[sdiy] AD/DA schematics? quantizers,etc

[Dan Snazelle]

ok that clears up a lot

the question from what you explained for me, as i am not so good with this stuff out of memory is how to add on the front, the exact (say its 01 volts) and on the output, subtract .01 volts


i had assumed i did this with an voltage divider offset in the opamp (on the + channel) but i was using THAT offset just to get the signal up to a high enough voltage to even drive vcos

 but i know that even once i get it up to the right voltage there is a heavy offset because even with nothing connected i get a shift on the vco


i too am too tired but i hope to get back to this first thing when i wake up around 7'

i think the thing i need to understand is BOTH how to make the Qunatizing accurate AND how to make its total voltage swing large enough when it gets out (maybe all this is is putting it through another opamp to add enough gain etc)


i have been looking at LOTS of schematics today but getting the output voltage high enough at the output is not an obvious thing from what i looked at


anyway...this will get solved!!

hooray

btw...even now as a qunatizer it sure does some weird stuff to cv and to audio



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----------------------------------------
> From: jerryge at cableone.net
> To: Synth-diy at dropmix.xs4all.nl
> Subject: RE: [sdiy] AD/DA schematics? quantizers,etc
> Date: Thu, 14 May 2009 00:02:50 -0500
> CC:
>
>
> 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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