Forums

add one resistor, quiet ugly smps ADC noise

Started by Winfield Hill June 28, 2018
On Fri, 29 Jun 2018 03:49:06 -0500, "Tim Williams"
<tiwill@seventransistorlabs.com> wrote:

>"John Larkin" <jjlarkin@highland_snip_technology.com> wrote in message >news:h0dajdh6jgchvfjm2bpo1uc8745sm8kb7q@4ax.com... >> 16 bits would be awful. Those LTM switcher bricks are pretty quiet. >> > >Having pushed one through EMC, and seen the ~ns edges from one, I laugh >derisively. > >Tim
The LTM8023 is a non-synchronous switcher, which doesn't have the giant spikes of a synchronous switcher. The switcher is a relatively slow NPN. All the nasty ground loops are internal to the brick. Used properly, it is very quiet. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On 29 Jun 2018 05:18:13 -0700, Winfield Hill
<hill@rowland.harvard.edu> wrote:

>sea moss wrote... >> >> I'm assuming you were already using a quiet ground >> for the ADC with a single-point tie to system ground? >> And possibly a resistor in series with its Vdd pin? >> And this wasn't enough? > > Nope, sorry, there was no room for best practices on > this long thin PCB. Yes, a separate "quiet" analog > supply, but was forced to be satisfied with a single > slightly-fattened ground trace running the length of > the board. And the TIA amp was next to the 14-volt > boost converter. Yes, very risky, but perfectly OK, > with the single-added-resistor smps shutoff trick.
Single-point grounding is usually not practical. An ADC has an analog input, clock, power, and digital i/o. All of those have their own grounds. It doesn't help to add inductance to the adc chip's common. -- John Larkin Highland Technology, Inc lunatic fringe electronics
On 06/28/18 20:38, John Larkin wrote:
> On Thu, 28 Jun 2018 19:59:53 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> On 06/28/18 15:40, John Larkin wrote: >>> On Thu, 28 Jun 2018 12:21:06 -0700 (PDT), Lasse Langwadt Christensen >>> <langwadt@fonz.dk> wrote: >>> >>>> torsdag den 28. juni 2018 kl. 21.09.47 UTC+2 skrev Winfield Hill: >>>>> Klaus Kragelund wrote... >>>>>> >>>>>> I don't really see the point of your efforts about this >>>>>> With 16bit, one LSB is 200uV >>>>>> >>>>>> You most likely have about 1uF output cap and a switching >>>>>> ripple of 100mV. So you will never measure this correctly >>>>>> if you don't filter the heck out of it, ... >>>>> >>>>> No, sorry, there's a high-res ADC on the circuit board >>>>> near the converter, measuring a TIA-amplifier signal. >>>>> The converter creates 14V for a 50mA LED current source. >>>>> The TIA amp, ADC and smps are sharing grounds, etc., and >>>>> the introduced converter noise looks huge on the amplified >>>>> signal, when viewed on a scope or with the ADC data. But >>>>> the noise disappears from the scope and ADC data while >>>>> the converter is momentarily stopped. BTW, we may only be >>>>> using 10 to 12 bits of the data, but the max noise seen >>>>> must have been at the 4 to 6-bit level! Anyway, I added >>>>> the extra 3rd resistor to the next rev of the PCB and now >>>>> we're happy. I re-purposed a controller bit that wasn't >>>>> busy doing anything else during the ADC conversions. >>>>> >>>>> I thought others might like to know about this little trick. >>>>> >>>> >>>> I've done similar and also paused things like display updates >>>> during burst of sampling >>>> >>>> I believe there also are some MCUs that can automatically go into >>>> "sleepmode" during sampling >>>> >>>> >>>> >>> >>> Like, computing trashes the on-chip ADC? Makes sense. >>> >>> The ARMs that we use can, I think, have the ADC programmed to sample >>> periodically and then interrupt. So the CPU could sleep-on-interrupt. >>> >>> An external ADC isn't bad; just keep the SPI bus quiet while the ADC >>> is busy. Some SAR ADCs are actually sequenced by the SPI burst, but >>> that's probably OK. >> >> We like the 14-bit TI ADC141S626, which works like that. It has this >> weird continuous mode where you leave CS' low, and it takes 18 clocks >> for a 14-bit conversion. That's a nuisance since the MCUs we mostly use >> (NXP Cortex Ms) can't do SPI transfers that long in hardware. >> >> However, if you work CS' on each acquisition, it only needs 16 clocks. >> It has super nice DNL and other good properties, and it's about $3 in >> thousands. >> >> Cheers >> >> Phil Hobbs > > We use ADS7866 for a cheap outboard ADC. SOT-23-6. 12 bits, $1.50 by > the reel. > > I once built a 10-bit ADC out of parts... transistors, no ICs. 3U > rackmount. It sold for more than my Austin Healey Sprite. > > I interfaced it to an IBM 1401. It was used to digitize waveforms from > monkey brains. Try to top that! > >
