Path: ...!Xl.tags.giganews.com!local-2.nntp.ord.giganews.com!nntp.supernews.com!news.supernews.com.POSTED!not-for-mail NNTP-Posting-Date: Wed, 05 Jun 2024 03:50:04 +0000 From: john larkin Newsgroups: sci.electronics.design Subject: Re: AD5791 Date: Tue, 04 Jun 2024 20:50:03 -0700 Message-ID: <4ujv5jlc6g2mtaucjc9elav2da56jlt81k@4ax.com> References: User-Agent: ForteAgent/8.00.32.1272 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Lines: 80 X-Trace: sv3-txEFG2RN66UHOE/ehF4BndCPQ+Gd2K+CjaAOmpbv1ipzgxTYc0C3E67KDYx8cC9msaQA3d/1RrmuyDU!VtGlETWb2AtGB+GXYvz3H7FUif3umWWcp38IVQuHYNtx8fpR4Txj5aW2rgcLc03Yu7DAZWRDnZHs!+/xBfg== 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: 3645 On Wed, 5 Jun 2024 01:11:33 -0000 (UTC), Phil Hobbs wrote: >john larkin wrote: >> On Tue, 4 Jun 2024 22:58:54 -0000 (UTC), Phil Hobbs >> wrote: >> >>> john larkin wrote: >>>> On Tue, 4 Jun 2024 21:53:13 +0200, Jeroen Belleman >>>> wrote: >>>> >>>>> On 6/4/24 19:48, 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. >>>>>> >>>>> >>>>> But you can power the chip from +/-16V and the LSB can be in >>>>> the 25uV ballpark. The Johnson noise of 7.5nV/rtHz doesn't >>>>> seem so bad then, does it? >>>>> >>>>> Jeroen Belleman >>>> >>>> That helps some. +-14v is about the limit on the references. We'd have >>>> to divide down to get our +-10v range back, and that would need some >>>> crazy stable resistors. >>>> >>>> Looks like the other way to get the noise down would be to parallel a >>>> number of DACs. Times 8 channels! Ballpark $100 per DAC, which is >>>> actually feasible. >>>> >>>> It will of course need crazy-low-noise hyper-stable references. >>>> >>>> I wonder how ADI tests these parts. I can't buy a 1 PPM accurate DVM. >>>> >>>> >>> >>> Its quoted rise time is 1us, corresponding to a 3 dB bandwidth of about 350 >>> kHz, or 550 kHz noise bandwidth. >>> >>> With 7.5 nV 1-Hz noise, the total RMS noise should be about 5.6 uV, just >>> about half a LSB at 10V FS. >>> >>> Not that shabby. >>> >>> Cheers >>> >>> Phil Hobbs >> >> Three DACs in parallel with +-16 refs, divided down to +-10, pencils >> out around 3.2 nv/rthz. >> >> I'm going to need a very good preamp to measure the noise, something >> below 1 nv/rthz. Any ideas? >> >> > >;) > >I believe you may have got one in your stocking in January. Yes, I forgot somehow. Thanks. We'll likely need another, if we go ahead with this project, and I'll have the customer buy some too. > >(For others: we sell a nice 20-MHz AC-coupled preamp for noise >measurements, the LA-20 Lab Amplifier. Works great, and is cheap like >borscht.) > >Cheers > >Phil Hobbs