[SOLVED] wrong simulation results for lowpass filter in OrCAD Capture

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P.Copper

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Hi all

I've designed a 4th order lowpass filter using filterPro. when i simulated the circuit using OrCad i don't get the expected lowpass filter transfer function. I used AC sweep for analysis with start frequency of 1Hz, end frequency of 650Hz and total points of 10. the output just gives me 0. see the attached circuitView attachment lowpass filter.bmpView attachment lowpass filter.bmp

thanks in advance
 

First try it from 1 Hz to 100 MHz, with 100 points per decade.

Note that if the frequency response plot is showing 0 dB, then that means the output is the same amplitude as the input. And so maybe you are not yet looking in the correct frequency range and have not seen the cutoff region.
 

Hi tgootee

I've tried simulating with the values you mentioned, the response is still not correct. Do i have to change other settings in OrCad besides those that i mentioned?
 

I am not able to verify if you have the correct supply voltages (plus/minus).
 

tgootee said:
Can you get it to work for a simple passive RC low-pass filter?
Click to expand...

yeah with passive lowpass filter the response is ok.

- - - Updated - - -

LvW said:
I am not able to verify if you have the correct supply voltages (plus/minus).
Click to expand...

I don't know if you mean the supply for the opamp (LM324 from OrCAD) or you mean the supply for the entire circuit. if you're talking about the supply voltages for LM324, i used VDC with amplitude of 15V (both positive), i figured out that LM324 has both +/- open. so i connected my 15V on both +/-
 


And I don`t know what you mean with "entire circuit"? I think, the opamp is the only part that must be powered, is it not?
More than that - VDC with an "amplitude"?
And what is the meaning of the last sentence ("LM324 has both +/- open. so i connected my 15V on both +/-") ?
(I can see the dc supply symbols but not the corresponding values: Are they +-15 Volt?)

Did you try another opamp model?
 

Yes they are +-15Volts. No i haven't considered another model.
 

P.Copper said:
Yes they are +-15Volts. No i haven't considered another model.
Click to expand...

Did you use the correct input source for ac analyses (not SIN but AC)?
What are the quiescent dc voltages at the outputs?
 

yes i used the AC source. please see the attached file for my outputs
 

Attachments

  • SCHEMATIC1 _ PAGE1.pdf
    9.4 KB · Views: 117

My first impression: there are wrong power supplies: +15V at the inv. node and vice versa. Am I right?
 
LvW said:
My first impression: there are wrong power supplies: +15V at the inv. node and vice versa. Am I right?
Click to expand...

Yes you are. i didn't notice that.

- - - Updated - - -

Thanks LvW now everything is working perfectly.
 

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