Hi,

I am completing schematics for our design where I have a an analog signal coming in going through a differential driver with a low pass filter and then on to the ADC.

I am having a little trouble understanding the low pass filter.

I have attached a part of the schematic where the differential driver and the low pass filter can be seen.

I understand that this filter is a 3rd order capacitor input low pass butterworth filter (from looking here). My doubt lies in how the cutoff frequency is calculated. According to the datasheet for the differential driver the cut off frequency of the filter is 125MHz and from looking here an explanation of the formulas is provided. I do not understand the terms Cproto and Lproto or the denormailzed C and L values well. I was hoping to find a formula where I could set lets say the capacitor values and solve the equation for the inductor value.

Could someone please explain how I can achieve this. The plan is that I need to be able to alter the cut off frequency of the filter and would like to know what values are required?

Also as an aside, I understand that the 33 ohm resistor at the output of the differential driver does not affect the filter operation? but is there to match the impedence between the output of the differential driver and the input of the ADC, is this true to say? So it can be changed to match both impedances. This is something i have yet to do, as it does not give me the input impedence of the ADC in the datasheet.

Thank you for your help

I am completing schematics for our design where I have a an analog signal coming in going through a differential driver with a low pass filter and then on to the ADC.

I am having a little trouble understanding the low pass filter.

I have attached a part of the schematic where the differential driver and the low pass filter can be seen.

I understand that this filter is a 3rd order capacitor input low pass butterworth filter (from looking here). My doubt lies in how the cutoff frequency is calculated. According to the datasheet for the differential driver the cut off frequency of the filter is 125MHz and from looking here an explanation of the formulas is provided. I do not understand the terms Cproto and Lproto or the denormailzed C and L values well. I was hoping to find a formula where I could set lets say the capacitor values and solve the equation for the inductor value.

Could someone please explain how I can achieve this. The plan is that I need to be able to alter the cut off frequency of the filter and would like to know what values are required?

Also as an aside, I understand that the 33 ohm resistor at the output of the differential driver does not affect the filter operation? but is there to match the impedence between the output of the differential driver and the input of the ADC, is this true to say? So it can be changed to match both impedances. This is something i have yet to do, as it does not give me the input impedence of the ADC in the datasheet.

Thank you for your help