Removing DC offset from ADC Buffer

On Tuesday, September 22, 2020 at 4:30:53 AM UTC-4, Jasen Betts wrote:
> On 2020-09-21, Ricketty C <gnuarm.deletethisbit@gmail.com> wrote:
> > On Monday, September 21, 2020 at 12:30:52 AM UTC-4, Jasen Betts wrote:
> >> On 2020-09-21, jlarkin@highlandsniptechnology.com <jlarkin@highlandsniptechnology.com> wrote:
> >> > On Sun, 20 Sep 2020 16:29:30 -0700 (PDT), whit3rd <whit3rd@gmail.com>
> >> > wrote:
> >> >>
> >> >>But not any that I mentioned; what ARE those other "convoluted arguments"?
> >> >>I'd like to judge their merits for myself...
> >> >
> >> >
> >> > I was just thinking how crazy it woud be to use, say, a 5000-tap FIR
> >> > filter to compute a good autozero average value of the last 5000
> >> > samples.
> >> >
> >> > Those 5000 multiply-by-1-and-add blocks will need a lot of logic.
> >> 
> >> Clearly not the best way to make a boxcar filter...
> >> LOL. I see what you did there.
> >> 
> >> -- 
> >>   Jasen.
> >
> > You mean to construct a straw man design and then shoot it down? 
> 
> I'm remarking on his wordplay on "convoluted": he describes a boxcar
> filter implemneted as a convolution.
> 
> It struck me that John knows DSP better than that, but it took me a
> while to catch his meaning.

Ok, wordplay.  Got it.  

Yes, the sort of joke that is only humorous to an engineer and the more... extreme ones at that.  

I have a math joke that has to be explained to most mathematicians, not because they don't understand the math aspects.  They don't understand humor.  It appears that when carried to an extreme "math joke" is a self contradiction.  Not so much different from "engineering joke". 

-- 

  Rick C.

  -+ Get 1,000 miles of free Supercharging
  -+ Tesla referral code - https://ts.la/richard11209

tirsdag den 22. september 2020 kl. 03.47.59 UTC+2 skrev Phil Hobbs:
> On 2020-09-21 21:32, Lasse Langwadt Christensen wrote:
> > tirsdag den 22. september 2020 kl. 02.47.37 UTC+2 skrev Phil Hobbs:
> >> On 2020-09-21 00:06, Jasen Betts wrote:
> >>> On 2020-09-21, jlarkin@highlandsniptechnology.com
> >>> <jlarkin@highlandsniptechnology.com> wrote:
> >>>> On Sun, 20 Sep 2020 16:29:30 -0700 (PDT), whit3rd
> >>>> <whit3rd@gmail.com> wrote:
> >>>>
> >>>>> On Sunday, September 20, 2020 at 2:01:05 PM UTC-7,
> >>>>> jla...@highlandsniptechnology.com wrote:
> >>>>>> On Sun, 20 Sep 2020 13:33:11 -0700 (PDT), whit3rd
> >>>>>> <whi...@gmail.com> wrote:
> >>>>>>
> >>>>>>> On Sunday, September 20, 2020 at 3:26:13 AM UTC-7,
> >>>>>>> jla...@highlandsniptechnology.com wrote:
> >>>>>>>> On Sun, 20 Sep 2020 00:26:47 -0700 (PDT), whit3rd
> >>>>>>>> <whi...@gmail.com> wrote:
> >>>>>>>>> On Saturday, September 19, 2020 at 9:49:47 PM UTC-7,
> >>>>>>>>> jla...@highlandsniptechnology.com wrote:
> >>>>>>>
> >>>>>>>>>> Just average the samples and subtract that average from
> >>>>>>>>>> each new sample. There are several ways to do that
> >>>>>>>>>> average:
> >>>>>>>>>>
> >>>>>>>>>> Sum the last N samples and divide by N.
> >>>>>>>>>
> >>>>>>>>> that's a FIR filter (finite impulse response) ... if you
> >>>>>>>>> choose the sample size and know the likely interference
> >>>>>>>>> sources (like, 60 Hz ripple), it allows you to place a
> >>>>>>>>> null appropriately
> >>>>>>>>>
> >>>>>>>>>> Exponential smoothing: Avg = Avg + (new-Avg) / N
> >>>>>>>>>
> >>>>>>>>> that's a IIR filter (infinite impulse response); usually
> >>>>>>>>> not a great choice
> >>>>>>>
> >>>>>>>> Why not? I see a lot of irrational prejuduce against simple
> >>>>>>>> IIR filters, in code and in FPGAs. Some people would rather
> >>>>>>>> write a hundred lines of code instead of one.
> >>>>>>>
> >>>>>>> Oh, it's simple, all right, but it has a long startup
> >>>>>>> transient.
> >>>>>> Any lowpass filter or averager does. Just poke a starting value
> >>>>>> into the integrator node if you're in a hurry, ADC midscale in
> >>>>>> this case.
> >>>>>
> >>>>> A lowpass needn't be considered appropriate during startup (and
> >>>>> brute-force setting a starting value helps).   FIR has a
> >>>>> time-limit on its history, which is often completely appropriate
> >>>>> and useful.
> >>>>>
> >>>>>
> >>>>>>> That means it doesn't deal with lightning-strike artifacts
> >>>>>>> well, either.
> >>>>>> Presumably an ADC rails on a huge transient. Why would an IIR
> >>>>>> filter be worse than a FIR for a spike?
> >>>>>
> >>>>> Small signal in big digitizer range, of course.  Your 'rails'
> >>>>> scenario is a measurement failure, and there's multiple ways to
> >>>>> treat such a thing, which FIR does by... ignoring the spike a few
> >>>>> samples afterward. IIR doesn't do that, so saturating the
> >>>>> digitizer is an alternate solution that you don't seem to
> >>>>> dislike.
> >>>>>
> >>>>>> It's impressive...
> >>>>>
> >>>>> I'm pleased that my response impresses you.
> >>>>>
> >>>>>> how many convoluted arguments people make to avoid IIR digital
> >>>>>> filters. Most of them reduce to "It's too simple and I don't
> >>>>>> like it."
> >>>>>
> >>>>> But not any that I mentioned; what ARE those other "convoluted
> >>>>> arguments"? I'd like to judge their merits for myself...
> >>>>
> >>>>
> >>>> I was just thinking how crazy it woud be to use, say, a 5000-tap
> >>>> FIR filter to compute a good autozero average value of the last
> >>>> 5000 samples.
> >>>>
> >>>> Those 5000 multiply-by-1-and-add blocks will need a lot of logic.
> >>>
> >>> Clearly not the best way to make a boxcar filter... LOL. I see what
> >>> you did there.
> >>>
> >>
> >> You still need a bunch of memory, because on each cycle you have to add
> >> the new sample and subtract the one from 5000 cycles ago.  IIR avoids
> >> that problem.
> >>
> > 
> > https://en.wikipedia.org/wiki/Cascaded_integrator%E2%80%93comb_filter
> > 
> 
> CICS is an IIR filter.
> 

it is recursive but is it IIR? ;)

