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[SOLVED] Why mu OpAmp oscillates sometimes?

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tony_lth

tony_lth

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Hi,
I made a OpAmp circuits, but it seems that it oscillates sometimes, could someone help me?
1. Is the root -cause is C6?
2. Or other issues?
Many thanks.

- - - Updated - - -

Or because R7 is only 1K, too small? Should I increase it to solve the OSC issue?
 

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The rated Phase Margin is ΦM=60 Degrees @ CL = 20 pF for 220pF the graph shows 40 deg PM
YOu have RC 16k5 and 2.2uF which probably reduces the PM near 0.
COnsider 1st Opamp with non-inverting gain of ~4 and eliminate 2nd Op AMp to improve phase margin then reduce C8 and double R9

The reason is the CMOS output Z is 5V/7mA (shorted cct) = 0.7k which is affected by the RC load.
 
Last edited:
Hi,

I expect the first opamp stage to be uncritical.

I assume a 100pF across R13 should also prevent oscillation.


Klaus
 
Capacitive load is perfectly isolated in this circuit. There's no reason for oscillations that could be seen from the schematic, may be there are other "real-world" effects. According to the rather low OP GBW (500 kHz) this must be more than a bit of parasitic capacitance.
 
HI, Gurus,
Thank you very much.
Best,
Tony Liu

- - - Updated - - -

I will try and ley you know the results tomorrow.

- - - Updated - - -

Hi, KlausSt,
Sorry to tell you that my OSC signal is a square wave, about 1Hz. Do you think 100pF across R13 will work for such low freq oscillation?
I haven't try, just doubt it, OR should I increase 100pF? What is the limit for the capacitor across R13 that I can try?
Many thanks.

- - - Updated - - -

And, SunnySkyguy,
Since 16.5K+2.2uF may affect OpAmp output, so 1K+1UF(C7+R7) should affect AD8226 output more?
Thanks for your kindness.
 

Hi,

Sorry to tell you that my OSC signal is a square wave, about 1Hz. Do you think 100pF across R13 will work for such low freq oscillation?
Click to expand...
You never mentioned 1Hz, so how can we know?

For sure for a 1Hz oscillation the 100pF will not help.

*****
Usually an OPAMP circuit oscillation is more in the MHz than in th kHz range. But 1Hz is very unusual.

To be true I have no idea where it may come from.

Can you show us your PCB layout?
Where is the VRef circuit?
What supply do you use?
Does it oscillate with and without input connection?

Klaus
 
Motorboating
The vref labelled from your circuit is from where?
Is this 1hz signal present on the first and second stage amplifier?
PCB layout?
There is just a lot that can cause this
 

Thanks, it should be more related to not enough AC decoupling, i guess. I will try and let you know the results.
many thanks.
 

Hi, Gurus,
The root cause is grounding issue, so basic lesson.
Anyway, I learned a lot from SunnySkyguy, Samson A., KlausST, FvM. You gave me light to solve the issue.
Many thanks.
 

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