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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

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

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

added bit.

John

*<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

added bit.

John

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