[SOLVED] how to analyze a passive filter

[Johnny_YU]

Hi all, I am trying to analyze the passive filter in this circuit as below.
The input is differential signal.
Is this circuit a differential mode filter?
Who can help me write circuit's the trans-function.

 

[asadi.siyavash]

it is two low pass filter with Differential low pass filter

 

[FvM]

The circuit has different low-pass cut off frequencies for common and differntial mode signals. Preferably you draw equivalent circuits for both operation modes and calculate it.

 

[Johnny_YU]

The circuit has different low-pass cut off frequencies for common and differntial mode signals. Preferably you draw equivalent circuits for both operation modes and calculate it.

HI，FvM
How can I separate the circuit into differential mode filter ande common mode filter?
Let me guess
Differential mode filter:

Common mode filter 1 :

or Common mode filter 2 :

 

[FvM]

Common mode filter 1 is correct.

For the differential mode, the first circuit is roughly correct, but the 1MOhm and 100 pF must be considered for exact transfer function. What you name "common mode filter 2" is acually the exact single ended equivalent circuit of the differential mode filter.

 

[Johnny_YU]

Common mode filter 1 is correct.

For the differential mode, the first circuit is roughly correct, but the 1MOhm and 100 pF must be considered for exact transfer function. What you name "common mode filter 2" is acually the exact single ended equivalent circuit of the differential mode filter.

HI , FvM
Thank you for your help!
Before I separate the circuit, i'v read two documentations from TI for reference.
One is <Design of differential filters for High-Speed Signal Chiains>. It says: "the process of designing a differential filter requires the design of a single-ended filter requirements;The single-ended LC low-pass,high-pass,and bandpass filter can be converted to a differential filter by repeating and folding the design around GND to create the V+ and V-;It is possible to design differential LC filters to have the same frequency response as the single-ended filter. "
I wonder, if this method applied in RC filter to convert a single-ended filter into a differential-ended filter is correct?
According to this Method , I drew the circuit which naned "common mode filter 2". As you say ,it's exact a single ended filter equivalent circuit of the defferential mode filter.
Another documentation that I read is <RTD Ratiometric Measurements and filtering Using the ADS1148 and ADS1248 Family of Devices>.
According to second documentation , i drew the circuit which naned "Differential mode filter".
So i'm Puzzled.

 

[Johnny_YU]

Hi,FvM
Could you tell me more about how to analyze this passive filter?
Is there any books or documentations I can get to read and learn more ?
Thank you !