Alternative to MIDI-CV

The Old Crow oldcrow at Access.Mountain.Net
Wed Feb 14 10:05:26 CET 1996



On Tue, 13 Feb 1996, J.D. McEachin wrote:

> Thinking about the best DAC to use for MIDI-CV...
> 
> Why not use a 16bit R-2R ladder?  Since linearity and clock rate aren't 
> as crucial w/ CVs as for audio, couldn't you get away w/ an R-2R built w/ 
> 10k resistor networks?  Is there some reason R-2Rs don't work w/ 16 bits?

  Yes, there is a problem.  Say the Dac is to output 0 to 10 volts.  At a
16-bit resolution, this means the MSB would, with +5 volts feeding the
ladder elements, require a resolution from the LSB of 5 x (1/65536) volts
= 76.3 microvolts or so.  Since the LSB is at the bottom of the ladder
(with the output at the top), it has to pass through 16 resistors (15
in-line plus the input resistor.  The tolerance of all those resistors is
going to well violate the 76.3uV LSB resolution.  (Even 1% resistors aren't
going to be good enough.  That's why most IC ladder Dacs use laser-trimmed
resistors all mounted close together in a thermally stable package.)

  The best Dac solution would be a Dac "windowing" circuit.  The Dac itself
is only responsible for a portion of the slope; it's output is summed with
a simple 2 or 3 bit R-2R type Dac which selects the portion of the slope 
(window) the monolithic Dac is to resolve upon.  This way, the "coarse" 
(window setting R-2R) Dac and the "fine" (slope setting monolithic) Dacs 
can be calibrated to provide either a log or linear CV.


   --Crow

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