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

David G Dixon dixon at mail.ubc.ca
Wed Dec 5 07:55:46 CET 2018

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

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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