Author Topic: Why does the passive highpass filter give such results in spice analysis?  (Read 704 times)

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Offline electronxTopic starter

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I am designing a passive high pass filter, here are the values: 2M ohms and 1uf cut-off frequency is 0.07Hz +-20V and when I apply a 1 khz sinus signal, I get such a result. What is the reason for this? The voltage level looks like this ım using pspice


 

Online iMo

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Re: Why does the passive highpass filter give such results in spice analysis?
« Reply #1 on: December 01, 2023, 09:52:05 am »
Because your resistor is 2 Milliohm?
Try with 2Meg or 2000k..
 
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Offline Chalcogenide

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Re: Why does the passive highpass filter give such results in spice analysis?
« Reply #2 on: December 01, 2023, 09:53:28 am »
There is a chance that you placed a 2 mOhm resistor instead of a 2MOhm; check the current across the resistor.
SPICE is typically not case sensitive, so the 1e6 multiplier is usually written as "MEG" (or just use scientific notation to be always sure).
 
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Offline electronxTopic starter

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Re: Why does the passive highpass filter give such results in spice analysis?
« Reply #3 on: December 01, 2023, 10:03:20 am »
Thank you  all .I feel super noob now :D
 

Offline MarkT

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Re: Why does the passive highpass filter give such results in spice analysis?
« Reply #4 on: December 02, 2023, 08:27:35 pm »
I'd say Spice is just dumb here, its a real gotcha...  I suppose the justification is Spice originally was written in Fortran using punched cards...  I doubt it understands p v. P either (pico / peta) - not that petaohm resistors are a thing!
 

Offline TimFox

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Re: Why does the passive highpass filter give such results in spice analysis?
« Reply #5 on: December 02, 2023, 08:35:15 pm »
SPICE is backwards-compatible for its notation.
The original SPICE, in compiled Fortran on Hollerith cards, was not case-sensitive, so "m" or "M" always means "milli" and for 106 one needs to write "Meg" or "MEG".
Similarly, "p" always means "pico" and "u" always means "micro".
 

Offline MrAl

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Re: Why does the passive highpass filter give such results in spice analysis?
« Reply #6 on: December 03, 2023, 11:44:40 am »
Hi,

Just to verify that it is a 0.002 Ohm resistor and not a 2M resistor (2M as we usually write 2 megohms) the theoretical calculation comes out to:
Vout=251.327 nV

which matches your plot result closely.

If you had an actual 2 megohm resistor here, it would have came out very very close to 1v.

For reference, the theoretical calculation comes out to:
Vout=(2*pi*f*C*R)/sqrt(4*pi^2*f^2*C^2*R^2+1)

with f, C, and R positive, and pi=3.14159265...
 


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