Path: ...!3.eu.feeder.erje.net!feeder.erje.net!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Phil Hobbs Newsgroups: sci.electronics.design Subject: Re: AD5791 Date: Wed, 5 Jun 2024 14:06:07 -0400 Organization: A noiseless patient Spider Lines: 90 Message-ID: <6499bdfe-3fd4-38ca-a059-3f82d36a4c08@electrooptical.net> References: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Injection-Date: Wed, 05 Jun 2024 20:06:08 +0200 (CEST) Injection-Info: dont-email.me; posting-host="a65847f65bc94ff7d36a41dee3f5cf5f"; logging-data="1135746"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1+SX0vvjFopueI+JQzz8MqS" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.0 Cancel-Lock: sha1:1O5/utZtD27rc2Ov8ibq6YvvhMA= In-Reply-To: Bytes: 3805 On 2024-06-04 21:11, 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. > > (For others: we sell a nice 20-MHz AC-coupled preamp for noise > measurements, the LA22 > Lab Amplifier. Works great, and is cheap like > borscht.) https://hobbs-eo.com/products/la-22-lab-amplifier > > 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