[sdiy] DIY Parallel DAC

[David G Dixon]

> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl 
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of 
> Brock Russell
> Sent: Sunday, July 31, 2011 6:17 PM
> To: Synth DIY
> Subject: Re: [sdiy] DIY Parallel DAC
> 
> 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_la
> dder_netwo
> >rk_.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

Thanks, Brock!