[sdiy] AD/DA schematics? quantizers,etc

[Dan Snazelle]

ok i am now awake! going to try and figure out how to get the voltage back up to 0 to 10 volts at the output AND add the small offset needed for quantazation accuracy

i read the chapter in horrowitz and hill aoe about a2d and d2a but it does not get into how to make cv come out right!

maybe chamberlins book?




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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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