OpAmp high capacitive load drive simulation

Hi mohammadsdtmnd,

In this particular case, you don't need the 0.5ohm series resistor on the output as LTspice simulation of AD8605 with 10uF cap load shows that it's stable (except some ringing) without it. 0.5ohm series resistance is not large enough to affect anything.

mohammadsdtmnd said:

Also how this can be like this: In simulation, higher cap i.e 5uf needs lower series resistor 2.2 but lower capacitance i.e. 0.5uf needs higher resistance to stablize 10 ohm. This is contrast of what has been formed in my mind: higher cap need higher resistor. This is conflict could someone also explain this conflict?

Response: Yes, that is a typical behavior. If I may quote from this article link below:

https://www.electronicdesign.com/power-management/article/21798578/whats-all-this-capacitive-loading-stuff-anyhow?gclid=Cj0KCQjwzqSWBhDPARIsAK38LY-c4piBNVY9e7t-hMJSP9TlbwvxItR3xyJ2HQt6UFs7AtpiWNoCarAaAiVaEALw_wcB

"4. Some amplifiers will break into oscillations with a little capacitive loading, but get stable again as a large capacitive load drags down the gain."

So, there is a worst-case value of cap load that is most unstable!

I also wanted to share this link to an application note on cap load driving:

https://www.analog.com/en/analog-dialogue/articles/ask-the-applications-engineer-25.html

Regards,

Hooman

Hi mohammadsdtmnd,

I'm sorry. I misspoke earlier. I showed simulations with 1000pF (not 10uF) above.

Yes, I see that LTspice predicts that with 10uF cap load, adding 0.5ohm series output resistance gets rid of the tiny ringing at the output as shown below:

Simulation file here:

AD8605 cap load EZ 7_9_22.asc

I'd say that this is possibly because with no series output isolation resistor, you are on the edge of instability and not fully unstable. Therefore, the 0.5ohm isolation must be enough to get rid of the tiny ringing after output transition. Most likely, you'd need to verify the results on the bench and not just rely on simulation alone.

Regards,

Hooman

Hi mohammadsdtmnd,

I'm sorry. I misspoke earlier. I showed simulations with 1000pF (not 10uF) above.

Yes, I see that LTspice predicts that with 10uF cap load, adding 0.5ohm series output resistance gets rid of the tiny ringing at the output as shown below:

Simulation file here:

AD8605 cap load EZ 7_9_22.asc

I'd say that this is possibly because with no series output isolation resistor, you are on the edge of instability and not fully unstable. Therefore, the 0.5ohm isolation must be enough to get rid of the tiny ringing after output transition. Most likely, you'd need to verify the results on the bench and not just rely on simulation alone.

Regards,

Hooman