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Re: 50 cent DDS synthesizer

Cross-posted to 2 groups.

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On Fri, 24 Oct 2025 13:40:25 +0200, Lasse Langwadt <llc@fonz.dk>
wrote:

>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?

Use Ohm's Law!

>
>you can probably get it from the IBIS model
>

This is the IBIS of one gpio pin of the efinix T20.

https://www.dropbox.com/scl/fi/r97plmw3yvls3kgo0liy1/trion_drive_strength.pdf?rlkey=c3evk5zn7amn9ejx2dbtywter&dl=0

That's pullup to 3.3 volts, the pfet side.

You can see a small glitch at the midpoint of this sine wave

https://www.dropbox.com/scl/fi/n12quxajp59egahv1b48i/DDS_1KHz.jpg?rlkey=8w0tlr3h1rxeif3byepzub947&dl=0

which is the classic dac MSB crossover error. It's not enough to
matter, but I might tweak the first resistor to beautify that.


John Larkin
Highland Tech Glen Canyon Design Center
Lunatic Fringe Electronics

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Thread

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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