[sdiy] DIY Parallel DAC

[Simon Brouwer]

Hi Brock,

Brock Russell schreef:
> At 02:29 PM 31/07/2011, Simon Brouwer wrote:
>>Op 31-7-2011 19:59, David G Dixon schreef:
>>>>You can drive an R/2R ladder from a CMOS buffer or a register
>>>>such as 74HCT377. Make sure the IC has its own well
>>>>decoupled/stabilized voltage rail and use large resistors
>>>>(50..100k or so) to minimize the influence of the output
>>>>resistances which can be up to ca. 70 ohms. With some 0.1%
>>>>resistors for the most significant bits you could get around
>>>>8 bit accuracy.
>>>...or use the smaller resistances but drive the ladder with a unity gain
>>>buffer.
>>
>>Um, no. The ladder in an R/2R DAC is not driven with a single analog
>>voltage, it's driven with multiple digital voltages (by "CMOS
>>buffer" I mean a multi bit buffer IC such as 74HCT240).
>>See
>>http://en.wikipedia.org/wiki/Resistor_ladder#R-2R_resistor_ladder_network_.28digital_to_analog_conversion.2C_or_DAC.29
>>
>>--
>>Vriendelijke groet, Simon Brouwer.
>
> Um, no, really. While it is possible to make a cheap and nasty R2R
> DAC with the ladder driven by a CMOS buffer, if you want to have any
> chance of making a reasonably accurate R2R DAC you will want to drive
> the ladder in voltage or current mode from a single reference voltage
> and switch the "rungs" (or arms, legs, bits) with low resistance switches.

That is exactly what you do when driving the ladder from CMOS buffers fed
by their own, accurate as needed, supply voltage, as I described. CMOS
output stages consist of just such low resistance switches connected to
the supply rails of the IC.

Of course, if around-8-bit precision is not "reasonably acccurate" enough
for your taste, or if you find this solution too cheap and nasty for
whatever other reason, feel free to build your DIY DAC from more exotic
and expensive components.

-- 
Vriendelijke groet,

Simon Brouwer
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