Idea for a CV quantizer

Haible_Juergen#Tel2743 HJ2743 at denbgm3xm.scnn1.msmgate.m30x.nbg.scn.de
Thu Feb 29 02:30:00 CET 1996


Hi!

Some time ago there was a question about a simple CV quantizer.
My answer back then was using an ADC abd a DAC, the DAC having
much more bits than the 6 or 7 that give you the desired resolution,
but with only these 6 or 7 bits *connected* to the ADC. This was to
achieve the desired accuracy which has to be far better than the
resolution.

Meanwhile I have found a better technique, both in an old Electronotes
issue and in the ARP Sequencer circuits. Here there is only a DAC
required. A counter cycles thru all possible values of the DAC
permanently, producing a staircase voltage.
A comparator compares the input CV with the staircase voltage and
fires a S&H as soon as the staircase approaches the CV. This
quantized CV is then held until the next cycle.
This design has two great advantages:
(1) You don't need an ADC,  and
(2) you only need one DAC for as many quantizer channels you want:
      just give each channel its own Comparator and S&H.
You still need a good DAC for this, however. So either you have to buy
a 12 or better 14-bit DAC (not easy to come by if you don't want a serial
input, nowadays!), or you have to build your own discrete DAC (low
resolution, but high accuracy).

Now here is the idea (don't really think that it is new; but I haven't seen
it anywhere, so far):
All you need is a precise staircase; no need to implement it with a
DAC and counter!
I was thinking about this old Integrator + Charge pump approach.
Build an integrator with an opamp, add a thyristor or some other
reset-device that discharges the cap way above the desired CV
range. Then take a pulse oscillator, clip the amplitude with some
precise voltage reference (adjustable, for exact 1/12 V steps),
and feed this into the integrator.

I am not sure how good this would work. Would I get the desired accuracy
of +/- 1 cent with standard components?
I would aim for a conversion rate of 1ms. So with a clock rate of 100kHz
I should be able to cover 88 notes.

What do you think ??
(I *feel* that there must be some week spot in this idea, but I just
can't see it at the moment!)
How precicely can a charge pump/integrator system work?
What would I have to consider?

Any comments welcome,

JH.



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