On Tuesday, September 22, 2020 at 1:35:58 PM UTC-4, Lasse Langwadt Christensen wrote:
> tirsdag den 22. september 2020 kl. 03.47.59 UTC+2 skrev Phil Hobbs:
> > On 2020-09-21 21:32, Lasse Langwadt Christensen wrote:
> > > tirsdag den 22. september 2020 kl. 02.47.37 UTC+2 skrev Phil Hobbs:
> > >> On 2020-09-21 00:06, Jasen Betts wrote:
> > >>> On 2020-09-21, jlarkin@highlandsniptechnology.com
> > >>> <jlarkin@highlandsniptechnology.com> wrote:
> > >>>> On Sun, 20 Sep 2020 16:29:30 -0700 (PDT), whit3rd
> > >>>> <whit3rd@gmail.com> wrote:
> > >>>>
> > >>>>> On Sunday, September 20, 2020 at 2:01:05 PM UTC-7,
> > >>>>> jla...@highlandsniptechnology.com wrote:
> > >>>>>> On Sun, 20 Sep 2020 13:33:11 -0700 (PDT), whit3rd
> > >>>>>> <whi...@gmail.com> wrote:
> > >>>>>>
> > >>>>>>> On Sunday, September 20, 2020 at 3:26:13 AM UTC-7,
> > >>>>>>> jla...@highlandsniptechnology.com wrote:
> > >>>>>>>> On Sun, 20 Sep 2020 00:26:47 -0700 (PDT), whit3rd
> > >>>>>>>> <whi...@gmail.com> wrote:
> > >>>>>>>>> On Saturday, September 19, 2020 at 9:49:47 PM UTC-7,
> > >>>>>>>>> jla...@highlandsniptechnology.com wrote:
> > >>>>>>>
> > >>>>>>>>>> Just average the samples and subtract that average from
> > >>>>>>>>>> each new sample. There are several ways to do that
> > >>>>>>>>>> average:
> > >>>>>>>>>>
> > >>>>>>>>>> Sum the last N samples and divide by N.
> > >>>>>>>>>
> > >>>>>>>>> that's a FIR filter (finite impulse response) ... if you
> > >>>>>>>>> choose the sample size and know the likely interference
> > >>>>>>>>> sources (like, 60 Hz ripple), it allows you to place a
> > >>>>>>>>> null appropriately
> > >>>>>>>>>
> > >>>>>>>>>> Exponential smoothing: Avg = Avg + (new-Avg) / N
> > >>>>>>>>>
> > >>>>>>>>> that's a IIR filter (infinite impulse response); usually
> > >>>>>>>>> not a great choice
> > >>>>>>>
> > >>>>>>>> Why not? I see a lot of irrational prejuduce against simple
> > >>>>>>>> IIR filters, in code and in FPGAs. Some people would rather
> > >>>>>>>> write a hundred lines of code instead of one.
> > >>>>>>>
> > >>>>>>> Oh, it's simple, all right, but it has a long startup
> > >>>>>>> transient.
> > >>>>>> Any lowpass filter or averager does. Just poke a starting value
> > >>>>>> into the integrator node if you're in a hurry, ADC midscale in
> > >>>>>> this case.
> > >>>>>
> > >>>>> A lowpass needn't be considered appropriate during startup (and
> > >>>>> brute-force setting a starting value helps).   FIR has a
> > >>>>> time-limit on its history, which is often completely appropriate
> > >>>>> and useful.
> > >>>>>
> > >>>>>
> > >>>>>>> That means it doesn't deal with lightning-strike artifacts
> > >>>>>>> well, either.
> > >>>>>> Presumably an ADC rails on a huge transient. Why would an IIR
> > >>>>>> filter be worse than a FIR for a spike?
> > >>>>>
> > >>>>> Small signal in big digitizer range, of course.  Your 'rails'
> > >>>>> scenario is a measurement failure, and there's multiple ways to
> > >>>>> treat such a thing, which FIR does by... ignoring the spike a few
> > >>>>> samples afterward. IIR doesn't do that, so saturating the
> > >>>>> digitizer is an alternate solution that you don't seem to
> > >>>>> dislike.
> > >>>>>
> > >>>>>> It's impressive...
> > >>>>>
> > >>>>> I'm pleased that my response impresses you.
> > >>>>>
> > >>>>>> how many convoluted arguments people make to avoid IIR digital
> > >>>>>> filters. Most of them reduce to "It's too simple and I don't
> > >>>>>> like it."
> > >>>>>
> > >>>>> But not any that I mentioned; what ARE those other "convoluted
> > >>>>> arguments"? I'd like to judge their merits for myself...
> > >>>>
> > >>>>
> > >>>> I was just thinking how crazy it woud be to use, say, a 5000-tap
> > >>>> FIR filter to compute a good autozero average value of the last
> > >>>> 5000 samples.
> > >>>>
> > >>>> Those 5000 multiply-by-1-and-add blocks will need a lot of logic.
> > >>>
> > >>> Clearly not the best way to make a boxcar filter... LOL. I see what
> > >>> you did there.
> > >>>
> > >>
> > >> You still need a bunch of memory, because on each cycle you have to add
> > >> the new sample and subtract the one from 5000 cycles ago.  IIR avoids
> > >> that problem.
> > >>
> > > 
> > > https://en.wikipedia.org/wiki/Cascaded_integrator%E2%80%93comb_filter
> > > 
> > 
> > CICS is an IIR filter.
> > 
> 
> it is recursive but is it IIR? ;)

The CIC filter is neither recursive or IIR.  But it can be designed using a structure that is similar to an IIR for efficiency much in the same way the FFT utilizes patterns in the sine function to calculate the discrete version of the Fourier transform.  The FFT is still calculating the same function as the DFT.  

Not sure what the CICS filter is so I can't say anything about that. 

-- 

  Rick C.

  +- Get 1,000 miles of free Supercharging
  +- Tesla referral code - https://ts.la/richard11209

tirsdag den 22. september 2020 kl. 20.22.06 UTC+2 skrev Ricketty C:
> On Tuesday, September 22, 2020 at 1:35:58 PM UTC-4, Lasse Langwadt Christensen wrote:
> > tirsdag den 22. september 2020 kl. 03.47.59 UTC+2 skrev Phil Hobbs:
> > > On 2020-09-21 21:32, Lasse Langwadt Christensen wrote:
> > > > tirsdag den 22. september 2020 kl. 02.47.37 UTC+2 skrev Phil Hobbs:
> > > >> On 2020-09-21 00:06, Jasen Betts wrote:
> > > >>> On 2020-09-21, jlarkin@highlandsniptechnology.com
> > > >>> <jlarkin@highlandsniptechnology.com> wrote:
> > > >>>> On Sun, 20 Sep 2020 16:29:30 -0700 (PDT), whit3rd
> > > >>>> <whit3rd@gmail.com> wrote:
> > > >>>>
> > > >>>>> On Sunday, September 20, 2020 at 2:01:05 PM UTC-7,
> > > >>>>> jla...@highlandsniptechnology.com wrote:
> > > >>>>>> On Sun, 20 Sep 2020 13:33:11 -0700 (PDT), whit3rd
> > > >>>>>> <whi...@gmail.com> wrote:
> > > >>>>>>
> > > >>>>>>> On Sunday, September 20, 2020 at 3:26:13 AM UTC-7,
> > > >>>>>>> jla...@highlandsniptechnology.com wrote:
> > > >>>>>>>> On Sun, 20 Sep 2020 00:26:47 -0700 (PDT), whit3rd
> > > >>>>>>>> <whi...@gmail.com> wrote:
> > > >>>>>>>>> On Saturday, September 19, 2020 at 9:49:47 PM UTC-7,
> > > >>>>>>>>> jla...@highlandsniptechnology.com wrote:
> > > >>>>>>>
> > > >>>>>>>>>> Just average the samples and subtract that average from
> > > >>>>>>>>>> each new sample. There are several ways to do that
> > > >>>>>>>>>> average:
> > > >>>>>>>>>>
> > > >>>>>>>>>> Sum the last N samples and divide by N.
> > > >>>>>>>>>
> > > >>>>>>>>> that's a FIR filter (finite impulse response) ... if you
> > > >>>>>>>>> choose the sample size and know the likely interference
> > > >>>>>>>>> sources (like, 60 Hz ripple), it allows you to place a
> > > >>>>>>>>> null appropriately
> > > >>>>>>>>>
> > > >>>>>>>>>> Exponential smoothing: Avg = Avg + (new-Avg) / N
> > > >>>>>>>>>
> > > >>>>>>>>> that's a IIR filter (infinite impulse response); usually
> > > >>>>>>>>> not a great choice
> > > >>>>>>>
> > > >>>>>>>> Why not? I see a lot of irrational prejuduce against simple
> > > >>>>>>>> IIR filters, in code and in FPGAs. Some people would rather
> > > >>>>>>>> write a hundred lines of code instead of one.
> > > >>>>>>>
> > > >>>>>>> Oh, it's simple, all right, but it has a long startup
> > > >>>>>>> transient.
> > > >>>>>> Any lowpass filter or averager does. Just poke a starting value
> > > >>>>>> into the integrator node if you're in a hurry, ADC midscale in
> > > >>>>>> this case.
> > > >>>>>
> > > >>>>> A lowpass needn't be considered appropriate during startup (and
> > > >>>>> brute-force setting a starting value helps).   FIR has a
> > > >>>>> time-limit on its history, which is often completely appropriate
> > > >>>>> and useful.
> > > >>>>>
> > > >>>>>
> > > >>>>>>> That means it doesn't deal with lightning-strike artifacts
> > > >>>>>>> well, either.
> > > >>>>>> Presumably an ADC rails on a huge transient. Why would an IIR
> > > >>>>>> filter be worse than a FIR for a spike?
> > > >>>>>
> > > >>>>> Small signal in big digitizer range, of course.  Your 'rails'
> > > >>>>> scenario is a measurement failure, and there's multiple ways to
> > > >>>>> treat such a thing, which FIR does by... ignoring the spike a few
> > > >>>>> samples afterward. IIR doesn't do that, so saturating the
> > > >>>>> digitizer is an alternate solution that you don't seem to
> > > >>>>> dislike.
> > > >>>>>
> > > >>>>>> It's impressive...
> > > >>>>>
> > > >>>>> I'm pleased that my response impresses you.
> > > >>>>>
> > > >>>>>> how many convoluted arguments people make to avoid IIR digital
> > > >>>>>> filters. Most of them reduce to "It's too simple and I don't
> > > >>>>>> like it."
> > > >>>>>
> > > >>>>> But not any that I mentioned; what ARE those other "convoluted
> > > >>>>> arguments"? I'd like to judge their merits for myself...
> > > >>>>
> > > >>>>
> > > >>>> I was just thinking how crazy it woud be to use, say, a 5000-tap
> > > >>>> FIR filter to compute a good autozero average value of the last
> > > >>>> 5000 samples.
> > > >>>>
> > > >>>> Those 5000 multiply-by-1-and-add blocks will need a lot of logic.
> > > >>>
> > > >>> Clearly not the best way to make a boxcar filter... LOL. I see what
> > > >>> you did there.
> > > >>>
> > > >>
> > > >> You still need a bunch of memory, because on each cycle you have to add
> > > >> the new sample and subtract the one from 5000 cycles ago.  IIR avoids
> > > >> that problem.
> > > >>
> > > > 
> > > > https://en.wikipedia.org/wiki/Cascaded_integrator%E2%80%93comb_filter
> > > > 
> > > 
> > > CICS is an IIR filter.
> > > 
> > 
> > it is recursive but is it IIR? ;)
> 
> The CIC filter is neither recursive or IIR. 

