Background: I'm a student, working on my final year project. I am designing a front end for a software defined radio. We have an IF frequency at 36Mz (and a BW ~8MHz). We are bandpass sampling this signal at 29MSPs (our signal falls in the 3rd nyquist zone). The ADC takes a differential input and requires a common mode offset. This DC offset is provided by a differential op-amp. We need to filter out the out of band noise from the op-amp and the previous stages before feeding it into the ADC.
In order to achieve this filtering, I have cascaded a bandstop filter (stop bands are at ~1MHz and ~29MHz) with a lowpass filter (cutoff at ~40MHz). This filters out the out of band noise above 40MHz, as well as below 29MHz while still passing the common mode DC offset.
The problem, is that the bandstop filter requires large inductors (6.2uH). These inductors all seem to have very low self-resonant frequencies and are thus basically useless.
The question: Is it possible to design a highpass filter, that also passes DC? Or is there some other strategy to let the common mode offset pass through my filter, but still cut out the noise in the 0-30MHz range? Or, to design a bandstop filter with (much) smaller inductors?