Editing question to add details:

The "analog resources" are programmable DACs which I described as voltage sources on the original question. The speed is not critical as the circuit intended to control switches for a DC bench measurement (I can spare 10s of seconds if needed). The digital outputs don't need to drive "heavy" loads, but digital switches, so the current should be around 1uA or less. I was very conservative on the original question and I believe the tri-state zone of the switches I need to control is probably around 2.25v to 3v, so anything < 2.2v should be a logic 0 and anything > 3.1v should be a logic 1. The idea I have in mind is to use a low voltage comparator with voltage dividers at the inputs.

Original question:

I need some suggestions to solve my problem without complications. I have 3 analog resources which can force voltage anywhere from 0 to 5v. I need 4 digital signals which logic 1 is >=3.3v and logic 0 is <= 0.5v.

I cannot use microcontrollers. Only those 3 lines to convert to 16 different binary states. I was thinking an ADC, where 2 resources are used for supplies and the last resource for input, but I don't know if this is doable.

Ideally, if I had another resource then my problem would be solved, but I don't have it and the solution is limited to those 3 analog voltage resources. Thanks!

  • \$\begingroup\$ Are there any other restrictions? Is this a homework problem? Is there a current limit or resolution on the analog sources that should be considered? \$\endgroup\$
    – pjc50
    May 17, 2019 at 15:13
  • \$\begingroup\$ You have left out a specification for the digital state when the voltage is <3.3V and >0.5v. \$\endgroup\$ May 17, 2019 at 15:14
  • \$\begingroup\$ @josecanuc That's called a tristate (high-Z) region if the voltage is stuck in between those values. \$\endgroup\$
    – user103380
    May 17, 2019 at 15:19
  • 1
    \$\begingroup\$ Without some constraints specifying how the digital outputs are supposed to represent the state of the analog inputs this is unanswerable - or at least, could be answered in so many 100's of different ways it makes the question pointless. \$\endgroup\$
    – brhans
    May 17, 2019 at 15:26
  • 2
    \$\begingroup\$ What do you mean by "resource"? DACs? Elves with potentiometers? Something in between? What mapping between analog and digital do you mean? Or do you mean that you have some analog outputs and you need to turn them to digital? Do you have any speed constraints? How fast can you change the analog, and how fast does the digital need to change? How disastrous is it if the digital outputs go through "wrong" states on their way to the right ones? Please edit your question to supply this information. \$\endgroup\$
    – TimWescott
    May 17, 2019 at 16:11

1 Answer 1


4 bit Analog to Digital

What you are looking for is typically called a Flash A/D.
You use comparators to detect the input signal level and convert this to your required 4 bit digital output.

Shown below is a 3 bit convertor, but you'll get the idea:

enter image description here

There are many flash convertors available and you can get ideas for implementations from devices such as the ADC0820, a decades old workhorse.


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