Forums

Deglitching a DAC

Started by Spehro Pefhany July 17, 2012
On Tue, 17 Jul 2012 16:32:24 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>Spehro Pefhany wrote: >> >> On Tue, 17 Jul 2012 15:46:35 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >> >On 07/17/2012 03:43 PM, Spehro Pefhany wrote: >> >> Any clever ideas on getting rid of glitches in a high precision DAC >> >> output? Update rate is a fixed rate, probably in the 10-100kHz range, >> >> and I'd like to keep the glitches (especially variation in glitches) >> >> to<< 25ppm, preferably< 5ppm. A S&H with a low charge injection >> >> switch? >> >> >> > >> >I'm a fan of dual-gate MOSFETs for that sort of job, because the charge >> >injection is so low. >> > >> >Cheers >> > >> >Phil Hobbs >> >> Is the idea to use an opposite going pulse on the second gate to >> compensate for the capacitive coupling? > >No, you bypass the second gate, or connect it to the source via an RC. >The effective output capacitance then is about 0.02 pF or less, a good >factor of 10 less than you can get in other discrete parts. > >Cheers > >Phil Hobbs
So it's effectively a cascode?
On Tue, 17 Jul 2012 14:16:32 -0700, John Larkin
<jlarkin@highlandtechnology.com> wrote:

> > >Could you possibly digitally limit the amount of code change, >essentially digitally slew limit? I have an idea for a deglitcher but >it could only tolerate a half volt or so of voltage step.
Yes, that's certainly possible, but the update rate might have to be juiced up.
Spehro Pefhany wrote:
> > On Tue, 17 Jul 2012 16:32:24 -0400, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > > >Spehro Pefhany wrote: > >> > >> On Tue, 17 Jul 2012 15:46:35 -0400, Phil Hobbs > >> <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> > >> >On 07/17/2012 03:43 PM, Spehro Pefhany wrote: > >> >> Any clever ideas on getting rid of glitches in a high precision DAC > >> >> output? Update rate is a fixed rate, probably in the 10-100kHz range, > >> >> and I'd like to keep the glitches (especially variation in glitches) > >> >> to<< 25ppm, preferably< 5ppm. A S&H with a low charge injection > >> >> switch? > >> >> > >> > > >> >I'm a fan of dual-gate MOSFETs for that sort of job, because the charge > >> >injection is so low. > >> > > >> >Cheers > >> > > >> >Phil Hobbs > >> > >> Is the idea to use an opposite going pulse on the second gate to > >> compensate for the capacitive coupling? > > > >No, you bypass the second gate, or connect it to the source via an RC. > >The effective output capacitance then is about 0.02 pF or less, a good > >factor of 10 less than you can get in other discrete parts. > > > >Cheers > > > >Phil Hobbs > > So it's effectively a cascode?
Yes, that's right. The cascode device is on-chip, though, so it works better. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
Phil Hobbs wrote:
> Spehro Pefhany wrote: >> On Tue, 17 Jul 2012 16:32:24 -0400, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> Spehro Pefhany wrote: >>>> On Tue, 17 Jul 2012 15:46:35 -0400, Phil Hobbs >>>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>>> >>>>> On 07/17/2012 03:43 PM, Spehro Pefhany wrote: >>>>>> Any clever ideas on getting rid of glitches in a high precision DAC >>>>>> output? Update rate is a fixed rate, probably in the 10-100kHz range, >>>>>> and I'd like to keep the glitches (especially variation in glitches) >>>>>> to<< 25ppm, preferably< 5ppm. A S&H with a low charge injection >>>>>> switch? >>>>>> >>>>> I'm a fan of dual-gate MOSFETs for that sort of job, because the charge >>>>> injection is so low. >>>>> >>>>> Cheers >>>>> >>>>> Phil Hobbs >>>> Is the idea to use an opposite going pulse on the second gate to >>>> compensate for the capacitive coupling? >>> No, you bypass the second gate, or connect it to the source via an RC. >>> The effective output capacitance then is about 0.02 pF or less, a good >>> factor of 10 less than you can get in other discrete parts. >>> >>> Cheers >>> >>> Phil Hobbs >> So it's effectively a cascode? > > Yes, that's right. The cascode device is on-chip, though, so it works > better. >
That's just the problem with DACs, on chip things get worse because the miniscule remnants of digital control pulses find their way into everything. Almost like spilled oil. The best de-glitcher would be off chip and, most importantly, differential. Compensating charge injection via a "counter charge" is one of the tricks to muffle any newly induced spikes. I have always done it in discretes but maybe the ADG12xx series switches is good enough for Spehro's project. -- Regards, Joerg http://www.analogconsultants.com/
John Larkin wrote:
> > We used to use AD1862 (20 bit parallel) DACs, but we had bad popcorn > noise problems, and AD eventually discontinued them. > > We have 140 in stock, popcorn fallouts.
Would they be useful for a digitally controlled power supply?
Joerg wrote:

