Re: 50 cent DDS synthesizer

From	Jan Panteltje <alien@comet.invalid>
Newsgroups	sci.electronics.design, comp.arch.fpga
Subject	Re: 50 cent DDS synthesizer
Date	2025-10-24 06:56 +0000
Organization	A noiseless patient Spider
Message-ID	<10df7vg$2dvoi$1@dont-email.me> (permalink)
References	(1 earlier) <10dcmfd$1fcrk$1@dont-email.me> <jgekfklf9s7khf3kqf07a1jdcuegik14bi@4ax.com> <20251023a@crcomp.net> <10ddo5a$1v5hk$1@dont-email.me> <BDyKQ.44049$O5fb.6397@fx01.ams4>

Cross-posted to 2 groups.

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>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?

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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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