[sdiy] AD/DA schematics? quantizers,etc

Matthias.Herrmann at jeppesen.com Matthias.Herrmann at jeppesen.com
Thu May 14 17:02:25 CEST 2009


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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