Hi ! I have the following question: If I use an AD7738 (Analog Devices) or ADS1256 (TI) 24bit delta/sigma ADC with "fast channel switching" (I think it's some mechanism to flush the digital filter so that you don't have to throw away 6+ samples before you get a valid one), what effect on the anti- aliasing filter requirements and transfer function does the channel switching have ? It works like this: select Channel 1 -- wait for input stage to settle -- sample -- select channel 2 - wait for input stage to settle -- sample ... and so on. Which frequencies do I have to remove with the anti-aliasing filter now ? Usually (without channel switching) an RC filter is often sufficient. Only frequencies around multiples of 1/4 of the clock frequencies (ADS1256) have to be removed. But is this really also the case when channel switching is used ? Please give some advice on literature, if possible. Moritz v. Buttlar -- ---------- remove 123 for correct email address
Delta Sigma ADC + channel switching
Started by ●October 28, 2003
Reply by ●October 30, 20032003-10-30
Most of these "fast-switching" sigma-delta ADCs simply use an decimation filter with a shorter impulse response, i.e. no longer than the output word rate. The result is often a sinc^3 instead of a sinc^5 frequency response, which increases your noise bandwidth a little bit, and makes the nulls less wide. Sometimes they use a composite filter like sinc(f/f1)*sinc^3(f/f2), where the first factor is just there to get a good 50/60 Hz null. The resulting decimation filter "is what it is", and you don't have to do anything special if you like what you see in the data sheet. As always, your analog anti-alias filter needs to have good attenuation at multiples of the modulator rate. In the case of multiple channels, you should have multiple antialias filters. If you try to do muxing in front of a single antialias filter, then your mux speed will be limited by its step response. David L. Rick Hach Company The "reply-to" address in my header has been disabled following the last round of spam-bot attacks. Humans can now email me at davidDOTrickAThachDOTcomREMOVE. "Moritz v. Buttlar" <info123@baltic-microsolutions.de> wrote in message news:<p8q271-o13.ln1@deepsea2.baltic-microsolutions.de>...> Hi ! > > I have the following question: If I use an AD7738 (Analog Devices) or > ADS1256 (TI) 24bit delta/sigma ADC with "fast channel switching" (I think > it's some mechanism to flush the digital filter so that you don't have to > throw away 6+ samples before you get a valid one), what effect on the anti- > aliasing filter requirements and transfer function does the channel > switching have ? > > It works like this: > select Channel 1 -- wait for input stage to settle -- sample -- select > channel 2 - wait for input stage to settle -- sample ... and so on. > > Which frequencies do I have to remove with the anti-aliasing filter now ? > > > Usually (without channel switching) an RC filter is often sufficient. > Only frequencies around multiples of 1/4 of the clock frequencies (ADS1256) > have to be removed. But is this really also the case when channel switching > is used ? > > Please give some advice on literature, if possible. > > Moritz v. Buttlar