Path: ...!news.mixmin.net!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Bill Sloman Newsgroups: sci.electronics.design Subject: Re: AD5791 Date: Wed, 5 Jun 2024 14:37:29 +1000 Organization: A noiseless patient Spider Lines: 64 Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Injection-Date: Wed, 05 Jun 2024 06:37:35 +0200 (CEST) Injection-Info: dont-email.me; posting-host="3a6cf4d24500ee8d704e7ae052c36925"; logging-data="875057"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1+UEk5Q5o33sC2OdIGpqgeEtJihaIImSYY=" User-Agent: Mozilla Thunderbird Cancel-Lock: sha1:C/33dK1nilYDeY/0tEJYtqQCU/I= Content-Language: en-US X-Antivirus: Norton (VPS 240604-2, 4/6/2024), Outbound message In-Reply-To: X-Antivirus-Status: Clean Bytes: 3587 On 5/06/2024 6:56 am, 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? > > 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. In fact it needs a stable thin-film array. Provided the thin-film resistors are on a common substrate, the divide ratio can be quite a bit more stable than the individual resistances which are at the same temperature and made of metal despoited at the same time. > 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. Very stable four terminal references can be bought - they aren't cheap but there's nothing crazy about the prices or availability. You do have to be careful of voltage drops in the relevant printed circuit traces - I once had to fix a circuit where the voltage reference was grounded at the wrong end of trace carrying the return current from a big EPROM. The quick fix was soldering a chunk of copper wire onto the track, but changing the layout to something closer to star grounding was the long term solution. > I wonder how ADI tests these parts. I can't buy a 1 PPM accurate DVM. Not a enough money? No access to liquid helium? NIST seems to have managed it in 1984. https://www.nist.gov/system/files/documents/calibrations/im-34-2a.pdf They were still working on the Josephson junction array 10V reference back then https://en.wikipedia.org/wiki/Josephson_voltage_standard and it back commercially available in 1989. -- Bill Sloman, Sydney -- This email has been checked for viruses by Norton antivirus software. www.norton.com