https://en.wikipedia.org/wiki/Recursive_filter

On Tuesday, September 22, 2020 at 4:16:25 PM UTC-4, Lasse Langwadt Christensen wrote:
> tirsdag den 22. september 2020 kl. 20.22.06 UTC+2 skrev Ricketty C:
> > On Tuesday, September 22, 2020 at 1:35:58 PM UTC-4, Lasse Langwadt Christensen wrote:
> > > tirsdag den 22. september 2020 kl. 03.47.59 UTC+2 skrev Phil Hobbs:
> > > > On 2020-09-21 21:32, Lasse Langwadt Christensen wrote:
> > > > > tirsdag den 22. september 2020 kl. 02.47.37 UTC+2 skrev Phil Hobbs:
> > > > >> On 2020-09-21 00:06, Jasen Betts wrote:
> > > > >>> On 2020-09-21, jlarkin@highlandsniptechnology.com
> > > > >>> <jlarkin@highlandsniptechnology.com> wrote:
> > > > >>>> On Sun, 20 Sep 2020 16:29:30 -0700 (PDT), whit3rd
> > > > >>>> <whit3rd@gmail.com> wrote:
> > > > >>>>
> > > > >>>>> On Sunday, September 20, 2020 at 2:01:05 PM UTC-7,
> > > > >>>>> jla...@highlandsniptechnology.com wrote:
> > > > >>>>>> On Sun, 20 Sep 2020 13:33:11 -0700 (PDT), whit3rd
> > > > >>>>>> <whi...@gmail.com> wrote:
> > > > >>>>>>
> > > > >>>>>>> On Sunday, September 20, 2020 at 3:26:13 AM UTC-7,
> > > > >>>>>>> jla...@highlandsniptechnology.com wrote:
> > > > >>>>>>>> On Sun, 20 Sep 2020 00:26:47 -0700 (PDT), whit3rd
> > > > >>>>>>>> <whi...@gmail.com> wrote:
> > > > >>>>>>>>> On Saturday, September 19, 2020 at 9:49:47 PM UTC-7,
> > > > >>>>>>>>> jla...@highlandsniptechnology.com wrote:
> > > > >>>>>>>
> > > > >>>>>>>>>> Just average the samples and subtract that average from
> > > > >>>>>>>>>> each new sample. There are several ways to do that
> > > > >>>>>>>>>> average:
> > > > >>>>>>>>>>
> > > > >>>>>>>>>> Sum the last N samples and divide by N.
> > > > >>>>>>>>>
> > > > >>>>>>>>> that's a FIR filter (finite impulse response) ... if you
> > > > >>>>>>>>> choose the sample size and know the likely interference
> > > > >>>>>>>>> sources (like, 60 Hz ripple), it allows you to place a
> > > > >>>>>>>>> null appropriately
> > > > >>>>>>>>>
> > > > >>>>>>>>>> Exponential smoothing: Avg = Avg + (new-Avg) / N
> > > > >>>>>>>>>
> > > > >>>>>>>>> that's a IIR filter (infinite impulse response); usually
> > > > >>>>>>>>> not a great choice
> > > > >>>>>>>
> > > > >>>>>>>> Why not? I see a lot of irrational prejuduce against simple
> > > > >>>>>>>> IIR filters, in code and in FPGAs. Some people would rather
> > > > >>>>>>>> write a hundred lines of code instead of one.
> > > > >>>>>>>
> > > > >>>>>>> Oh, it's simple, all right, but it has a long startup
> > > > >>>>>>> transient.
> > > > >>>>>> Any lowpass filter or averager does. Just poke a starting value
> > > > >>>>>> into the integrator node if you're in a hurry, ADC midscale in
> > > > >>>>>> this case.
> > > > >>>>>
> > > > >>>>> A lowpass needn't be considered appropriate during startup (and
> > > > >>>>> brute-force setting a starting value helps).   FIR has a
> > > > >>>>> time-limit on its history, which is often completely appropriate
> > > > >>>>> and useful.
> > > > >>>>>
> > > > >>>>>
> > > > >>>>>>> That means it doesn't deal with lightning-strike artifacts
> > > > >>>>>>> well, either.
> > > > >>>>>> Presumably an ADC rails on a huge transient. Why would an IIR
> > > > >>>>>> filter be worse than a FIR for a spike?
> > > > >>>>>
> > > > >>>>> Small signal in big digitizer range, of course.  Your 'rails'
> > > > >>>>> scenario is a measurement failure, and there's multiple ways to
> > > > >>>>> treat such a thing, which FIR does by... ignoring the spike a few
> > > > >>>>> samples afterward. IIR doesn't do that, so saturating the
> > > > >>>>> digitizer is an alternate solution that you don't seem to
> > > > >>>>> dislike.
> > > > >>>>>
> > > > >>>>>> It's impressive...
> > > > >>>>>
> > > > >>>>> I'm pleased that my response impresses you.
> > > > >>>>>
> > > > >>>>>> how many convoluted arguments people make to avoid IIR digital
> > > > >>>>>> filters. Most of them reduce to "It's too simple and I don't
> > > > >>>>>> like it."
> > > > >>>>>
> > > > >>>>> But not any that I mentioned; what ARE those other "convoluted
> > > > >>>>> arguments"? I'd like to judge their merits for myself...
> > > > >>>>
> > > > >>>>
> > > > >>>> I was just thinking how crazy it woud be to use, say, a 5000-tap
> > > > >>>> FIR filter to compute a good autozero average value of the last
> > > > >>>> 5000 samples.
> > > > >>>>
> > > > >>>> Those 5000 multiply-by-1-and-add blocks will need a lot of logic.
> > > > >>>
> > > > >>> Clearly not the best way to make a boxcar filter... LOL. I see what
> > > > >>> you did there.
> > > > >>>
> > > > >>
> > > > >> You still need a bunch of memory, because on each cycle you have to add
> > > > >> the new sample and subtract the one from 5000 cycles ago.  IIR avoids
> > > > >> that problem.
> > > > >>
> > > > > 
> > > > > https://en.wikipedia.org/wiki/Cascaded_integrator%E2%80%93comb_filter
> > > > > 
> > > > 
> > > > CICS is an IIR filter.
> > > > 
> > > 
> > > it is recursive but is it IIR? ;)
> > 
> > The CIC filter is neither recursive or IIR. 
> 
> https://en.wikipedia.org/wiki/Recursive_filter

Not sure what your point is.  The CIC is the sum of the last N samples, typically applied successively in small sections with decimation.  That is not a recursive filter or an IIR.  The fact that you can construct an equivalent filter using a recursive structure does not say anything about the nature of the filter.  

If there is something someone wishes to say about recursive filters and then try to apply that to a CIC filter, I'd like to hear it.  Otherwise we are just counting angels dancing on the head of a pin.  

-- 

  Rick C.

