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'[EE]: two 8 bit DAC to make 16 bit?'
2002\05\08@170049 by Richard Sloan

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Am I nuts?

Can this be done?

In the audio world if you were to playback just the MSB? what would it sound like, the LSB only accounts for 1/256 of the votage out, if the data was 0x5500 vs 0x55ff the difference would only be 1/256 of full scale? So it sounds like it would make no difference.

R.

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2002\05\08@173205 by Jinx

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> Am I nuts?

Possibly. However, I am not a doctor

> Can this be done ?

Not really, because each DAC has its own inaccuracies. The
LSb error in the MSB DAC is greater than the value of the
entire LSB DAC, so actually rendering the LSB DAC surplus
to requirements, effectively leaving you with just the MSB DAC.
You need a proper 16-bit or greater DAC

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2002\05\08@173215 by Bob Ammerman

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>Am I nuts?

Not nuts, just a bit misguided. The inaccuracy of the MSBits DAC would swamp
the LSBits DACs output completely (most DACs are specced at about +/- 1LSBit
accuracy).

>Can this be done?

Sorry, no.

> In the audio world if you were to playback just the MSB? what would it
sound like, the LSB only accounts for 1/256 of the votage out, if the data
was 0x5500 vs 0x55ff ?> the difference would only be 1/256 of full scale? So
it sounds like it would make no difference.

The difference is the same as the difference between an 8-bit and a 16-bit
WAV file. Very noticable indeed. This is because of the much higher
quantization noise, especially as it affects quieter parts of the audio.

Bob Ammerman
RAm Systems

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2002\05\08@203314 by M. Adam Davis

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Strictly speaking, two 8 bit DACs would be equivilant to one 9 bit DAC.
Each DAC can only represent 256 different levels, so two of them can
only represent 512 levels.

As far as how much of a difference it would make, it depends on what
you're doing with it.  If you are doing sound, the averag person can
tell the difference between an 8bit DAC output and a 16bit DAC output
given a good output stage and amplifier.  The major problem is that
having such large steps between each sample create lower frequency
harmonics which are not as easily filtered out without removing wanted
audio information.

Think of it as the color depth of a monitor.  Sure, 65536 colors is a
lot, and more than adequate for many (if not most) uses.  But when you
look at a photo of someone's face you can see the lines where the color
changes.  Dithering helps, but you need higher and higher resolutions to
make dithering worthwhile.  Even at very high resolutions with good
dithering the average person would rather see the photo on a true color
display.

But it depends on the application.

-Adam

Richard Sloan wrote:

{Quote hidden}

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2002\05\08@213711 by Richard Sloan

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Thanks,

what about a source for a off the shelf 16 bit parallel DAC? or dual? Thats my problem... is to find a good source of them.

Richard.

>>  > Am I nuts?

>>  Possibly. However, I am not a doctor

>>  > Can this be done ?

>>  Not really, because each DAC has its own inaccuracies. The
>>  LSb error in the MSB DAC is greater than the value of the
>>  entire LSB DAC, so actually rendering the LSB DAC surplus
>>  to requirements, effectively leaving you with just the MSB DAC.
>>  You need a proper 16-bit or greater DAC

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2002\05\08@213730 by Richard Sloan

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Makes sense.

Thanks.

>>  Strictly speaking, two 8 bit DACs would be equivilant to one 9 bit DAC.
>>  Each DAC can only represent 256 different levels, so two of them can
>>  only represent 512 levels.

>>  As far as how much of a difference it would make, it depends on what
>>  you're doing with it.  If you are doing sound, the averag person can
>>  tell the difference between an 8bit DAC output and a 16bit DAC output
>>  given a good output stage and amplifier.  The major problem is that
>>  having such large steps between each sample create lower frequency
>>  harmonics which are not as easily filtered out without removing wanted
>>  audio information.

