Nonlinear Overshoot in 0.18um CMOS Op-Amp

[matchasm]

Hi,

I am designing an Op-Amp with around 200MHZ GBW and 100-dB DC gain and I notice that when I reduce the noise by width-scaling I get some overshoot in the transient response. The AC phase margin is alright, so anybody got any other suggestions?

 

[steer]

Not enough details. Post schematics and transient response graph, then chances to get help would be higher.

 

[darkk]

My BLIND guess is that your phase margin shrinks due to the width-scaling. Check it.

 

[pixel]

I would rather say that scaling could be sucessfully used only in digital circuits. In the case of analog electronics simple scaling is not enough because of sub treshold effects.

 

[matchasm]

Not enough details. Post schematics and transient response graph, then chances to get help would be higher.

Alright. Here's a rough schematic and transient response. The amp is used in a switched-cap MDAC configuration.

 

[pfd001]

how about the phase margin? did you add all load when you simulate the phase margin?

 

[matchasm]

how about the phase margin? did you add all load when you simulate the phase margin?

yep, and switches are in the right position too. i can change the compensation network to adjust the phase margin any way i want, but the overshoot always remains.