Well, I did build diodes that worked from DC to nearly daylight (200 THz). ;) But only one bit at a time. 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
On Fri, 29 Jun 2018 11:11:42 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>On 06/28/18 20:38, John Larkin wrote: >> On Thu, 28 Jun 2018 19:59:53 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> On 06/28/18 15:40, John Larkin wrote: >>>> On Thu, 28 Jun 2018 12:21:06 -0700 (PDT), Lasse Langwadt Christensen >>>> <langwadt@fonz.dk> wrote: >>>> >>>>> torsdag den 28. juni 2018 kl. 21.09.47 UTC+2 skrev Winfield Hill: >>>>>> Klaus Kragelund wrote... >>>>>>> >>>>>>> I don't really see the point of your efforts about this >>>>>>> With 16bit, one LSB is 200uV >>>>>>> >>>>>>> You most likely have about 1uF output cap and a switching >>>>>>> ripple of 100mV. So you will never measure this correctly >>>>>>> if you don't filter the heck out of it, ... >>>>>> >>>>>> No, sorry, there's a high-res ADC on the circuit board >>>>>> near the converter, measuring a TIA-amplifier signal. >>>>>> The converter creates 14V for a 50mA LED current source. >>>>>> The TIA amp, ADC and smps are sharing grounds, etc., and >>>>>> the introduced converter noise looks huge on the amplified >>>>>> signal, when viewed on a scope or with the ADC data. But >>>>>> the noise disappears from the scope and ADC data while >>>>>> the converter is momentarily stopped. BTW, we may only be >>>>>> using 10 to 12 bits of the data, but the max noise seen >>>>>> must have been at the 4 to 6-bit level! Anyway, I added >>>>>> the extra 3rd resistor to the next rev of the PCB and now >>>>>> we're happy. I re-purposed a controller bit that wasn't >>>>>> busy doing anything else during the ADC conversions. >>>>>> >>>>>> I thought others might like to know about this little trick. >>>>>> >>>>> >>>>> I've done similar and also paused things like display updates >>>>> during burst of sampling >>>>> >>>>> I believe there also are some MCUs that can automatically go into >>>>> "sleepmode" during sampling >>>>> >>>>> >>>>> >>>> >>>> Like, computing trashes the on-chip ADC? Makes sense. >>>> >>>> The ARMs that we use can, I think, have the ADC programmed to sample >>>> periodically and then interrupt. So the CPU could sleep-on-interrupt. >>>> >>>> An external ADC isn't bad; just keep the SPI bus quiet while the ADC >>>> is busy. Some SAR ADCs are actually sequenced by the SPI burst, but >>>> that's probably OK. >>> >>> We like the 14-bit TI ADC141S626, which works like that. It has this >>> weird continuous mode where you leave CS' low, and it takes 18 clocks >>> for a 14-bit conversion. That's a nuisance since the MCUs we mostly use >>> (NXP Cortex Ms) can't do SPI transfers that long in hardware. >>> >>> However, if you work CS' on each acquisition, it only needs 16 clocks. >>> It has super nice DNL and other good properties, and it's about $3 in >>> thousands. >>> >>> Cheers >>> >>> Phil Hobbs >> >> We use ADS7866 for a cheap outboard ADC. SOT-23-6. 12 bits, $1.50 by >> the reel. >> >> I once built a 10-bit ADC out of parts... transistors, no ICs. 3U >> rackmount. It sold for more than my Austin Healey Sprite. >> >> I interfaced it to an IBM 1401. It was used to digitize waveforms from >> monkey brains. Try to top that! >> >> >Well, I did build diodes that worked from DC to nearly daylight (200 >THz). ;) > >But only one bit at a time. > >Cheers > >Phil Hobbs
It worked something like this. https://www.dropbox.com/s/f8b831iz1afa1oz/Old_SAR_ADC.JPG?raw=1 I may have clamped the summing node with some diodes. It's been a while. I did a bunch of 16-bit DACs with discretes too. They drove a giant display in some military Dr Strangelove sort of thing. I'be been lucky in working for companies with terrible marketing people who would take purchase orders for anything they could get. -- John Larkin Highland Technology, Inc lunatic fringe electronics
John Larkin wrote...
> > It worked something like this.
I'm surprised you didn't use the BJT switches in inverted mode. -- Thanks, - Win
On 29 Jun 2018 09:14:25 -0700, Winfield Hill
<hill@rowland.harvard.edu> wrote:

