[SOLVED] Cadence virtuoso AC simulation

[didid]

When I simulate AC response of my circuit, I get 2 different measurements.

After I run the ac simulation, there are two ways to get the results.
First, I click 'Results - Direct Plot - Main form' and select 'Different Nets' to get the 'dB20' results.

I click VOUT and VIN to get the dB20 data.

Secondly, I click 'Results - Direct Plot - AC Gain & Phase' then I also click VOUT and VIN to get the gain and phase results.

As you can see the picture below, I get the different results. I wonder which one is the right result of the AC response.

 

[The_pearl]

The one using main form must be the correct result as you specify the analysis there. When directly using AC Gain and phase we don't know what DC condition it is taking. Try simulating the AC analysis alone without the DC/ transient analysis and check if you're getting different results.

 

[BigBoss]

I couldn't see AC Voltage Source here..

 

[didid]

I couldn't see AC Voltage Source here..

I set VIN and VIP just like the pictures above, respectively.

--- Updated Jul 14, 2024 ---

The one using main form must be the correct result as you specify the analysis there. When directly using AC Gain and phase we don't know what DC condition it is taking. Try simulating the AC analysis alone without the DC/ transient analysis and check if you're getting different results.

I tried AC simulation alone, but the difference between two results keeps occuring.

 

[dick_freebird]

With both inputs AC-coupled in forward and feedback, how
can you ensure sane DC input conditions at the amplifier-
core diff pair inputs for your small signal analysis?

Let alone reality?

 

[didid]

With both inputs AC-coupled in forward and feedback, how
can you ensure sane DC input conditions at the amplifier-
core diff pair inputs for your small signal analysis?

Let alone reality?

this amp is for neural recording. That's why I use AC-coupled for blocking dc offset which has larger value than the value of the recorded neural signals.
I wanted to make the closed loop gain with capacitive feedback.

I know that when the inputs are AC-coupled, the dc goes to 0 V. Is this the reason why the results graph look weird?

 

[dick_freebird]

You need some ohmic or clocked means to ensure the front
end "knows what's zero". If this is part of a sampled data
lineup then maybe consider turning this into a T'H amp that
during "hold" is also applying a shorting switch to the core
inputs and puts the inboard side of the caps to a resting
reference (Vdiff=0, Vcm=fortunate) and then on return to
"track", reconnect as shown without this.

The inputs as is, could drift anywhere short of breakdowns
and you should be looking at core input common mode vs
common mode "works right" range.