> Phil Hobbs wrote: > >>Spehro Pefhany wrote: >> >>>On Tue, 17 Jul 2012 16:32:24 -0400, Phil Hobbs >>><pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>> >>>>Spehro Pefhany wrote: >>>> >>>>>On Tue, 17 Jul 2012 15:46:35 -0400, Phil Hobbs >>>>><pcdhSpamMeSenseless@electrooptical.net> wrote: >>>>> >>>>> >>>>>>On 07/17/2012 03:43 PM, Spehro Pefhany wrote: >>>>>> >>>>>>>Any clever ideas on getting rid of glitches in a high precision DAC >>>>>>>output? Update rate is a fixed rate, probably in the 10-100kHz range, >>>>>>>and I'd like to keep the glitches (especially variation in glitches) >>>>>>>to<< 25ppm, preferably< 5ppm. A S&H with a low charge injection >>>>>>>switch? >>>>>>> >>>>>> >>>>>>I'm a fan of dual-gate MOSFETs for that sort of job, because the charge >>>>>>injection is so low. >>>>>> >>>>>>Cheers >>>>>> >>>>>>Phil Hobbs >>>>> >>>>>Is the idea to use an opposite going pulse on the second gate to >>>>>compensate for the capacitive coupling? >>>> >>>>No, you bypass the second gate, or connect it to the source via an RC. >>>>The effective output capacitance then is about 0.02 pF or less, a good >>>>factor of 10 less than you can get in other discrete parts. >>>> >>>>Cheers >>>> >>>>Phil Hobbs >>> >>>So it's effectively a cascode? >> >>Yes, that's right. The cascode device is on-chip, though, so it works >>better. >> > > > That's just the problem with DACs, on chip things get worse because the > miniscule remnants of digital control pulses find their way into > everything. Almost like spilled oil. > > The best de-glitcher would be off chip and, most importantly, > differential. Compensating charge injection via a "counter charge" is > one of the tricks to muffle any newly induced spikes. I have always done > it in discretes but maybe the ADG12xx series switches is good enough for > Spehro's project. >
This thread reminds of a tube of serial DACs that were ship to us by mistake. We kept them for prosperity. Jamie
Jamie wrote:
> > This thread reminds of a tube of serial DACs that were ship to us by > mistake. We kept them for prosperity.
Rather than return them to their owners. That's not surprising.
On Tue, 17 Jul 2012 19:50:15 -0400, "Michael A. Terrell"
<mike.terrell@earthlink.net> wrote:

> >John Larkin wrote: >> >> We used to use AD1862 (20 bit parallel) DACs, but we had bad popcorn >> noise problems, and AD eventually discontinued them. >> >> We have 140 in stock, popcorn fallouts. > > > Would they be useful for a digitally controlled power supply?
Sure, if you don't mind the occasional 10 PPM bump. -- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
On Tuesday, July 17, 2012 3:43:21 PM UTC-4, Spehro Pefhany wrote:
> Any clever ideas on getting rid of glitches in a high precision DAC > output? Update rate is a fixed rate, probably in the 10-100kHz range, > and I&#39;d like to keep the glitches (especially variation in glitches) > to &lt;&lt; 25ppm, preferably &lt; 5ppm. A S&amp;H with a low charge inje=
ction
> switch?
One clever idea is the so-called re-glitching technique used by National In= struments for the past 15 years or so. Less is more, no T/H is used. The id= ea there is to uniformize the glitch energy, or make it code independent, a= nd shift its spectrum to the sampling frequency where the so-called anti-im= aging LPF can eliminate it. This can't be done with code-dependent glitch e= nergy since quite a lot of it remains withing the signal band. See US564662= 0...guess it's still protected though.
John Larkin wrote:
> > On Tue, 17 Jul 2012 19:50:15 -0400, "Michael A. Terrell" > <mike.terrell@earthlink.net> wrote: > > > > >John Larkin wrote: > >> > >> We used to use AD1862 (20 bit parallel) DACs, but we had bad popcorn > >> noise problems, and AD eventually discontinued them. > >> > >> We have 140 in stock, popcorn fallouts. > > > > > > Would they be useful for a digitally controlled power supply? > > Sure, if you don't mind the occasional 10 PPM bump.
That wouldn't bother me. I want to build some new bench supplies that I can program the voltage & current limiting. I would need two per output and I'd like at least four outputs.