>John Larkin wrote... >> >> It worked something like this. > > I'm surprised you didn't use the BJT switches in inverted mode.
I did in my 16 bit DAC. Complementary over-driven emitter followers driving an r-2r ladder network. The simple NPNs were good enough for the 10 bit ADC. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Winfield Hill wrote:
> Switching converters and sensitive SAR ADCs don't > go well together.
Do modern designs still use SARs? It's been many years since I saw anything in print about them. Of course I understood SAR and dual slope when I was a teenager but still don't really grasp delta sigma.
On 30.6.18 16:58, Tom Del Rosso wrote:
> Winfield Hill wrote: >> Switching converters and sensitive SAR ADCs don't >> go well together. > > Do modern designs still use SARs? It's been many years since I saw > anything in print about them. > > Of course I understood SAR and dual slope when I was a teenager but > still don't really grasp delta sigma.
Delta-sigma is fine for slow signals. For excellent resolution, it needs hefty oversampling, leading to long conversion times. For quite fast sampling, SAR is OK; and for fast sampling, the way to go is a flash converter. -- -TV
On Saturday, June 30, 2018 at 10:43:23 AM UTC-4, Tauno Voipio wrote:
> On 30.6.18 16:58, Tom Del Rosso wrote: > > Winfield Hill wrote: > >> Switching converters and sensitive SAR ADCs don't > >> go well together. > > > > Do modern designs still use SARs? It's been many years since I saw > > anything in print about them. > > > > Of course I understood SAR and dual slope when I was a teenager but > > still don't really grasp delta sigma. > > Delta-sigma is fine for slow signals. For excellent resolution, it > needs hefty oversampling, leading to long conversion times. > > For quite fast sampling, SAR is OK; and for fast sampling, the > way to go is a flash converter.
It is easy to do the oversampling in delta sigma and can be done at many MHz. If you need MHz sample rate, then no, delta sigma is not the right technology. SAR can be done quite fast and these days there are very few true flash converters below some GHz. Instead of flash it is a hybrid of flash and SAR. It has some name that I can't recall at the moment. Rick C.
On Sat, 30 Jun 2018 09:58:34 -0400, "Tom Del Rosso"
<fizzbintuesday@that-google-mail-domain.com> wrote:

>Winfield Hill wrote: >> Switching converters and sensitive SAR ADCs don't >> go well together. > >Do modern designs still use SARs? It's been many years since I saw >anything in print about them. > >Of course I understood SAR and dual slope when I was a teenager but >still don't really grasp delta sigma. > >
Sure, there are lots of SAR ADCs around, up to 18 bits [1]. The three popular ADC architectures now are SAR Pipeline, which is multi-stage flash Delta-sigma. Dual-slope is rare nowadays. Capacitor DA is probably why. Delta-sigma just uses a duty-cycle-based DAC to balance the input signal into an integrator. When it's balanced, the duty cycle tells you the input voltage. Some tricks are played to reduce the duty cycle ripple and speed things up. [1] and some questionable claims of more. http://www.analog.com/en/products/analog-to-digital-converters/standard-adc/precision-adc-20msps/single-channel-ad-converters/ltc2380-24.html -- John Larkin Highland Technology, Inc lunatic fringe electronics