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. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Removing DC offset from ADC Buffer
Started by ●September 20, 2020
Reply by ●September 21, 20202020-09-21
Reply by ●September 21, 20202020-09-21
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. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●September 21, 20202020-09-21
On Monday, September 21, 2020 at 2:21:44 PM UTC-4, 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....I think the DC is present to center the AC in the ADC range. If you remove the bias you get the positive half of the sine wave only. Unless we know more about what he was measuring when he got the numbers he provided we won't know if he even needs to do any filtering or if he could just subtract 2048 from each sample. -- Rick C. -- Get 1,000 miles of free Supercharging -- Tesla referral code - https://ts.la/richard11209
Reply by ●September 21, 20202020-09-21
tirsdag den 22. september 2020 kl. 02.50.53 UTC+2 skrev Phil Hobbs:> 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. >you could say delta-sigma is nothing but dither and that is what makes it work
Reply by ●September 21, 20202020-09-21
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
Reply by ●September 21, 20202020-09-21
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. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●September 21, 20202020-09-21
On 2020-09-21 21:30, Lasse Langwadt Christensen wrote:> tirsdag den 22. september 2020 kl. 02.50.53 UTC+2 skrev Phil Hobbs: >> 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. >> > > you could say delta-sigma is nothing but dither and that is what makes > it work >Sort of, assuming that your noise has good properties, which it often does not. Anyway, you can't usefully dither a 1-bit delta-sigma. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●September 21, 20202020-09-21
On Saturday, September 19, 2020 at 8:38:09 PM UTC-7, 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. > 2- Sample the DC bias and average it. > 3- Don't remove the DC bias. If I calculate RMS then I would get the DC value when there's no input signal. > 4- Use a more sophisticated software high pass filter? > > Example of DC bias readings: > > ADC_buffer > [0] 2048 > [1] 2046 > [2] 2049 > [3] 2051 > [4] 2051 > [5] 2052 > [6] 2050 > [7] 2050 > [8] 2050 > [9] 2047 > [10] 2049 > [11] 2050 > [12] 2049 > > I appreciate any help or suggestion.You didn't say how fast you want to converge on a solution. Or what is the bandwidth of your output? If your result is DC and has a long time to respond to a change, then just average. But if you need a quick response to a step change, then you will need to do more sophisticated digital filtering. A simple solution is an FIR highpass filter to get rid of the DC component. But you have to specify how fast and how accurately you need a result to design the filter.
Reply by ●September 21, 20202020-09-21
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?Right. You have to know the dither contribution to every sample, so generate it with a DAC. Some people dither a lot, many ADC bits. I generally used an async triangle wave as the dither in my electric meters. I got utility-grade power metering, a huge dynamic range, with a dithered 7-bit single-slope ADC. In power/energy metering, you multiply the voltage and current samples and integrate that long-term, so all sorts of errors just go away.> >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. > >Cheers > >Phil Hobbs-- John Larkin Highland Technology, Inc Science teaches us to doubt. Claude Bernard
Reply by ●September 22, 20202020-09-22
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. -- Jasen.
