# required : sallen key filter unity gain model for PSpice

#### KFUPMer

Joined Nov 4, 2015
8
Hi guys ,

I am trying here to design 4th order Lowpass Chebyshev filter

and the given data :
1/3dB Frequency: fo= 3KHz

2/Pass Band Ripple = 1dB

I have used this sites to design my filter :

(http://www.daycounter.com/Filters/Sallen-Key-LP-Calculator.phtml)

as u can see I have used unity gain

If R= 2.5K Ω , then c1= 54.62nF and C2=11.1nF (The first stage)

The second stage should be identical to the first stage .

the circuit :

#Problems:
1/after this long introduction , I want to simulate this circuit by using PSpice but I do not have a model for sallen key filter unity gain

2/ Should the gain be exactly =1 in the simulation ???

#### MikeML

Joined Oct 2, 2009
5,444
What prevents you from just entering the component topology, the resistor and capacitor values, and a suitable opamp model into PSpice?

Cascading two identical filter stages does not produce the correct 4th order response.

Go to the MicroChip website and download FilterLab. It synthesizes active filters from specs, and will handle the two cascaded stages correctly....

Here's what FilterLab comes up with:

Last edited:

#### KFUPMer

Joined Nov 4, 2015
8
What prevents you from just entering the component topology, the resistor and capacitor values, and a suitable opamp model into PSpice?

Cascading two identical filter stages does not produce the correct 4th order response.

Go to the MicroChip website and download FilterLab. It synthesizes active filters from specs, and will handle the two cascaded stages correctly....

Here's what FilterLab comes up with:

View attachment 95859

So , they should not be identical ???

and this is my SPice file :
Code:
project

Vs 8 0 AC 1 sin(0 1 1k)

R1 7 8 2500
R2 7 2 2500
R3 1 9 2500
R4 9 5 2500

C1 1 7 54.62n
C2 2 0 11.1n
C3 4 9 54.62n
C4 5 0 11.1n

X1 2 1 1 OPAMP1
X2 5 4 4 OPAMP1

.SUBCKT OPAMP1
(what should I write here ?????)
.ENDS

.AC dec 100 10 40meg
.TRAN 0.01m 2m
.PROBE
.END

#### KFUPMer

Joined Nov 4, 2015
8

By the way, is this an example or this is what my filter should be ????

#### KFUPMer

Joined Nov 4, 2015
8
What prevents you from just entering the component topology, the resistor and capacitor values, and a suitable opamp model into PSpice?

Cascading two identical filter stages does not produce the correct 4th order response.

Go to the MicroChip website and download FilterLab. It synthesizes active filters from specs, and will handle the two cascaded stages correctly....

Here's what FilterLab comes up with:

View attachment 95859
one last question , how can I enter the value of (Pass Band Ripple) in FilterLab ????

#### MikeML

Joined Oct 2, 2009
5,444
one last question , how can I enter the value of (Pass Band Ripple) in FilterLab ????

Do you know how to scale resistor and capacitor values if you do not like what FilterLab comes up with?

#### KFUPMer

Joined Nov 4, 2015
8

Do you know how to scale resistor and capacitor values if you do not like what FilterLab comes up with?

Filter >Design > then what ?

Actually I like FilterLab thanx ^_^

#### MikeML

Joined Oct 2, 2009
5,444
Filter >Design > then what ?

Actually I like FilterLab thanx ^_^
Filter/Design/FilterSpecs/...

Check all of the parameters that can be specified.