The delta-sigma advantage to anti-aliasing filters

RE: The delta-sigma advantage to anti-aliasing filters

Ryan Andrews — Wed, 16 Aug 2017 19:29:28 GMT

Hi Otavio,

Thanks for reading!

How much attenuation your antialiasing filter (AAF) should provide really depends on your application. I think the theoretical target you mentioned holds true for any ADC architecture - that is, your AAF should attenuate the noise seen after the first Nyquist frequency to a level which is insignficant when compared to the ADC's inherent quantization and thermal noise. For a SAR, this attenuation level must be reached very close to Nyquist. However, for a delta-sigma ADC, this attenuation is really necessary closer to the modulator sampling frequency (fMOD). Between the data rate Nyquist frequency (fDATA / 2) and fMOD, the digital filter stopband will suppress most of the out-of-band noise.

The signal bandwidth of interest determines the passband that you need for your filters (AAF and digital). The difference in frequency between the signal bandwidth and the aliasing band determines the type of AAF and the response you need. Ideally, your AAF and digital filter should remain relatively flat throughout your bandwidth of interest. For low-speed applications where the bandwidth is only a few hundred hertz, a first- or second-order RC filter may be enough since the modulator of the ADC is often running at frequencies greater than hundreds of kilohertz. Conversely, a high-speed application with hundreds of kilohertz of signal bandwidth may need a high-order active filter to reach the attenuation target before a modulator frequency of a few megahertz.

I don't know of any new filter design tools out there, but we used to promote this FilterPro Webench tool (see link below). This may help you as a starting point for active filter design.

[View:http://www.ti.com/tool/filterpro&DCMP=hpa_tools&HQS=Tools+PR+filterpro-pr:1230:0]

Best Regards,

Ryan

RE: The delta-sigma advantage to anti-aliasing filters

Otavio Augusto Gomes — Sun, 13 Aug 2017 17:05:01 GMT

Hey Ryan,

Thanks for the blog post! I have been reading several e2e forum posts and blog articles that you have wrote besides another literatures about anti-aliasing filter design for delta-sigma ADC's. But I still in doubt about how much attenuation my anti-aliasing filter must provide and where. For example, in traditional SAR ADC's the AA filter must provide, theoretically at least, 6.02*N + 1.76 dB of attenuation in Fsample/2 in order to reduce out-of-band noise at quantization noise level order, or below noise floor. But I don't know how to determine mathematically how much attenuation my anti-aliasing filter must provide and in which frequency this attenuation must be. I have read several times that oversampling delta-sigma ADC's relaxes the requirements of AA filter, but I never found a consistent guideline in order to calculate AA filter parameters so that I can design a AA filter using Butterworth or another filter approximation. In most literatures, the information is that a simple RC passive filter is enough, just it. Can you help-me?

RE: The delta-sigma advantage to anti-aliasing filters

Ryan Andrews — Thu, 09 Feb 2017 21:43:56 GMT

Hello user4372489,

Thanks for your question! I'm glad the blogs are useful to you. :)

The quantization noise produced by the delta-sigma modulator is mostly filtered by the digital filter, not the analog antialiasing filter at the inputs. As you can see in Figure 3, the quantization noise is shaped until fMOD/2, where it reaches its peak magnitude. This falls directly in the stopband of the digital filter and becomes heavily attenuated.

Best Regards,

Ryan

RE: The delta-sigma advantage to anti-aliasing filters

user4372489 — Thu, 09 Feb 2017 03:11:39 GMT

Dear Ryan Andrews :

Your blogs are so detailed that I have learned a lot about the anti-aliasing filter for delta-segma ADCs!

I have a question about that since the digital filter could not attenuate the nosie around the sampling rate of ADCs,as fMOD,so the front-end anti-aliasing filter is necessary!Thus, the anti-aliasing filter is at the front of ADCs,and the quantizing noise is produced by modulation of ADCs after the filter,so my question is how the front filter to attenuate nosie produced later?