[sdiy] CD4050 hex buffer based DAC questions

[Mattias Rickardsson]

Hi,

under the hood, a typical Roland microprocessor controlled analog 
synth can look like this:

	http://www.kolumbus.fi/janne.husu/specs/202ana.gif

To the upper left you see IC11 and IC12 (4050 hex buffers). Together 
with an R-2R ladder they act as a D/A converter for the pitch CV. The 
least significant bits have one buffer each, and the most significant 
bits have 2, 3, and 4 buffers in parallel - because the precision is 
extra important for these bits.

But in what way is it better to have several buffers in parallel?
I can imagine that

1) output voltage differs among individuals and you want an average

2) output voltage depends on output current and you want to split the current

I'm guessing that high/low [binary] output voltages are very near 
supply/ground and alternative 2 feels more probable. (But the choice 
of 1/2/3/4 buffers then feels illogical (sic) compared to for example 
1/2/4 eller 1/2/4/8.)

Does anybody have a good answer here?
I can't remember it ever being discussed. :-)

And further thoughts...

the microprocessor's own output pins could be used unbuffered 
instead, what are the main reasons for avoiding that? (Fluctuating 
supply, disturbances from the rest of the processor, less ideal 
output voltages & currents, ...?)

Is the 4050 solution as good as a comparator based solution, or is it 
just chosen for its low price?


/mr