[sdiy] DAC selection in MIDI-CV Part #1

[Ingo Debus]

harrybissell wrote:
> Many early monosynths had 1% resistors in the keyboards, if you were
> lucky they were hand selected.

Resistor ladder keyboard, i.e. 1 resistor per key?

I think in this case resistor tolerance is less critical than in a 
R-2R-ladder DAC.

Ok, worst case for a resistor ladder keyboard is when the lower half 
of the keyboard has only resistor at one extreme of the spec and the 
upper half has only resistors at the other extreme of the spec. So, 
with 100 ohms resistors in the lower half only 99 ohms and in the 
upper half only 101 ohms. Then the middle key will indeed be off by 
one percent, while the lowest and the highest key are in tune. Say the 
middle key should give an output of 3 volts, then the error is 30mV. 
About 1/3 semitone, too much.

But how likely is that? Usually the resistor tolerances would 
compensate each other a lot. This sure can be calculated with 
statistics. Homework: if 10000 synths are produced, what is the 
probability that one has the worst case error described above?

Ingo