[sdiy] AD/DA schematics? quantizers,etc

[Jerry Gray-Eskue]

Let me try to clear up the Vref thing for you.

If you are just running audio signals through the converters all you need to
do is scale down the 0-10v to 0-5v run it into the A to D run the Data lines
into the D to A and scale the output back up to 0-10v, the drawback here is
that the audio signal is running with a 5v offset and needs to have that
removed either by subtracting the voltage or using capacitive coupling into
your amplifier.

If your analog signal IN is centered on 0v i.e. -5v to +5v your scaling will
need to scale the input down to -2.5v to +2.5v and Add a 2.5v offset so that
the signal into the A to D is 0v to 5v. In this setup vref+ = 5v and Vref- =
0v is good. Again the offset needs to be removed.

Now If you are tiring to quantize a Control Voltage and want it to "Hit
Notes" you have other considerations. Take a control Voltage of 1 volt per
Octave from 0 - 10v. In this case Each Volt represents 12 "Notes" for 10
Octaves Or 12 * 10 = 120 distinct notes. An 8 bit converter has 0-255 as the
output or 256 "steps" 256/120 = 2.133 in other words it is a mismatch. you
need to match the 120 notes to "steps" So you can set it us to use 240 of
the 256 steps with 16 "steps" unused. In other words Using a vref+ of 5v and
a vref- of 0v you want 10v in to result in the converter outputting "Step"
240. Each Step of the A to D is 0.01953125 volts, we want 240 of these =
4.6875 volts, So the input to the A to D should scale the 10volt input at
4.6875/10 or 0.46875 .
If the D to A is also using vref+ 5v and vref- 0v we need to do the reverse
to scale back up to 10 volts 10/4.6875 or 2.1333...
I am just about to tired to think but I also believe that we have at this
point centered the "Notes" on the Step transitions, we really want them
centered between the transitions, so as part of the Front end (A to D) after
the scaling we need to ADD 1/2 of a Step to the Voltage into the Converter,
or 0.01953125/2, 0.01v is close enough, and coming out of the Back end (D to
A) we need to SUBTRACT 0.01v before scaling the voltage back up.

I hope this helps clear things up and not create total confusion.




-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of Dan Snazelle
Sent: Wednesday, May 13, 2009 10:39 PM
To: sdiy
Subject: RE: [sdiy] AD/DA schematics? quantizers,etc



ok



i feel like a fool

in all my reading and testing today i had somehow missed the fact that
on an ADCchip, the vref cant go below grnd (or not much below)



so i am confused. usually when i see a - sign i think of putting the
negative rail. but in this case it seems you pick a top voltage and a
you pick a bottom but that has to be within 0-vref



is this correct? and the vcc is always 5volts?



how high can the vref be?



a few people have mentioned that for cv, i want a 10v vref.



would a high vref be useful for audio as well? or a lower vref useful even?



thanks

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