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.
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.