AW: [sdiy] AD/DA schematics? quantizers,etc

Matthias Herrmann matthias.herrmann at fonik.de
Thu May 14 17:50:02 CEST 2009


> Make perfect sense to me and sounds like a solid design.
> As long as you are not using the control voltage to run a VCO at
> 1v/o you do
> not have to be concerned about where the output voltages make transitions.

i do! and the staircase tracks nice over 10 octaves (at least for me it is
okay, i did not do exact measurements, though).

cheers,
matthias
___
www.modular.fonik.de


>
> It had slipped my mind that some control voltages are -5 to +5 and others
> are 0 to +10v.
>
>
> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl
> [mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of
> Matthias.Herrmann at jeppesen.com
> Sent: Thursday, May 14, 2009 10:02 AM
> To: synth-diy at dropmix.xs4all.nl
> Subject: RE: [sdiy] AD/DA schematics? quantizers,etc
>
>
> i did not follow the whole thread, however i am currently working on a
> quantizer that would be usable for control votages in the range of -5V to
> +5V.
>
> i used a DAC0800, that is setup to operate in a bipolar mode (output from
> -5V to +5V using a REF102 10V reference). as we already learned the output
> is not exactly 10V p-p when using only 7 bits, therefor i added an
> invering mixer with offset and gain control to trim the DACs output to
> exactly -5V to +5V.
>
> hope that makes any sense in this context. as said above i did not follow
> the thread toroughly. sorry.
>
> cheers,
> matthias
> ___
> www.modular.fonik.de
>
>
>
>
> synth-diy-bounces at dropmix.xs4all.nl wrote on 05/14/2009 04:46:36 PM:
> > <<The Scaling is already in place on the BACK END D to A using the 10v
> vref
> > in
> > this schematic.
> > This does not take care of any Offset you may have.>>
> > ====
> > That's not quite right - with a 10v vref you still need to scale the
> output
> > but the factor is
> > (OUT 10v/9.412v = 1.06)
> > ====
> >
> > -----Original Message-----
> > From: synth-diy-bounces at dropmix.xs4all.nl
> > [mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of Jerry
> > Gray-Eskue
> > Sent: Thursday, May 14, 2009 9:33 AM
> > To: Synth-diy at dropmix.xs4all.nl
> > Subject: RE: [sdiy] AD/DA schematics? quantizers,etc
> >
> >
> >
> > I should not try to do math at midnight when I am tired. I have a couple
> of
> > inaccuracies in the message below.
> >
> > 1 The "Step" value of an 8 bit converter with 5v ref should be 5v/255
> not
> > 5v/256 ~= 0.0196078 volts. This in turn modifies the Scale Factors (IN
> > 4.706v/10v = .4706) (OUT 10v/4.706v = 2.125) Note:"Step" 240 = 0.01960v
> *
> > 240 steps = 4.705872 ~= 4.706
> >
> > 2 Do not "SUBTRACT 0.01v" on the back end, this would null out Centering
> the
> > desired values on the steps and throw all the output "Steps" off 1/2
> "Step"
> > and off "Note"
> > ========
> > IF you setup your DAC0808 as in the Data Sheet- FIGURE 1. +10V Output
> > Digital to Analog Converter You will see that the DAC0808 Can take 10v
> for
> > Vref+ SO things get simpler You only need to do the FRONT END scaling
> into
> > the A to D
> >
> > The Scaling is already in place on the BACK END D to A using the 10v
> vref in
> > this schematic.
> > This does not take care of any Offset you may have.
> > ========
> > To clear up Offset and Scaling -
> >
> > Offset is a DC voltage in an audio signal that pushes the waveform away
> from
> > centering on 0 volts, Typical audio signals run balanced amounts of +
> and -
> > voltages. Mathematically this is ADDITION or SUBTRACTION of a Fixed
> Value to
> > ZERO out the offset, Decoupling Caps may also be used to remove offset.
> >
> > Scaling is MULTIPLYING or DIVIDING a value to change its overall peak,
> or
> > peak to peak voltage.
> > =======
> >
> >
> > -----Original Message-----
> > From: synth-diy-bounces at dropmix.xs4all.nl
> > [mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of Jerry
> > Gray-Eskue
> > Sent: Thursday, May 14, 2009 12:03 AM
> > To: Synth-diy at dropmix.xs4all.nl
> > Subject: RE: [sdiy] AD/DA schematics? quantizers,etc
> >
> >
> >
> > 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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