  ++ Get 1,000 miles of free Supercharging
  ++ Tesla referral code - https://ts.la/richard11209

tirsdag den 22. september 2020 kl. 22.28.27 UTC+2 skrev Ricketty C:
> On Tuesday, September 22, 2020 at 4:16:25 PM UTC-4, Lasse Langwadt Christensen wrote:
> > tirsdag den 22. september 2020 kl. 20.22.06 UTC+2 skrev Ricketty C:
> > > On Tuesday, September 22, 2020 at 1:35:58 PM UTC-4, Lasse Langwadt Christensen wrote:
> > > > tirsdag den 22. september 2020 kl. 03.47.59 UTC+2 skrev Phil Hobbs:
> > > > > On 2020-09-21 21:32, Lasse Langwadt Christensen wrote:
> > > > > > tirsdag den 22. september 2020 kl. 02.47.37 UTC+2 skrev Phil Hobbs:
> > > > > >> On 2020-09-21 00:06, Jasen Betts wrote:
> > > > > >>> On 2020-09-21, jlarkin@highlandsniptechnology.com
> > > > > >>> <jlarkin@highlandsniptechnology.com> wrote:
> > > > > >>>> On Sun, 20 Sep 2020 16:29:30 -0700 (PDT), whit3rd
> > > > > >>>> <whit3rd@gmail.com> wrote:
> > > > > >>>>
> > > > > >>>>> On Sunday, September 20, 2020 at 2:01:05 PM UTC-7,
> > > > > >>>>> jla...@highlandsniptechnology.com wrote:
> > > > > >>>>>> On Sun, 20 Sep 2020 13:33:11 -0700 (PDT), whit3rd
> > > > > >>>>>> <whi...@gmail.com> wrote:
> > > > > >>>>>>
> > > > > >>>>>>> On Sunday, September 20, 2020 at 3:26:13 AM UTC-7,
> > > > > >>>>>>> jla...@highlandsniptechnology.com wrote:
> > > > > >>>>>>>> On Sun, 20 Sep 2020 00:26:47 -0700 (PDT), whit3rd
> > > > > >>>>>>>> <whi...@gmail.com> wrote:
> > > > > >>>>>>>>> On Saturday, September 19, 2020 at 9:49:47 PM UTC-7,
> > > > > >>>>>>>>> jla...@highlandsniptechnology.com wrote:
> > > > > >>>>>>>
> > > > > >>>>>>>>>> Just average the samples and subtract that average from
> > > > > >>>>>>>>>> each new sample. There are several ways to do that
> > > > > >>>>>>>>>> average:
> > > > > >>>>>>>>>>
> > > > > >>>>>>>>>> Sum the last N samples and divide by N.
> > > > > >>>>>>>>>
> > > > > >>>>>>>>> that's a FIR filter (finite impulse response) ... if you
> > > > > >>>>>>>>> choose the sample size and know the likely interference
> > > > > >>>>>>>>> sources (like, 60 Hz ripple), it allows you to place a
> > > > > >>>>>>>>> null appropriately
> > > > > >>>>>>>>>
> > > > > >>>>>>>>>> Exponential smoothing: Avg = Avg + (new-Avg) / N
> > > > > >>>>>>>>>
> > > > > >>>>>>>>> that's a IIR filter (infinite impulse response); usually
> > > > > >>>>>>>>> not a great choice
> > > > > >>>>>>>
> > > > > >>>>>>>> Why not? I see a lot of irrational prejuduce against simple
> > > > > >>>>>>>> IIR filters, in code and in FPGAs. Some people would rather
> > > > > >>>>>>>> write a hundred lines of code instead of one.
> > > > > >>>>>>>
> > > > > >>>>>>> Oh, it's simple, all right, but it has a long startup
> > > > > >>>>>>> transient.
> > > > > >>>>>> Any lowpass filter or averager does. Just poke a starting value
> > > > > >>>>>> into the integrator node if you're in a hurry, ADC midscale in
> > > > > >>>>>> this case.
> > > > > >>>>>
> > > > > >>>>> A lowpass needn't be considered appropriate during startup (and
> > > > > >>>>> brute-force setting a starting value helps).   FIR has a
> > > > > >>>>> time-limit on its history, which is often completely appropriate
> > > > > >>>>> and useful.
> > > > > >>>>>
> > > > > >>>>>
> > > > > >>>>>>> That means it doesn't deal with lightning-strike artifacts
> > > > > >>>>>>> well, either.
> > > > > >>>>>> Presumably an ADC rails on a huge transient. Why would an IIR
> > > > > >>>>>> filter be worse than a FIR for a spike?
> > > > > >>>>>
> > > > > >>>>> Small signal in big digitizer range, of course.  Your 'rails'
> > > > > >>>>> scenario is a measurement failure, and there's multiple ways to
> > > > > >>>>> treat such a thing, which FIR does by... ignoring the spike a few
> > > > > >>>>> samples afterward. IIR doesn't do that, so saturating the
> > > > > >>>>> digitizer is an alternate solution that you don't seem to
> > > > > >>>>> dislike.
> > > > > >>>>>
> > > > > >>>>>> It's impressive...
> > > > > >>>>>
> > > > > >>>>> I'm pleased that my response impresses you.
> > > > > >>>>>
> > > > > >>>>>> how many convoluted arguments people make to avoid IIR digital
> > > > > >>>>>> filters. Most of them reduce to "It's too simple and I don't
> > > > > >>>>>> like it."
> > > > > >>>>>
> > > > > >>>>> But not any that I mentioned; what ARE those other "convoluted
> > > > > >>>>> arguments"? I'd like to judge their merits for myself...
> > > > > >>>>
> > > > > >>>>
> > > > > >>>> I was just thinking how crazy it woud be to use, say, a 5000-tap
> > > > > >>>> FIR filter to compute a good autozero average value of the last
> > > > > >>>> 5000 samples.
> > > > > >>>>
> > > > > >>>> Those 5000 multiply-by-1-and-add blocks will need a lot of logic.
> > > > > >>>
> > > > > >>> Clearly not the best way to make a boxcar filter... LOL. I see what
> > > > > >>> you did there.
> > > > > >>>
> > > > > >>
> > > > > >> You still need a bunch of memory, because on each cycle you have to add
> > > > > >> the new sample and subtract the one from 5000 cycles ago.  IIR avoids
> > > > > >> that problem.
> > > > > >>
> > > > > > 
> > > > > > https://en.wikipedia.org/wiki/Cascaded_integrator%E2%80%93comb_filter
> > > > > > 
> > > > > 
> > > > > CICS is an IIR filter.
> > > > > 
> > > > 
> > > > it is recursive but is it IIR? ;)
> > > 
> > > The CIC filter is neither recursive or IIR. 
> > 
> > https://en.wikipedia.org/wiki/Recursive_filter
> 
> Not sure what your point is.  The CIC is the sum of the last N samples, typically applied successively in small sections with decimation.  That is not a recursive filter or an IIR.  The fact that you can construct an equivalent filter using a recursive structure does not say anything about the nature of the filter.  
> 
> If there is something someone wishes to say about recursive filters and then try to apply that to a CIC filter, I'd like to hear it.  Otherwise we are just counting angels dancing on the head of a pin.  
> 