>>  Think of it as the color depth of a monitor.  Sure, 65536 colors is a
>>  lot, and more than adequate for many (if not most) uses.  But when you
>>  look at a photo of someone's face you can see the lines where the color
>>  changes.  Dithering helps, but you need higher and higher resolutions to
>>  make dithering worthwhile.  Even at very high resolutions with good
>>  dithering the average person would rather see the photo on a true color
>>  display.

>>  But it depends on the application.

>>  -Adam

>>  Richard Sloan wrote:

>>  >Am I nuts?
>>  >
>>  >Can this be done?
>>  >
>>  >In the audio world if you were to playback just the MSB? what would it
>>  sound like, the LSB only accounts for 1/256 of the votage out, if the
>>  data was 0x5500 vs 0x55ff the difference would only be 1/256 of full
>>  scale? So it sounds like it would make no difference.
>>  >
>>  >R.
>>  >
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2002\05\08@225801 by Jinx

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> What about a source for a off the shelf 16 bit parallel DAC?

Analogue Devices AD1851, 669, 7846, 712

Burr-Brown DAC703 / 712

for starters

================================================

Try here

http://www.embeddedlinks.com/chipdir/

"Other chips lists", local - by function > Digital To Analog Convertor

http://www.embeddedlinks.com/chipdir/f/dac.htm

"one or two" DACs here

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2002\05\09@073511 by Olin Lathrop

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> Strictly speaking, two 8 bit DACs would be equivilant to one 9 bit DAC.
>  Each DAC can only represent 256 different levels, so two of them can
> only represent 512 levels.

Strictly speaking, two 8 bit DACs could be equivalent to one 16 bit DAC
theoretically.  Each DAC can create 256 different levels, so two of them can
create 256 x 256 = 65535 different levels.

If the two D/As were perfect, you could scale one by 1/256 of the other and
get a 16 bit D/A.  In reality, errors in the high D/A will cause gaps and
overlaps in the final output range.  You can, however, scale the low D/A by
a larger number, like 1/64 for example.  It doesn't need to be a power of
two, but does need to be large enough that the range of the low D/A can
cover the worst case range between adjacent LSBs of the high D/A.  That
would guarantee you can cover the full dynamic range with 14 bit resolution,
within a 16 bit number space.  Due to errors in the low D/A and crossover
problems between different values of the high D/A, you won't actually get 14
bit accuracy, but 12 and maybe 13 bits are achievable.  Another way of
looking at this is that you can send 65536 different numbers to this
circuit, of which 4096 of them can be chosen to emulate a 12 bit D/A.  There
is no way to know which 4096 of the 65536 numbers are the right ones without
measuring.  You could measure all 65536 voltages, then pick the best 4096 of
them and store those 16 bit values in a table.  You would now have a quite
serviceable 12 bit D/A as long as things don't drift much.

I did something like this once where I made a D/A using the standard R / 2R
ladder, but the ratio wasn't exactly 2.  Each lower bit was worth a little
more than 1/2 the previous, enough to cover the additional error range of
the previous resistor.  In this case I strung together 16 bits like this,
did all the measurements with a computer controlled voltmeter, and
demonstrated an accurate 10 bit D/A.  This method has other problems, and
this was only a proof that you don't *need* resistors accurate to 1 part in
1000 to make a 10 bit D/A with +- 1/2 LSB error.


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2002\05\09@074136 by Olin Lathrop

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> In the audio world if you were to playback just the MSB? what would it
> sound like,

I would sound like a signal that was overamplified then clipped.  Some
"musicians" do this on purpose for various effects.


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2002\05\09@113051 by M. Adam Davis

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You are right.  I was pondering this further after I sent my first
message and realized that you could set the second DAC to 1/256 the
total output of the first DAC then add them together.  That does require
extra active components and board space though.  I was on this train of
thought mainly because you cannot do the same thing with ADCs nearly as
easily.

-Adam

Olin Lathrop wrote:

{Quote hidden}

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