# Basic DAC Question - Page 2

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Re: Basic DAC Question
snipped-for-privacy@example.net says...

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

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Keith

Re: Basic DAC Question

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

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Re: Basic DAC Question

Why do you want to know the _total_ _harmonic_ distortion for a
sampled audio? system ?.

There is always the classical formula for SNR in dB = 1.76 dB +6.02n,
in which n is the number of bits.

Some of the noise components are outside the required audio passband,
especially when some form of noise shaping is used and thus filtered
out.

In a sampled system, you will only get strong _harmonic_ components,
when the produced waveform is a subharmonic of the sampling frequency,
at other generated frequencies, the same noise power is distributed
among a very large number of frequencies, creating a noise floor.

Look at the spectrum for a DDS system, there are usually a noise
floor, but at some frequencies, the noise power is concentrating on a
few discrete spurs, while the frequencies in between are very quiet.

Paul

Re: Basic DAC Question

snipped-for-privacy@jjdesigns.fsnet.co.uk wrote:

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?

Re: Basic DAC Question

Yes. :)

Re: Basic DAC Question

Some (most?) spice engines allow table entry, and will do fourier plots,
so you could enter the sine LUT into a table and
run the fourier ?
I recall ~1yr ago, bumping into a table limit in B2Spice,
and got them to fix it for this type of use, but no, I have
not done this specific table usage.

-jg

Re: Basic DAC Question
On Fri, 25 May 2007 13:38:44 +0000, Guy Macon
<http://www.guymacon.com/ wrote:

The usual expression is that

s/n = 6.02N - 1.249   dB

and is independent of sample rate if the usual sampling rules are
followed, and assuming an ideal dac. This assumes that the signal has
a gaussian distribution and averages 1/4 of ADC full scale. Whatever
the definition of "signal", the improvement in s/n remains 6.02 dB per