Question: Filter Frequency Response

[hw_junkie]

Hi,

I'm trying to design a series of cascaded filters. I was wondering, if I have one filter to remove high frequencies (in the Mhz range) and another to remove 2nd harmonic (< 10KHz) and there is a unity gain buffer (op amp) in between, will the phase shift of the second stage add up or affect the phase of the first stage (at any frequency)? or are their frequency responses isolated due to the buffer in between them?

Thx

 

[LvW]

Hi,

I'm trying to design a series of cascaded filters. I was wondering, if I have one filter to remove high frequencies (in the Mhz range) and another to remove 2nd harmonic (< 10KHz) and there is a unity gain buffer (op amp) in between, will the phase shift of the second stage add up or affect the phase of the first stage (at any frequency)? or are their frequency responses isolated due to the buffer in between them?
Thx

Yes, they are isolated. That means, the 2nd stage does not influence the first one. However, since the output of the first stage is the input for the 2nd stage both gains are multiplied (added in dB) and both phase shifts are added.

 

[vfone]

The input and output impedance of the cascaded filters are not the same at different frequencies, and in this way the filters characteristics are somehow affected.

 

[LvW]

Hi, hw-junkie !

Forget the above comment from vfone. It is not correct resp. applicable.

 

[hw_junkie]

Thanks LvW

An Active 1st order LPF (Inverting Integrator) should have a phase of -90°.
But when I simulate it in Pspice I get +90°.
If I simulate a non-inverting Integrator, I get a phase shift of -90°

Why is that??

Please help!

 

[LvW]

..........
An Active 1st order LPF (Inverting Integrator) should have a phase of -90°.

How do you arrive at -90 deg?

The process of integration shifts the phase to -90 deg.
When you have an inverting integrator it adds additional ±180deg.
Thus, you have either -90+180=+90 or -90-180=-270 deg, which is identical to +90 deg.
So, everything looks good.

 

[hw_junkie]

Thanks!
Makes perfect sense