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TRNG Tinkering

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Dabbot:

--- Quote from: magic on July 08, 2020, 02:40:12 pm ---Well, an analog solution has the further disadvantage that you will never get it to output 50/50 unless you get offset voltages perfectly right.

--- End quote ---
I realise that's not going to be possible, but I can always see how close I can get it.


--- Quote from: magic on July 08, 2020, 02:40:12 pm ---If you want to play with it, be sure to include some hysteresis (positive feedback) in the comparator.

--- End quote ---
I might give it a try using a separate comparator IC. Having the comparator thrashing about on the same silicon with all that gain present can't be helping either. Any hysteresis will need to be small, given the noise is only roughly 50mv RMS.

magic:
I doubt it oscillated due to inter-channel crosstalk. Some amount of hysteresis is necessary IMO, even if it's just 1/1000 of the output.
A real comparator will have the advantage of much faster slew rate. Try TLC3702 or similar with a push-pull output stage (not open collector).

A silly little simulation below. Looks a bit like a delta-sigma ADC but reacting to slope rather than absolute value.

Dabbot:
I just tried an open collector comparator with a 1K resistor (I don't have any push-pull types handy) and got respectable results.
I didn't see any obvious difference by including hysteresis, according to observations on the oscilloscope. This might change once I start sending random samples to the PC.

RoGeorge:
Again, why don't you apply a Von Neumann Debiaser to the ADC bit(s) you said they were unbalanced?

Anyway, if you insist to keep the noise balanced in the analog domain, put a series capacitor.  Also, a revese polarized BE junctions from a bipolar transistor makes a great noise generator, all those OpAmps are not needed.

Dabbot:

--- Quote from: RoGeorge on July 09, 2020, 05:49:19 am ---Again, why don't you apply a Von Neumann Debiaser to the ADC bit(s) you said they were unbalanced?

--- End quote ---
That's part of the challenge. It's easy enough to do debiasing in software, but I wanted to have a good starting point.


--- Quote from: RoGeorge on July 09, 2020, 05:49:19 am ---Anyway, if you insist to keep the noise balanced in the analog domain, put a series capacitor.

--- End quote ---
This doesn't treat the ADC's bias.


--- Quote from: RoGeorge on July 09, 2020, 05:49:19 am ---Also, a revese polarized BE junctions from a bipolar transistor makes a great noise generator, all those OpAmps are not needed.

--- End quote ---
The reason I came up with this circuit is it will run down to 3 volts. No charge pumps necessary to place BE junctions into avalanche. Besides, all those op amps are very cheap and come in a single package.

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