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Low Pass Filter Implementations

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Ardni

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Hi all,
I am currently working on a design where I have a differential driver (ADA4937-2) which drives an ADC (AD9640). I am having problems understanding how the output of the driver should be connected to the ADC.

We want to put a low pass filter in place of about 100MHz before the entry of the ADC. I have attached the datasheet of the Differential ADC driver. On page 23, 24 & 25 there are 3 examples of three different configurations of a low pass filter. The problem I have is that my basic conception of a low pass filter was a resistor and then a capacitor in parellel which could be repeated to increase the order.

On page 23 of the datasheet they have a 24.4 ohm resistor in series with a 30mH inductor all in parellel with a 47 pF cap. they say this is a second order lpf with 100 MHz cutoff. This will do me nicely but I would like to understand what the inductor does? why it is a 2nd order filter and how can the cut off frequency be calculated?
I understood that the cut -off freq was 1/2*pi*R*C but i am not sure how it is done in this case.

From looking on the internet it seems that an inductor alone can be used as a lpf, but how can a cut off freq be calculated?


Also I probably should say that the diagram given in the datasheet are for single ended inputs wheras mine is differential, however I dont believe this will have any affect on the filter phase.

Many thanks for any help.
 

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Tesla23

Member
Hi all,
On page 23 of the datasheet they have a 24.4 ohm resistor in series with a 30mH inductor all in parellel with a 47 pF cap. they say this is a second order lpf with 100 MHz cutoff. This will do me nicely but I would like to understand what the inductor does? why it is a 2nd order filter and how can the cut off frequency be calculated?
I understood that the cut -off freq was 1/2*pi*R*C but i am not sure how it is done in this case.
Many thanks for any help.
Seems to be a standard 2nd order butterworth as they claim, see for example link. Note that in the ADI design they have a 25 ohm source impedance and infinite load impedance so you need a different table of values (e.g. Zerev). Without hte inductor you can only get one pole, second order filter has 2 poles, you need to read up on filter design.
 
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fernando_g

New Member
Beware!
The inductor's value is not in milihenries (mH) but nanohenries (nH).

Also, At those frequencies, parasitic elements and layout are as important as the component themselves.
 

Ardni

New Member
Thank you for the replies.
I was doing a little reading on internet and from one of the links provided, I came across this page:
Here it gives some formulas for the different filters but I do not understand the equation provided for calculating L in the lowpass filter. I do not understand what is meant by Lproto. It is explain a little on the RHS but I can't grasp it. Basically I just want to verify and understand the filter implemented in the datasheet.

Also could you please elaborate on the following
Also, At those frequencies, parasitic elements and layout are as important as the component themselves.
I'm still a beginner to much of this.

I really appreciate your help.
Many Thanks
 
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