CIC is a recursive implementation of specific FIR filter  

On Mon, 21 Sep 2020 20:50:42 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>On 2020-09-21 19:12, John Larkin wrote:
>> On Mon, 21 Sep 2020 15:36:26 -0700 (PDT), jrwalliker@gmail.com wrote:
>> 
>>> On Monday, 21 September 2020 19:21:44 UTC+1, Kevin Aylward  wrote:
>>>>> "Ricketty C"  wrote in message
>>>>> news:97ab1d7f-07cf-4caa-9faa-1e66f2b86b4eo@googlegroups.com...
>>>>
>>>>> On Saturday, September 19, 2020 at 11:38:09 PM UTC-4, Gold_Spark wrote:
>>>>> I'm using a STM32 Cortex M0+ to read an AC signal from a CT. I'm sampling
>>>>> at 6kHz and storing 400 samples. The signal has a DC bias equal to Vcc/2 =
>>>>>   >1.65V. In the digital domain this is 2048. In hardware this DC value is
>>>>> very precise, but when sampling it, it varies from 2044 to 2052 inside the
>>>>> buffer. Now >if I want to do RMS in that set of data, I need to find a way
>>>>> to deal with this DC bias variation.
>>>>>
>>>>>> I have been thinking the following:
>>>>>
>>>>>> 1- Subtract a fixed value of 2048 from each ADC reading. This is no so
>>>>>> good as I said above this value may vary slightly. Also, if I want to
>>>>>> read zero cross it >>may cause errors to choose exactly 2048 as
>>>>>> reference.
>>>>
>>>>> 4- Use a more sophisticated software high pass filter?
>>>>
>>>>> Funny that people propose you use a low pass filter and subtract.  A high
>>>>> pass would just provide the AC without the DC.  It would take some time to
>>>>> start >up, but can work very effectively.  If this sampling is continuous
>>>>> that would work fine.
>>>>
>>>> Yeah... seems like the twilight zone here. All this FIR, averaging and
>>>> $hit... seems to be... the plot is lost....
>>>>
>>>>  From the description the poster is measurement an AC signal, thus stick a
>>>> cap on the input to block the DC and you're done....
>>>>
>>> There is no dc to remove from the input.  Its a current transformer.
>>> The problem is that the ADC needs to be biased at the midpoint of
>>> the positive-only supply so that it can digitise positive and negative
>>> outputs from the CT.  The converted values of that midpoint bias are
>>> not completely stable and need to be removed before the rms calculation, otherwise small alternating signals are swamped by the offset error.
>>>
>>> John
>> 
>> Exactly. You can auto-zero to a fraction of an ADC LSB. Then your RMS
>> measurement is limited by ADC quantization and linearity. ADC offset
>> usually drifts slowly, so the autozero can use tons of samples and
>> need not be especially fast. Averaging the last 65536 samples at your
>> 6K rate would work fine. You'd get a new az value every 10 seconds.
>> 
>> I did software az in my electric meters. And added a little noise
>> dither to the ADC front end. That added a little baseline offset to
>> the reported RMS currents, but vastly improved low-level power
>> measurement. It's actually hard to design electronics that's as good
>> as an old disk-type meter. The only spec I really beat them on was
>> tilt.
>> 
>> There's a trick to adding dither without increasing the apparent RMS
>> floor much. The nuclear guys do that in pulse-height spectroscopy.
>
>Subtracting the dither off again digitally?
>
>Dithering is surprisingly subtle occasionally--for instance, you can't 
>do a good job dithering a 1-bit delta-sigma because it has no linear range.

Hmm.  The Sonar folk used one-bit correlators twenty or thirty years
ago, and were able to reach very deep into the background noise (the
oceans are very noisy from biological sources).  That noise acted as a
dither source.

Joe Gwinn

tirsdag den 22. september 2020 kl. 23.16.36 UTC+2 skrev Joe Gwinn:
> On Mon, 21 Sep 2020 20:50:42 -0400, Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
> 
> >On 2020-09-21 19:12, John Larkin wrote:
> >> On Mon, 21 Sep 2020 15:36:26 -0700 (PDT), jrwalliker@gmail.com wrote:
> >> 
> >>> On Monday, 21 September 2020 19:21:44 UTC+1, Kevin Aylward  wrote:
> >>>>> "Ricketty C"  wrote in message
> >>>>> news:97ab1d7f-07cf-4caa-9faa-1e66f2b86b4eo@googlegroups.com...
> >>>>
> >>>>> On Saturday, September 19, 2020 at 11:38:09 PM UTC-4, Gold_Spark wrote:
> >>>>> I'm using a STM32 Cortex M0+ to read an AC signal from a CT. I'm sampling
> >>>>> at 6kHz and storing 400 samples. The signal has a DC bias equal to Vcc/2 =
> >>>>>   >1.65V. In the digital domain this is 2048. In hardware this DC value is
> >>>>> very precise, but when sampling it, it varies from 2044 to 2052 inside the
> >>>>> buffer. Now >if I want to do RMS in that set of data, I need to find a way
> >>>>> to deal with this DC bias variation.
> >>>>>
> >>>>>> I have been thinking the following:
> >>>>>
> >>>>>> 1- Subtract a fixed value of 2048 from each ADC reading. This is no so
> >>>>>> good as I said above this value may vary slightly. Also, if I want to
> >>>>>> read zero cross it >>may cause errors to choose exactly 2048 as
> >>>>>> reference.
> >>>>
> >>>>> 4- Use a more sophisticated software high pass filter?
> >>>>
> >>>>> Funny that people propose you use a low pass filter and subtract.  A high
> >>>>> pass would just provide the AC without the DC.  It would take some time to
> >>>>> start >up, but can work very effectively.  If this sampling is continuous
> >>>>> that would work fine.
> >>>>
> >>>> Yeah... seems like the twilight zone here. All this FIR, averaging and
> >>>> $hit... seems to be... the plot is lost....
> >>>>
> >>>>  From the description the poster is measurement an AC signal, thus stick a
> >>>> cap on the input to block the DC and you're done....
> >>>>
> >>> There is no dc to remove from the input.  Its a current transformer.
> >>> The problem is that the ADC needs to be biased at the midpoint of
> >>> the positive-only supply so that it can digitise positive and negative
> >>> outputs from the CT.  The converted values of that midpoint bias are
> >>> not completely stable and need to be removed before the rms calculation, otherwise small alternating signals are swamped by the offset error.
> >>>
> >>> John
> >> 
> >> Exactly. You can auto-zero to a fraction of an ADC LSB. Then your RMS
> >> measurement is limited by ADC quantization and linearity. ADC offset
> >> usually drifts slowly, so the autozero can use tons of samples and
> >> need not be especially fast. Averaging the last 65536 samples at your
> >> 6K rate would work fine. You'd get a new az value every 10 seconds.
> >> 
> >> I did software az in my electric meters. And added a little noise
> >> dither to the ADC front end. That added a little baseline offset to
> >> the reported RMS currents, but vastly improved low-level power
> >> measurement. It's actually hard to design electronics that's as good
> >> as an old disk-type meter. The only spec I really beat them on was
> >> tilt.
> >> 
> >> There's a trick to adding dither without increasing the apparent RMS
> >> floor much. The nuclear guys do that in pulse-height spectroscopy.
> >
> >Subtracting the dither off again digitally?
> >
> >Dithering is surprisingly subtle occasionally--for instance, you can't 
> >do a good job dithering a 1-bit delta-sigma because it has no linear range.
> 
> Hmm.  The Sonar folk used one-bit correlators twenty or thirty years
> ago, and were able to reach very deep into the background noise (the
> oceans are very noisy from biological sources).  That noise acted as a
> dither source.
> 

