[sdiy] DIY Parallel DAC

[Brock Russell]

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.
See page 5: http://www.analog.com/static/imported-files/tutorials/MT-015.pdf

Sincerest regards, Brock Russell