A few weeks ago I was working on a prototype containing a MAX5443 single supply 3V 16-bit DAC in a circuit very similar to the one in its datasheet, using another MAX voltage ref. and a buffer and as far as I remember the output was pretty stable.
Mete On Tue, 22 Mar 2022 at 20:10, Bob kb8tq <[email protected]> wrote: > Hi > > > On Mar 22, 2022, at 12:14 PM, Erik Kaashoek <[email protected]> wrote: > > > > Hi Bob, > > By your advice I went for a 6.5 digit DVM and after logging and plotting > the DAC output its clear there are some stability issues in the DAC output. > The voltage is wandering around at about the level of frequency wandering > observed. > > That’s a very typical result as you dig into the DAC ( = the DAC isn’t > perfect ). > > > A different supply topology for the DAC and VCXO will have to be > created.. The DAC resolution is 50 uV (2.048 V / 400000 steps) but the > random variations are about 4 times p-p larger. > > Once this is done I hope longer term logging of frequency and voltage > will make sense. > > Or is it better to have a low pass filter between the DAC and the VCXO > Vtune input so the loop is fast enough to remove the remaining drift? > > You can go to a stable ADC and let it “servo” the DAC. That’s going to add > cost and > (likely) some noise from the ADC. The most common outcome is to find > another DAC > and possibly a better reference. > > > If so, what should the time constant of the filter be compared to the > intersect of the ADEV of the GPS and the VCXO (100 s)? > > Having a big time constant (10s?) will be a pain in the initial tuning > and will require active components. > > Does owning a 6.5 digit DVM qualify one as a volt-nut? Or should first > some voltage references be added? > > Erik. > > There’s *always* a way to spend more money on bench gear :) :) :). In > terms of this > project, what you have sounds like it’s doing the job just fine. > > Bob > > > > > On 5-3-2022 20:50, Bob kb8tq wrote: > >> Hi > >> > >> One suggestion on the frequency plot: > >> > >> TimeLab as you have it set does a “per pixel” process on its charts. > You have > >> a limited screen resolution and a ton of data. You have to do > something. There > >> is no way to display it all. > >> > >> It’s a good idea to zoom in to some of the “problem regions” and see > >> what is really going on. This reduces the number of data points that > >> map to one pixel and potentially changes what you see a bit. The closer > you > >> get to one data point per pixel, the closer you are to “reality” > without any > >> processing potentially getting in the way. > >> > >> On the DVM, eBay (with some time spent shopping) will sell you a working > >> 6 1/2 digit device for < $100 if you are patient. If you need it now, > it’s still > >> likely to be below $200. Yes it will be a big clunky box that takes up > bench > >> space. It will be HPIB and not serial i/o. The display might be a bit > fun in > >> bright lighting. > >> > >> While you are shopping, I would get set up for phase noise as well. > Often > >> it can spot issues that are tough to find with a second to second data > stream. > >> The setup does not have to be very fancy or terribly expensive. > >> > >> Bob > >> > > > _______________________________________________ > time-nuts mailing list -- [email protected] -- To unsubscribe send > an email to [email protected] > To unsubscribe, go to and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] -- To unsubscribe send an email to [email protected] To unsubscribe, go to and follow the instructions there.
