Basic DAC Question

Clueless.Newbie · 2007-05-25T00:20:42+00:00

I have three questions about theoretical THD+N distortion whilecreating sine waves with DACs.First question: Assuming a perfect DAC with perfect analogcomponents,obviously a DAC with fewer bits will create a sinewave with largersteps,and thus will have a higher percentage of THD+N, but how do Icalculatethe exact percentage?Second question: I know about the Nyquist limit and it seems to methatas the Nyquist limit is approached the sinewave will have biggerstepsno matter how many bits it has and thus will have a higher percentageof THD+N, but how do I calculate the exact percentage?Third question; In the real world I wouldn't have perfect analogcomponents; in fact I would purposely introduce a lowpass filterat the output of the DAC to attenuate the switching noise. Howmuch would that change the answers to the questions above?BACKGROUND:We need to replace an old system that generates 20 Hz to 20 kHz sinewaves with a 12 bit DAC that puts out a 4096-step sine wave -- thesame number of steps whether it is putting out 20 Hz or 20 kHz.A variable oscillator changes the clock rate of a counter thatgets the values from an EPROM lookup table.We were discussing replacing the above with a modern DAC -- either16 bits at 44.1 ksps or 24 bits at 96 ksps. The objection wasraised that at 20Khz we are putting out 4096 x 20,000 sps, or81.92 Msps. I am guessing that 96 ksps with a added filter atthe DAC output is good enough. The final power stage startsslew-rate limiting at 30-40 kHz with large signals and the small-signal response is 3dB down at 50Khz and way down in the mud at100KHz. I just don't see how it needs over 80 megasamples persecond to keep the THD+N reasonably low. Am I right?

Basic DAC Question

May 25, 2007 26 Replies

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Guy Macon 19 years ago

I don't think that the OP asked for that, and I certainly didn't have that in mind. I was hoping to calculate the no-filtering THD, which seems a lot easier to calculate. I can always do a SPICE simutation to figure it out for a particular DAC and filter, but having a formula and figuring out the reasons why the formula works is always a big help in getting a deeper understanding of a circuit. And it's an interesting puzzle.

Leaving off the filter, how did you calculate the 0.02% and 3% THD figures? Did you use the "multiply bits by magic number X" method posted elsewhere?

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john 19 years ago

I've a real problem with 'em. Keep policing. It's the only way I'll learn :)

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john 19 years ago

Yes. :)

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John Larkin 19 years ago

But

y'alls

does.

John

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John Larkin 19 years ago

You are *not* an authority on the syntax of yat.

John

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krw 19 years ago

How about "y'all's", as in: "Y'all's coming over tonight to watch NASCAR, right?"

Keith

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Michael A. Terrell 19 years ago

"Y'all's coming over tonight to watch NASCAR, ain'tcha?"

Service to my country? Been there, Done that, and I've got my DD214 to prove it. Member of DAV #85. Michael A. Terrell Central Florida

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