Here's a GPS receiver with a "1-bit ADC" (an LVDS receiver)

http://www.aholme.co.uk/GPS/Main.htm

On Tuesday, 22 September 2020 22:24:26 UTC+1, Lasse Langwadt Christensen  wrote:
> tirsdag den 22. september 2020 kl. 23.16.36 UTC+2 skrev Joe Gwinn:
> > On Mon, 21 Sep 2020 20:50:42 -0400, Phil Hobbs
> > <pcdhSpamMeSenseless@electrooptical.net> wrote:
> > 
> > >On 2020-09-21 19:12, John Larkin wrote:
> > >> On Mon, 21 Sep 2020 15:36:26 -0700 (PDT), jrwalliker@gmail.com wrote:
> > >> 
> > >>> On Monday, 21 September 2020 19:21:44 UTC+1, Kevin Aylward  wrote:
> > >>>>> "Ricketty C"  wrote in message
> > >>>>> news:97ab1d7f-07cf-4caa-9faa-1e66f2b86b4eo@googlegroups.com...
> > >>>>
> > >>>>> On Saturday, September 19, 2020 at 11:38:09 PM UTC-4, Gold_Spark wrote:
> > >>>>> I'm using a STM32 Cortex M0+ to read an AC signal from a CT. I'm sampling
> > >>>>> at 6kHz and storing 400 samples. The signal has a DC bias equal to Vcc/2 =
> > >>>>>   >1.65V. In the digital domain this is 2048. In hardware this DC value is
> > >>>>> very precise, but when sampling it, it varies from 2044 to 2052 inside the
> > >>>>> buffer. Now >if I want to do RMS in that set of data, I need to find a way
> > >>>>> to deal with this DC bias variation.
> > >>>>>
> > >>>>>> I have been thinking the following:
> > >>>>>
> > >>>>>> 1- Subtract a fixed value of 2048 from each ADC reading. This is no so
> > >>>>>> good as I said above this value may vary slightly. Also, if I want to
> > >>>>>> read zero cross it >>may cause errors to choose exactly 2048 as
> > >>>>>> reference.
> > >>>>
> > >>>>> 4- Use a more sophisticated software high pass filter?
> > >>>>
> > >>>>> Funny that people propose you use a low pass filter and subtract.  A high
> > >>>>> pass would just provide the AC without the DC.  It would take some time to
> > >>>>> start >up, but can work very effectively.  If this sampling is continuous
> > >>>>> that would work fine.
> > >>>>
> > >>>> Yeah... seems like the twilight zone here. All this FIR, averaging and
> > >>>> $hit... seems to be... the plot is lost....
> > >>>>
> > >>>>  From the description the poster is measurement an AC signal, thus stick a
> > >>>> cap on the input to block the DC and you're done....
> > >>>>
> > >>> There is no dc to remove from the input.  Its a current transformer.
> > >>> The problem is that the ADC needs to be biased at the midpoint of
> > >>> the positive-only supply so that it can digitise positive and negative
> > >>> outputs from the CT.  The converted values of that midpoint bias are
> > >>> not completely stable and need to be removed before the rms calculation, otherwise small alternating signals are swamped by the offset error.
> > >>>
> > >>> John
> > >> 
> > >> Exactly. You can auto-zero to a fraction of an ADC LSB. Then your RMS
> > >> measurement is limited by ADC quantization and linearity. ADC offset
> > >> usually drifts slowly, so the autozero can use tons of samples and
> > >> need not be especially fast. Averaging the last 65536 samples at your
> > >> 6K rate would work fine. You'd get a new az value every 10 seconds.
> > >> 
> > >> I did software az in my electric meters. And added a little noise
> > >> dither to the ADC front end. That added a little baseline offset to
> > >> the reported RMS currents, but vastly improved low-level power
> > >> measurement. It's actually hard to design electronics that's as good
> > >> as an old disk-type meter. The only spec I really beat them on was
> > >> tilt.
> > >> 
> > >> There's a trick to adding dither without increasing the apparent RMS
> > >> floor much. The nuclear guys do that in pulse-height spectroscopy.
> > >
> > >Subtracting the dither off again digitally?
> > >
> > >Dithering is surprisingly subtle occasionally--for instance, you can't 
> > >do a good job dithering a 1-bit delta-sigma because it has no linear range.
> > 
> > Hmm.  The Sonar folk used one-bit correlators twenty or thirty years
> > ago, and were able to reach very deep into the background noise (the
> > oceans are very noisy from biological sources).  That noise acted as a
> > dither source.
> > 
> 
> Here's a GPS receiver with a "1-bit ADC" (an LVDS receiver)
> 
> http://www.aholme.co.uk/GPS/Main.htm

Motorola were making GPS receivers with 1-bit adcs at least 20 years
ago, possibly considerably more.

