In our project, we developed and tested a auto-zero chopping op amps coupling with Sigma-delta ADC for low-frequency voltage signal measurement. Below is the FFT spectrum from the 1-bit data stream of the ADC. Now, I am designing a decimation filter to convert 1-bit to high-precision data word. I noticed that there are several noise peaks around our chopping frequency. I wonder if I have to design a notch filter to take care of those peaks or just a regular sinc3 filter would suppress the noise and do the conversion efficiently.
When you're designing a decimation filter, the purpose of the stopband is to reduce that part of the input signal spectrum below the level that would cause you problems if aliased down to baseband.
If your approach was only to design sufficient stopband to suppress the noise-shaping rise, then you would be short of attenuation for the noise peaks around your chopping frequency. Whether you go for additional attenuation by means of notch filters, or simply a more aggressive general stopband, is up to you. Note that if the chopping frequency were to change, perhaps due to substituting a different amplifier, then you would have to redesign the notch filter to cope.
My general strategy is to do limited sinc decimation to get the data rate down, followed by further polyphase FIR decimation to flatten the passband.