[sdiy] Help, I'm Desperate! (Charge Injection with DG408)

[Tom Wiltshire]

I have to say it - this really sounds like a job for $0.50 of microcontroller.

==================
       Electric Druid
Synth & Stompbox DIY
==================

> On 5 Dec 2018, at 06:55, David G Dixon <dixon at mail.ubc.ca> wrote:
> 
> Hello SDIY Team,
> 
> Today, I thought deeply about the switching glitch problem, and I think I
> concur with Steve Lenham.
> 
> The switching time of the DG408 is about 250ns.  The slew rate of TL074 is
> about 13V/us, so it takes about 2us or 2000ns for the comparators to change
> state.  Also, the thresholds cascade from one comparator to the next, so the
> times are probably roughly additive.  Hence, it could take as long as 6000ns
> for all three comparators to change state, and this is a veritable eternity
> from the perspective of the DG408 -- about 24 switchings could occur during
> that time.  Hence, when two or three comparators have to change state, the
> switches blow through a bunch of different audio inputs, creating classic
> audio pop.
> 
> So, I now believe that the best approach is to use Gray code to drive the
> switches, so that only one bit ever has to change.  This consists of two
> things: 1) changing the logic circuit to generate Gray code from the CV
> signal, and 2) reconnecting the switch inputs to conform to Gray code.  The
> second part is trivially easy.  The first part is not so easy.
> 
> However, I have discovered that 3-bit binary can be converted to 3-bit Gray
> code fairly directly, since the 1st Gray and binary bits are equivalent, the
> 2nd Gray bit is the XOR of the 1st and 2nd binary bits, and the 3rd Gray bit
> is the XOR of the 2nd and 3rd binary bits.  An analog XOR circuit like the
> one on Ian Fritz's website (inspired by Electronotes) would fulfill the XOR
> function from my existing 3-bit hack-job ADC.
> 
> However, I am going to rethink this entire logic and switching circuit,
> because it is getting fairly complicated.  I believe that proper comparators
> with pullup resistors to 5V would be better than the opamps with zener
> diodes, and I'll figure out some other way to generate the threshold
> voltages.  With proper logic levels, I'll probably just use proper logic
> chips.
> 
> In any case, I need to stop thinking about this project for a little while
> because I have other more pressing things to do, so I'll return to this in a
> few days.
> 
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