I'm working with a Analog Devices ADAR7251 Sigma-Delta ADC. It has multiple sample rates to choose from (1800 kSPS, 1200 kSPS, 900 kSPS, 600 kSPS, 450 kSPS, and 300 kSPS) but my application requires 112.5 kSPS. I plan on using the ADC in 450 kSPS mode and will decimate by a factor of 4 to reach the target sample rate. This ADC can simultaneously sample 4 channels and has a built in LNA+PGA for each channel. These features prevent me from changing to a new device.
I know the proper way to do it is to use a cascade of 2 half band filters. My question is can I do without them and simply take every fourth sample?
I'm aware that this particular ADC oversamples by a factor of 48xFs (21.6 MSPS in 450 kSPS mode) which relaxes the requirement for the anti-alias filter. I'm using a 3rd order Bessel Differential LPF (fPass @ 60 kHz, fStop @ 180 kHz with 20dB of attenuation). Additionally, my sensors are an array of electret microphones with a LPF frequency response rolling off around 20 kHz. I don't think there will be any significant signal power at a high enough frequency to cause aliasing that I have to worry about.