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

Thu Sep 19 17:19:01 CEST 2002

With 37 (or so) resistors in series (that was what I meant, not
an R-2R ladder) the chances of it being in tune are statistically
MUCH better than the tolerance of the single resistor.

If the parts are from the same manufacturing lot...they usually match
far better than 1%... they are just not closer to the "ideal"
value than 1% (i.e. they are all pretty close to 99 ohms, or 101 ohms)

The case you detail WILL eventually occur.  So buy 200 resistors
(for $4) and sort them, choose the closest 50...

H^) harry

--------Original Message--------  

From: debus at cityweb.de
To: synth-diy at dropmix.xs4all.nl
Sent: Sep 19 2002 15:55
Subject: Re: [sdiy] DAC selection in MIDI-CV Part #1

>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