[sdiy] AD/DA schematics? quantizers,etc

Jerry Gray-Eskue jerryge at cableone.net
Thu May 14 16:46:36 CEST 2009


<<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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