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

mark verbos markverbos at gmail.com
Tue Dec 4 11:09:30 CET 2018


What Steve said.

My opinion is that it is much more likely that your “logic” signals, coming from mere opamps have sloped edges. Since the thresholds of the DG and the comparators in the converters and everything all are different, then the timing of all of this is different. So, each time a transition is about to happen, a flam of changes happens and glitches of unrelated signals leak through. When I have had logic races like this, I just added caps to the fastest of the signals to get them to fall in line with the slower ones. I have done a sequencer that crossfades from the last step to the current step to create a slide as well as a multistage envelope that does something similar and these kinds of problems showed up.

Mark







> On Dec 4, 2018, at 10:28 AM, Steve Lenham <steve at bendentech.co.uk> wrote:
> 
> 
> 
> - Here is my main suggestion as to what it might be. It sounds like (we only have your description of the circuit to go on) you are generating the three-bit control word for the analogue switch from the outputs of several analogue comparators. How sure are you that the transition from one digital value to the next is clean, i.e. that all three bits change state at exactly the same time? If the three-bit word passes through any intermediate "runt" states on its way, the switch will briefly select a completely different input and potentially inject spikes of something else into the output. This is an issue in digital systems (search "race hazards") and is likely to be even worse if comparatively slow and imprecise analogue comparators are involved.
> 
> Your previous design used a DG409, which only has a two-bit address - the problem could theoretically still occur there, but there is less scope for it to do so.
> 
> 
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