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

john slee indigoid at oldcorollas.org
Wed Dec 5 10:29:51 CET 2018


Tom,

I was contemplating the same. However while browsing some AVR datasheets
I've not found a spec indicating any guarantees about what happens when you
write to a register representing multiple GPIO pins — eg. the PORTA, PORTB,
et al "ports" in AVR land that each represent 8 pins. If the GPIO edges are
not synchronised to a reasonable degree, the same problems may well manifest

Though I suppose at 16Mhz, the DG408's 250ns transition time is
approximately four cycles. It should still be way ahead of TL07x

Speaking of TL07x... David wrote:

> The slew rate of TL074 is about 13V/us, so it takes about 2us or 2000ns
for the comparators to change state

The Vishay DG408 datasheet I'm reading[1] appears to indicate that you only
need to swing the "logic high" voltage to 2.8V, which should give a slew
time of around 215ns, if my math is right. You may be able to get slightly
faster again by using a value above 0V for "logic low", but that's pushing
a bit close to the Vishay specs. Nowhere near as fast as a microcontroller,
but a lot faster than a full-range swing.

[1] https://www.vishay.com/docs/70062/dg408.pdf

John

On Wed, 5 Dec 2018 at 19:39, Tom Wiltshire <tom at electricdruid.net> wrote:

> 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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>
>
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