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

john slee indigoid at oldcorollas.org
Wed Dec 5 10:33:30 CET 2018


Ugh, ignore me, I neglected to check the rail specs for the TL074. Oopsie.
0-5V swing would take 384ns, definitely too slow

John

On Wed, 5 Dec 2018 at 20:29, john slee <indigoid at oldcorollas.org> wrote:

> 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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