Path: ...!Xl.tags.giganews.com!local-2.nntp.ord.giganews.com!nntp.supernews.com!news.supernews.com.POSTED!not-for-mail NNTP-Posting-Date: Fri, 07 Jun 2024 20:51:18 +0000 From: john larkin Newsgroups: sci.electronics.design Subject: Re: AD5791 Date: Fri, 07 Jun 2024 13:51:17 -0700 Message-ID: References: <6a51813e-c633-2ac7-b47d-bc38f8e7953e@electrooptical.net> User-Agent: ForteAgent/8.00.32.1272 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Lines: 74 X-Trace: sv3-W8KsfNKIKJg2aKUFxagfWtSFkiDrKpUUSpEEK5Z95U12KIEObbqRht0AzBpnUIxdi08HT5AEeJe6VXd!PoRl+ery4R/CzLEO+L814D794jiyVF2iISjAtko1c/fu6QwZtfksKSg4Vsz+mfBy2Qhc6yszNCAv!iqvoQw== X-Complaints-To: www.supernews.com/docs/abuse.html X-DMCA-Complaints-To: www.supernews.com/docs/dmca.html X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your complaint properly X-Postfilter: 1.3.40 Bytes: 3863 On Fri, 7 Jun 2024 14:28:18 -0400, Phil Hobbs wrote: >On 2024-06-06 22:38, john larkin wrote: >> On Thu, 6 Jun 2024 22:19:22 -0400, Phil Hobbs >> wrote: >> >>> On 2024-06-06 13:57, john larkin wrote: >>>> On Thu, 06 Jun 2024 10:15:45 -0700, boB wrote: >>>> >>>>> On Tue, 04 Jun 2024 10:48:00 -0700, john larkin wrote: >>>>> >>>>>> https://www.analog.com/en/products/ad5791.html >>>>>> >>>>>> That's an amazing part. 20 bit DAC with 1 PPM accuracy and 0.05 PPM >>>>>> per degree C tempco. >>>>>> >>>>>> My main gripe is its 3.4K output impedance, which makes a lot of >>>>>> Johnson noise. I suppose I could run a bunch in parallel. >>>>> >>>>> >>>>> Nice part but costs way too much for any products we make. >>>>> >>>>> boB >>>> >>>> What do you make? >>>> >>>> We live on the lunatic fringe of electronics, things that are really >>>> hard to do, things with extreme exponents. It makes money because it >>>> has little competition, but the money is a side effect. I do it >>>> because it's fun. >>>> >>>> There must be something cool that we can do with a 1 PPM accurate DAC. >>>> >>>> TI has a 20-bit delta-sigma DAC that's about $12, but it's only linear >>>> to 15 PPM. I don't understand how a d-s DAC or ADC can even be that >>>> good. It would seem to need femtosecond edge accuracies inside. >>>> >>> >>> I expect that the deterministic part of the jitter gets pushed out to >>> high frequency by the noise shaping. >>> >>> Random jitter you'd have to deal with by averaging. >>> >>> Cheers >>> >>> Phil Hobbs >> >> I was thinking about rise/fall time asymmetry, changing average values >> as duty cycles squirm all over the place. >> > >Yeah, part of which is deterministic and part random. DAC noise shaping >AIUI makes the the DS sum run in a limit cycle even for a fixed code, so >that most of the switching junk is up at high frequency where it's >easier to filter out. However, I'm not a delta-sigma expert. > >(They call them sigma-deltas for some reason--possibly related to gang >insignia.) ;) > >Cheers > >Phil Hobbs Looking at the classic d-s architecture, a midscale code will be almost a square wave from the feedback quantizer, but near full scale the duty cycle will be very small. Edge density will change with input signal. Every blip has a rise and a fall and they have to be match exactly to get the right average. Maybe real chips do something else, charge balancing maybe.