3-bit folding flash ADC uses only 3 comparators

[Taylor]

Adding a cap seems to tame some of the glitches.  Link

upping the 10n cap at the end to 1uf (heavy handed lpf-ing) cleans it up nicely... (i know:obviously )

But a cap that big completely removes the digital artifacts that ETA worked so hard to get!  So what would be the point of this circuit then?

[Quackzed]

true. it would be pointless to 'un-digitize' it at this point, but after waveshaping,  manipulating , folding etc while its in the digital domain ... you could 'then' heavy lpf the signal...to er-'un-digitize' it...
i just figured that was one of the possibilities, rather than a/d being the end point.
you know a/d - digital fun -d/a(low pass filter)...
i was just surprised how effective low passing a digital signal is...well, to a point...
 

[Taylor]

Ah, you're right. Lots of potential in here! Somebody needs to build this thing...

[earthtonesaudio]

@PRR: I have a layman's understanding of the generic different ADC/DAC types, maybe just a little more than what can be gleaned from Wikipedia.  While I was working on this I did look up what patent drawings I could find regarding folding flash ADCs but they were mostly for stuff that would be fabricated on a single slab of silicon, way out of my comfort zone.  I'm not really surprised that this ground has been covered before, but I am pleased that I was able to discover it independently. 

@Taylor: There are a couple things to watch out for when going from simulation to real parts with this build.  First, if you use an op-amp it has to tolerate whatever differential voltages may appear at its inputs.  For a small full-scale input range, this differential can be negligible.  For a large full-scale input range, it might be better to use comparators that can handle such conditions gracefully.
Second, the simulation assumes the output can swing from zero to nine volts exactly.  A rail-rail op-amp will be close enough but a general purpose one will at best swing to within a volt or two of the supplies.  This makes the voltage at the resistive divider different so you get a deviation from the expected values; the result is some bits do not appear in the sequence.  I'm still working on a way to calculate these values reliably.  With rail-rail outputs it's just ratios, but with not-quite rail-rail outputs I think there might be actual math involved. 
What I did was use DC input voltage that I could sweep manually (a pot across the rails), and LEDs on the op-amp outputs to monitor the binary output.  Then I subbed resistor values until I got all the binary combinations.  Now, I would start with the values in the sim and tweak with trimpots as needed.

@Quackzed: 

@toneman: I may just have to buy a copy of that issue from PAIA, but I don't know which issue...

@all: thanks for the kind words. 

I think there is a lot of potential in this thing as an audio mangler or noisemaker, but also as a way for folks to dabble with digital processing in a very raw and intimate way with discrete XOR/AND/OR chips as toneman suggested.

[toneman]

@toneman: I may just have to buy a copy of that issue from PAIA, but I don't know which issue... Huh

Possibly the july/august 1979 issue  .....

"Digitizer Projects and Software"

http://www.paia.com/Articles/polyphny.htm

suggestion: e-mail PAiA and see how many pages the article is.
And IF it is, in fact, the complete article (more than one page) 
I have an ACTUAL EK7 but am looking for the documentation   
afn
 

[loss1234]

I hope this is not a stupid question...would it be possible to convert this circuit to have LESS bits instead of more? ( a 2 bit flash ADC )
for controlling CMOS chips that only have an A and B input like the 4052?


any info greatly appreciated!

thanks

[Hides-His-Eyes]

You can indeed... Take the 2 most significant bits, get rid of the other two and then work out which op-amps have nothing connected to their outputs; delete those. Then see if any of the resistor divider pairs are left unconnected and get rid of those too.

[loss1234]

"get rid of the other two"

Sorry for the confusion...if there are only three bits (A, B and C) how can I take the two most significant bits and then remove the other two?

Did you mean just remove the third one?

thanks!

[earthtonesaudio]

@loss1234:
Referring to the schematic from post #1 of this thread, you can get rid of IC1a, R1 and R2 to make it into a 2-bit ADC.  Also, since no current is drawn into the inverting input of IC1B, you can also omit IC1D and do the whole thing with a dual op-amp or comparator package.

I think Hides-His-Eyes was talking about removing the other two sections from the 4-bit version, the most recently linked schematic.

[toneman]

taylor:
try using real comparators in place of the opamps for the bit conversions. 
Comparators have "built-in snap", whereas opamps need positive feedback to get "snap". 
An LM339 is a quad comparator, same pinout as the MC3302. 

earthtonesaudio:
I FOUND the EK-7 article  ...scanning now.....
If anyone wants a copy send me a PM with your email. 

A further note, I built he EK-7 out to 5bits with 1% resistors and it works fine.  The EK-7 was mainly invented/used for the Paia Proteus, a monophonic synth with saveable presets.  The EK-7, or actually TWO of them, were used to digitize the pot settings of the synth to make an 8bit word to be stored in memory, WITHOUT using any type of processor. It was cool 

 

[PRR]

> Comparators have "built-in snap", whereas opamps need positive feedback to get "snap".

General-purpose comparators (like '339) don't "snap". Hysteresis and positive feedback is left to the user.

Comparators are often (not always) rated for large differential inputs; but so are most common op-amps (such as '074) (and yes, I've blown '709 opamps with excess input diff).

Comparators are generally faster to switch. An op-amp's speed must be controlled so it will be stable under negative feedback. 

What is stunningly elegant about earthtonesaudio's plan is that the "point" of hysteresis is accomplished another way, with threshold shifts. Once any stage switches, all other stages get new thresholds. It is unlikely to "chatter" around a near-threshold level, sweet.

[toneman]

What is stunningly elegant about earthtonesaudio's plan is that the "point" of hysteresis is accomplished another way, with threshold shifts. Once any stage switches, all other stages get new thresholds

That's the way the PAIA 4bit digitizer worked.  Each stage fed previous/further stages, setting the biases for a new shift point. I think that is one of the reasons it's called a "folded" A2D.  The output bits are "folded back" to set the new thresholds for the inputs. Simonton used all comparators.  I found that there was still a little chattering just at the thresholds.  It worked OK for the Proteus.  I've included the digitizer page from the Proteus in the scans. I have scanned the EK-7 article and converted to pdf.  It's 6M.

check this article out:
http://www.analog.com/static/imported-files/application_notes/46875282066493AN_849.pdf