Path: ...!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 01:11:33 -0000 (UTC) Organization: A noiseless patient Spider Lines: 73 Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Injection-Date: Wed, 05 Jun 2024 03:11:34 +0200 (CEST) Injection-Info: dont-email.me; posting-host="d0de4698a6dd44801af72791eed81064"; logging-data="703560"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX19sFm3GPtatbHUM9DPErBAq" User-Agent: NewsTap/5.5 (iPhone/iPod Touch) Cancel-Lock: sha1:CQiut0y1+4vxrMxEPZYJfIyswu0= sha1:KCa2EUFekRS/rf3oMLeFP76MUD0= Bytes: 3413 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 LA-20 Lab Amplifier. Works great, and is cheap like borscht.) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics