that the noise spectrum coveres 160MHz to 600MHz.

In this situation make sure the power supply lines are decoupled with 100pF-1nF ceramic caps // with 10µF tantalum placed as close to the opamp as possible and use 1nH-10nH inductor between +/-Vcc and opampm supply pins.

On the opamp non-inverting input you can use ≤1nF cap and similar cap you can connect between the opamp's output and inverting input.
Also, you may need some shielding ..

 

[wwfieee]

Re: adding caps to opamps

well, the noise originates from the switching output of my board. the ripples travel back to the op-amp output, thus op-amps' power supply. since it's a voltage buffer, the inputs of the opamps are also rippled. it is at this input node i'm using as a reference to other parts in my board.

if this node ripples, it'll travel to my other part of the board. i've tried all kinds of things to prevent the high frequency noise and it seems i have to live with it. now the objective is i don't want to it ripple throughout my board. the opamp is very close to the output, thus probably are coupled through stray caps. my other ics however are quite far away from the output.

that is why i'm asking if its possible to put decoupling caps on the input nodes of the opamp. my board has decoupling caps all over the power supply nodes in the range of 10uF, 4.7uF, 0.1uF, 0.01uF.

so to put it simply, opamp output has high frequency noise that is unpreventable. input of this opamp is used as a reference for other part of the circuit. i want to filter out this noise.

i'll try the caps and inductor values mentioned.

 

[IanP]

Re: adding caps to opamps

You can always use R/L/C passive low-pass filter at the opamps output; you will loose a little bit from very low output impedance, but the noise should be eliminated ..

 

[mohammad mokhtar]

Re: adding caps to opamps

Hi
Just using 0.1 uf ceramic capacitor will be quite enough.