John

On Tuesday, September 22, 2020 at 5:10:31 PM UTC-4, Lasse Langwadt Christensen wrote:
> tirsdag den 22. september 2020 kl. 22.28.27 UTC+2 skrev Ricketty C:
> > On Tuesday, September 22, 2020 at 4:16:25 PM UTC-4, Lasse Langwadt Christensen wrote:
> > > tirsdag den 22. september 2020 kl. 20.22.06 UTC+2 skrev Ricketty C:
> > > > On Tuesday, September 22, 2020 at 1:35:58 PM UTC-4, Lasse Langwadt Christensen wrote:
> > > > > tirsdag den 22. september 2020 kl. 03.47.59 UTC+2 skrev Phil Hobbs:
> > > > > > On 2020-09-21 21:32, Lasse Langwadt Christensen wrote:
> > > > > > > tirsdag den 22. september 2020 kl. 02.47.37 UTC+2 skrev Phil Hobbs:
> > > > > > >> On 2020-09-21 00:06, Jasen Betts wrote:
> > > > > > >>> On 2020-09-21, jlarkin@highlandsniptechnology.com
> > > > > > >>> <jlarkin@highlandsniptechnology.com> wrote:
> > > > > > >>>> On Sun, 20 Sep 2020 16:29:30 -0700 (PDT), whit3rd
> > > > > > >>>> <whit3rd@gmail.com> wrote:
> > > > > > >>>>
> > > > > > >>>>> On Sunday, September 20, 2020 at 2:01:05 PM UTC-7,
> > > > > > >>>>> jla...@highlandsniptechnology.com wrote:
> > > > > > >>>>>> On Sun, 20 Sep 2020 13:33:11 -0700 (PDT), whit3rd
> > > > > > >>>>>> <whi...@gmail.com> wrote:
> > > > > > >>>>>>
> > > > > > >>>>>>> On Sunday, September 20, 2020 at 3:26:13 AM UTC-7,
> > > > > > >>>>>>> jla...@highlandsniptechnology.com wrote:
> > > > > > >>>>>>>> On Sun, 20 Sep 2020 00:26:47 -0700 (PDT), whit3rd
> > > > > > >>>>>>>> <whi...@gmail.com> wrote:
> > > > > > >>>>>>>>> On Saturday, September 19, 2020 at 9:49:47 PM UTC-7,
> > > > > > >>>>>>>>> jla...@highlandsniptechnology.com wrote:
> > > > > > >>>>>>>
> > > > > > >>>>>>>>>> Just average the samples and subtract that average from
> > > > > > >>>>>>>>>> each new sample. There are several ways to do that
> > > > > > >>>>>>>>>> average:
> > > > > > >>>>>>>>>>
> > > > > > >>>>>>>>>> Sum the last N samples and divide by N.
> > > > > > >>>>>>>>>
> > > > > > >>>>>>>>> that's a FIR filter (finite impulse response) ... if you
> > > > > > >>>>>>>>> choose the sample size and know the likely interference
> > > > > > >>>>>>>>> sources (like, 60 Hz ripple), it allows you to place a
> > > > > > >>>>>>>>> null appropriately
> > > > > > >>>>>>>>>
> > > > > > >>>>>>>>>> Exponential smoothing: Avg = Avg + (new-Avg) / N
> > > > > > >>>>>>>>>
> > > > > > >>>>>>>>> that's a IIR filter (infinite impulse response); usually
> > > > > > >>>>>>>>> not a great choice
> > > > > > >>>>>>>
> > > > > > >>>>>>>> Why not? I see a lot of irrational prejuduce against simple
> > > > > > >>>>>>>> IIR filters, in code and in FPGAs. Some people would rather
> > > > > > >>>>>>>> write a hundred lines of code instead of one.
> > > > > > >>>>>>>
> > > > > > >>>>>>> Oh, it's simple, all right, but it has a long startup
> > > > > > >>>>>>> transient.
> > > > > > >>>>>> Any lowpass filter or averager does. Just poke a starting value
> > > > > > >>>>>> into the integrator node if you're in a hurry, ADC midscale in
> > > > > > >>>>>> this case.
> > > > > > >>>>>
> > > > > > >>>>> A lowpass needn't be considered appropriate during startup (and
> > > > > > >>>>> brute-force setting a starting value helps).   FIR has a
> > > > > > >>>>> time-limit on its history, which is often completely appropriate
> > > > > > >>>>> and useful.
> > > > > > >>>>>
> > > > > > >>>>>
> > > > > > >>>>>>> That means it doesn't deal with lightning-strike artifacts
> > > > > > >>>>>>> well, either.
> > > > > > >>>>>> Presumably an ADC rails on a huge transient. Why would an IIR
> > > > > > >>>>>> filter be worse than a FIR for a spike?
> > > > > > >>>>>
> > > > > > >>>>> Small signal in big digitizer range, of course.  Your 'rails'
> > > > > > >>>>> scenario is a measurement failure, and there's multiple ways to
> > > > > > >>>>> treat such a thing, which FIR does by... ignoring the spike a few
> > > > > > >>>>> samples afterward. IIR doesn't do that, so saturating the
> > > > > > >>>>> digitizer is an alternate solution that you don't seem to
> > > > > > >>>>> dislike.
> > > > > > >>>>>
> > > > > > >>>>>> It's impressive...
> > > > > > >>>>>
> > > > > > >>>>> I'm pleased that my response impresses you.
> > > > > > >>>>>
> > > > > > >>>>>> how many convoluted arguments people make to avoid IIR digital
> > > > > > >>>>>> filters. Most of them reduce to "It's too simple and I don't
> > > > > > >>>>>> like it."
> > > > > > >>>>>
> > > > > > >>>>> But not any that I mentioned; what ARE those other "convoluted
> > > > > > >>>>> arguments"? I'd like to judge their merits for myself...
> > > > > > >>>>
> > > > > > >>>>
> > > > > > >>>> I was just thinking how crazy it woud be to use, say, a 5000-tap
> > > > > > >>>> FIR filter to compute a good autozero average value of the last
> > > > > > >>>> 5000 samples.
> > > > > > >>>>
> > > > > > >>>> Those 5000 multiply-by-1-and-add blocks will need a lot of logic.
> > > > > > >>>
> > > > > > >>> Clearly not the best way to make a boxcar filter... LOL. I see what
> > > > > > >>> you did there.
> > > > > > >>>
> > > > > > >>
> > > > > > >> You still need a bunch of memory, because on each cycle you have to add
> > > > > > >> the new sample and subtract the one from 5000 cycles ago.  IIR avoids
> > > > > > >> that problem.
> > > > > > >>
> > > > > > > 
> > > > > > > https://en.wikipedia.org/wiki/Cascaded_integrator%E2%80%93comb_filter
> > > > > > > 
> > > > > > 
> > > > > > CICS is an IIR filter.
> > > > > > 
> > > > > 
> > > > > it is recursive but is it IIR? ;)
> > > > 
> > > > The CIC filter is neither recursive or IIR. 
> > > 
> > > https://en.wikipedia.org/wiki/Recursive_filter
> > 
> > Not sure what your point is.  The CIC is the sum of the last N samples, typically applied successively in small sections with decimation.  That is not a recursive filter or an IIR.  The fact that you can construct an equivalent filter using a recursive structure does not say anything about the nature of the filter.  
> > 
> > If there is something someone wishes to say about recursive filters and then try to apply that to a CIC filter, I'd like to hear it.  Otherwise we are just counting angels dancing on the head of a pin.  
> > 
> 
> CIC is a recursive implementation of specific FIR filter

It has been many years since I implemented one of these filters, but I do recall reading Hoganhour's paper.  I guess I'm not recalling it correctly. 

-- 

  Rick C.

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