Hi for many 16 bits will do and one other alternative is do a design that will also work with the AD 1861. I know it is hard to get but in the last year I found a sizable volume on ebay, convinced them to lower their price, bought some my self and posted it on time-nuts. With a group effort I am sure we can find some sources. There are also old audio devices out there that have them enbedded. Bert In a message dated 6/29/2010 12:54:37 P.M. Eastern Daylight Time, [email protected] writes:
Hi Some of the TI (Burr Brown) 16 bit parts are 1/4 lsb DNL on > 98% of the transitions. Most of the time you have a "coarse" DAC that's at 18 bits. Some of the errors are predictable and you can take them out with a simple training process. You won't easily get 24 bits, but 20 is very achievable. 20 bits is a nice round million to one factor. A 1 ppm EFC becomes a 1 ppb step if everything is perfectly linear. Even with a 4:1 slope ratio it's still quite small. With a more restricted EFC of 2.5x10^-8 you are at 1x10^-13 with the 4:1 slope ratio. Bob -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Attila Kinali Sent: Tuesday, June 29, 2010 12:42 PM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] yet another GPSDO design, or so On Tue, 29 Jun 2010 09:17:39 -0700 (PDT) Stanley Reynolds <[email protected]> wrote: > If we lower the size of each step to over lap more would this > lower the error ? Software would adjust both converters at the > cross over point so neither would change it's full range at this point. > Two 12 bit converters would form one 18 or 20 bit converter. Yes, you can do that. With good D/A converters, the non-linearity is within 1-2LSB, so if a full range of the fine D/A is more than, lets say 4LSB of the coarse D/A you can minimize the step in control voltage, when you switch from one fine D/A range to the next. But it will be always there, you cannot get completely rid of it. There are two things that make this step problematic: 1) If it happens automatically, it will result in problems when doing long term measurements using that GPSDO as reference. When doing the switch manually triggered (ie the GPSDO says that it needs to adjust the coarse D/A and does so only after the user presses a button), then unattended operation over long periods is not possible anymore. 2) Each adjustment of the coarse D/A is a non-linear operation. It is basically a step function excitation of the control loop. With such a non-linear element within the control loop, the software has to take either precautions during the period of the non linearity or the control loop has to be able to cope with such steps, without starting to oscillate. > I guess taken to the extreme we could interweave any number of > converters so hopefully the errors would average out :-) 64 times > 12 bit converters making a 20 bit converter each with an equal > contribution at each step. Actually, the less components you have, the better. Not only do more components add more non-linearity (it is very difficult to average multiple components if they arent R's and C's), the whole circuitry involved produces also noise. Attila Kinali -- If you want to walk fast, walk alone. If you want to walk far, walk together. -- African proverb _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
