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How to perform AC analysis after designing filter?

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wjxcom

wjxcom

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Hi all: I have already designed a lowpass filter, this filter can be found in the book "CMOS Analog Circuit Design (2th edition)" writed by Allen, i.e. the Example 9.7-5.

Now I want to perform AC analysis for this filter, but I don't know how to do!

I added a voltage source whose DC voltage is 2.5V and AC voltage is 1V to the input port of the lowpass filter, i.e.

VAVIN VIN 0 DC=2.5V AC=1V

but It seems that this way is not right!

Please help me, how to perform AC analysis? thanx you!!!

The netlist can be found at the attachment. Extract the folder "filter" to D:\ or E:\ to simulate!
 

Your method is right.

VIN VIN 0 2.5V ac=1

then check the output magnitude and phase.

Yibin.
 

    wjxcom

    wjxcom

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wjxcom said:
Hi all: I have already designed a lowpass filter, this filter can be found in the book "CMOS Analog Circuit Design (2th edition)" writed by Allen, i.e. the Example 9.7-5.

Now I want to perform AC analysis for this filter, but I don't know how to do!

I added a voltage source whose DC voltage is 2.5V and AC voltage is 1V to the input port of the lowpass filter, i.e.

VAVIN VIN 0 DC=2.5V AC=1V

but It seems that this way is not right!

Please help me, how to perform AC analysis? thanx you!!!
Click to expand...


I think your settings are right, Why do you say your way is not right?

Maybe you should add this line
.AC lin 100 10 3Meg


Ryan
 

    wjxcom

    wjxcom

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Hi all: if the method is right, why the simulation result of the AC analysis is not right? at the same time, the result of TRAN is right!
 

what kind of results did you get by your AC simulation?
 

    wjxcom

    wjxcom

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Very strange! the wave can be found at the attachment!

regard!
 

I cannot understand your simulaiton results, because I don't think the LPF ac response will look like that, it seems that the filter did not work in the right state. I suggest that you should use a ideal opamp in your schematic, because the opamp's characterirstic will affect the performance of the whole filter, for example, the gain bandwidth of it will limit the cutoff frequency of the filter to a extent.

hope it helps!
 

    wjxcom

    wjxcom

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Hi abcyin： do you know how to use the ideal OPA in Hspice?
 

Er..., you can get ideal opamp from ahdl library in spectre, or somebody will use vcvs directly.
as to the hspice, I don't know whether there is an ideal opamp model, but I think you can build it by yourself, the following website will tell you how to build it:

http://www.ecircuitcenter.com/Circuits/opmodel1/opmodel1.htm

hope it helps!
regards!
 

Hi !!!
Could you post the schematic of your circuit ???
It seems to be a problem with initial condition.
I mean, the time response of your filter looks like a step response of a low pass.
It's the dampened oscillations. The oscillation are cut for high value because of the power supply limitation, 5V I suppose.
You shoud try the same simulation with initial condition : Vin=2,5V for t<0
Bye.
 

wjxcom said:
Hi all: I have already designed a lowpass filter, this filter can be found in the book "CMOS Analog Circuit Design (2th edition)" writed by Allen, i.e. the Example 9.7-5.

Now I want to perform AC analysis for this filter, but I don't know how to do!

I added a voltage source whose DC voltage is 2.5V and AC voltage is 1V to the input port of the lowpass filter, i.e.

VAVIN VIN 0 DC=2.5V AC=1V

but It seems that this way is not right!

Please help me, how to perform AC analysis? thanx you!!!
Click to expand...



Hi,
You are right, and add .ac analysis , and see magnitude and phase response.
 

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