[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: Tuesday, November 25, 2008 3:20 PM
To: harrybissell at wowway.com
Cc: Synth-DIY
Subject: Re: [sdiy] CD4050 hex buffer based DAC questions


But in this application they are neither in the same IC package nor
distributed as to give the same currents. (1/2/4/8 etc)

/mr

2008/11/25  <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
>
>
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