[sdiy] CD4050 hex buffer based DAC questions

[phillip m gallo]

Matt,

The idea is to allow the Most Sig Bits to produce a consistent voltage
independent of the load that varies when differing binary values are applied
to the network.

regards,
p

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of Mattias Rickardsson
Sent: Monday, November 24, 2008 5:02 PM
To: Synth-DIY
Subject: [sdiy] CD4050 hex buffer based DAC questions


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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