[sdiy] CD4050 hex buffer based DAC questions

[harrybissell at wowway.com]

paralled buffers (in the same IC package so they are well matched) allow
more drive current and can swing the load closer to the power supply rails...
critical in a DAC.  It was a common practice at that time, when good
monolythic DACs were expensive, or poorly performed

H^) harry



On Tue, 25 Nov 2008 02:01:52 +0100, Mattias Rickardsson wrote
> 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
> 
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Harry Bissell & Nora Abdullah 4eva