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Groups > comp.arch.fpga > #38682
| From | Lasse Langwadt <llc@fonz.dk> |
|---|---|
| Newsgroups | sci.electronics.design, comp.arch.fpga |
| Subject | Re: 50 cent DDS synthesizer |
| Date | 2025-10-24 13:40 +0200 |
| Organization | A noiseless patient Spider |
| Message-ID | <10dfoj9$2iaif$1@dont-email.me> (permalink) |
| References | (2 earlier) <jgekfklf9s7khf3kqf07a1jdcuegik14bi@4ax.com> <20251023a@crcomp.net> <10ddo5a$1v5hk$1@dont-email.me> <BDyKQ.44049$O5fb.6397@fx01.ams4> <10df7vg$2dvoi$1@dont-email.me> |
Cross-posted to 2 groups.
On 10/24/25 08:56, Jan Panteltje wrote: >> Chris Jones <lugnut808@spam.yahoo.com>wrote: >>> On 24/10/2025 4:20 am, Jan Panteltje wrote: >>>> "Don" <g@crcomp.net>wrote: >>>>> john larkin wrote: >>>>> Jan Panteltje wrote: >>>>>>> john larkin wrote: >>>>>>>> >>>>>>> We have a little proto board with a RP2040 Rpi cpu and an Efinix T20 >>>>>>> FPGA. The Pi configures the FPGA at powerup and then talks SPI to it. >>>>>>> >>>>>>> We often need programmable clocks so I added a few parts to make a DDS >>>>>>> frequency synthesizer. We use a PLL inside the FPGA to spin up a 204 >>>>>>> MHz clock for the 32-bit phase accumulator. The sine lookup is 4K >>>>>>> points and the "dac" is six resistors. >>>>>>> >>>>>>> This lowpass filter is tacky, but it all works. At 50 MHz, the jitter >>>>>>> is a few hundred ps p-p, and that looks mostly like amplitude noise to >>>>>>> me. >>>>>>> >>>>>>> https://www.dropbox.com/scl/fo/336qn643xyilkqz5rc9lp/ADC4ThBQa3B-L5auaTDh8fQ?rlkey=q2qrsbdow816id6wgc4eypuas&dl=0 >>>>>>> >>>>>>> The FPGA 1.2v core supply uses about 30 mA total when this is running >>>>>>> at 50 MHz out. >>>>>> >>>>>> Nice, 250 Ohm is a big load, good the FPGA can handle that. >>>>> >>>>> The MSB actually sees 500 ohms. >>>>> >>>>>> >>>>>> I was using R2R for video: >>>>>> https://www.electronics-tutorials.ws/combination/r-2r-dac.html >>>>> >>>>> One can buy a real r-2r network cheap, but it's easier to use 0603 >>>>> resistors that we have in stock. >>>> >>>> Also, the ability to adjust assembled resistor values works better for >>>> me as a DIYer. Here's mouser's offering for the geniuses in our midst, >>>> with the foresight to always choose the correct component: >>>> >>>> <https://www.mouser.com/c/passive-components/resistors/?circuit%20type=R%2F2R%20Ladder> >>>> >>>> Jan's cited tutorial says: >>>> >>>> Compared to the R-2R DAC, the binary weighted digital-to-analogue >>>> converter has an analogue output voltage which is the weighted >>>> sum of the individual inputs. Thus it requires a large range of >>>> precision resistors within its ladder network, making its design >>>> both expensive and impractical for most DAC's requiring lower >>>> levels of resolution. >>>> >>>> Discussions in this group left me with the impression precision resistor >>>> fabrication became trivial thanks to the trimming technology of modern >>>> lasers? Is the price of precision resistors still a factor? >>> >>> In the case of R2R you can use resistors from one batch and that may have less spreading in values >>> No precision resistors needed. >>> (2R is then 2 1R resistors in series). >>> >> >> You don't want them the same though: The MOSFETS of the CMOS logic >> outputs have some on-resistance (maybe 30-50 Ohms) and so the resistors >> in series with each of these outputs should be decreased by whatever you >> measure that output resistance to be, otherwise the resistance ratio >> isn't 1:2 in the R-2R network, and the DNL will be poor. >> >> You could use the average of the on-resistance of the NMOS and the PMOS >> (which are not quite the same), measured on a typical chip, at room >> temperature, so that it is usually at least roughly correct, rather than >> systematically and always wrong. >> >> The MOSFET on-resistance is usually a more important effect than the >> error of 1% resistor tolerances, so it makes sense to fix that before >> worrying about selecting resistors from the same batch. >> >> It is worth fixing it for R-2R and also for at least a few msbs of >> binary weighted resistor DACs. > > How would you go about measuring MOSFET on resistance? you can probably get it from the IBIS model
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50 cent DDS synthesizer john larkin <jl@glen--canyon.com> - 2025-10-22 14:49 -0700
Re: 50 cent DDS synthesizer Jan Panteltje <alien@comet.invalid> - 2025-10-23 07:45 +0000
Re: 50 cent DDS synthesizer john larkin <jl@glen--canyon.com> - 2025-10-23 07:36 -0700
Re: 50 cent DDS synthesizer Jan Panteltje <alien@comet.invalid> - 2025-10-23 15:16 +0000
Re: 50 cent DDS synthesizer "Don" <g@crcomp.net> - 2025-10-23 16:54 +0000
Re: 50 cent DDS synthesizer Jan Panteltje <alien@comet.invalid> - 2025-10-23 17:20 +0000
Re: 50 cent DDS synthesizer Martin Brown <'''newspam'''@nonad.co.uk> - 2025-10-23 19:33 +0100
Re: 50 cent DDS synthesizer Chris Jones <lugnut808@spam.yahoo.com> - 2025-10-24 10:12 +1100
Re: 50 cent DDS synthesizer Jan Panteltje <alien@comet.invalid> - 2025-10-24 06:56 +0000
Re: 50 cent DDS synthesizer Martin Brown <'''newspam'''@nonad.co.uk> - 2025-10-24 09:53 +0100
Re: 50 cent DDS synthesizer Lasse Langwadt <llc@fonz.dk> - 2025-10-24 13:40 +0200
Re: 50 cent DDS synthesizer john larkin <jl@glen--canyon.com> - 2025-10-24 07:09 -0700
Re: 50 cent DDS synthesizer Jan Panteltje <alien@comet.invalid> - 2025-10-24 17:17 +0000
Re: 50 cent DDS synthesizer antispam@fricas.org (Waldek Hebisch) - 2025-10-25 22:00 +0000
Re: 50 cent DDS synthesizer Jan Panteltje <alien@comet.invalid> - 2025-10-26 08:10 +0000
Re: 50 cent DDS synthesizer john larkin <jl@glen--canyon.com> - 2025-10-26 07:56 -0700
Re: 50 cent DDS synthesizer Bill Sloman <bill.sloman@ieee.org> - 2025-10-27 03:19 +1100
Re: 50 cent DDS synthesizer john larkin <jl@glen--canyon.com> - 2025-10-26 10:08 -0700
Re: 50 cent DDS synthesizer Bill Sloman <bill.sloman@ieee.org> - 2025-10-27 16:02 +1100
Re: 50 cent DDS synthesizer antispam@fricas.org (Waldek Hebisch) - 2025-10-27 00:10 +0000
Re: 50 cent DDS synthesizer Jan Panteltje <alien@comet.invalid> - 2025-10-27 09:52 +0000
Re: 50 cent DDS synthesizer antispam@fricas.org (Waldek Hebisch) - 2025-10-28 16:31 +0000
Re: 50 cent DDS synthesizer Jan Panteltje <alien@comet.invalid> - 2025-10-28 17:41 +0000
Re: 50 cent DDS synthesizer john larkin <jl@glen--canyon.com> - 2025-10-27 07:58 -0700
Re: 50 cent DDS synthesizer john larkin <jl@glen--canyon.com> - 2025-10-23 11:46 -0700
Re: 50 cent DDS synthesizer john larkin <jl@glen--canyon.com> - 2025-10-23 18:55 -0700
Re: 50 cent DDS synthesizer Jan Panteltje <alien@comet.invalid> - 2025-10-24 06:57 +0000
Re: 50 cent DDS synthesizer Jan Panteltje <alien@comet.invalid> - 2025-10-24 07:38 +0000
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