Re: [sdiy] cv and adc...

Roman modular at go2.pl
Fri Aug 8 11:07:36 CEST 2003


Hi,
some micros have problem converting voltages close to supply rails. You may read 0s from a/d when the input is at, say 50mV (that's more than 2LSB for 8-bit). Also it's nice to scale CV so each volt means some nice number of LSBs, say 64, or 48 if you want to quantize it later into semitones.
So I say, put inverting amplifier with adjustable gain and offset, so you can center expected CV range within a/d input range, and have convenient voltage/LSB ratio. From such amplifier place resistor and clamping diodes to gnd and +5. You won't need that if you use low power rail-to-rail amplifier supplied from 5V.
If you want more than 5 octaves range, then set gain lower.
Example assuming 8-bit a/d and 0-5V input range:
5 oct: Ri = 100k, Rf = 93.75k (91k+2,7k), LSB per octave = 48
10 oct: R1 = 100k, RF = 46.875k (47k), LSB per octave = 24
For offset add multiturn trimmer between 0 and -5V with 200k from taper to opamp's inverting input.

hope that helps

Roman



---- Wiadomość Oryginalna ----
Od: Bert Schiettecatte 
Do: synth-diy at dropmix.xs4all.nl
Data: Thu, 7 Aug 2003 21:34:24 +0200
Temat: [sdiy] cv and adc...


Hi all,
 
I’m doing a microcontroller project which involves a/d conversion of CV signals ... can anyone give me some idea of the circuitry I need between the CV input jacks and the a/d conversion pin of my microcontroller? I have no idea about the voltage levels I need to support or how I have to connect the grounds of the equipment connected to my circuit to the circuit ground 
 etc etc
 you guessed it: I am a clueless idiot with a computer science education :-D
 
Thanks,
bert



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