ADC on a Key-chain

I wanted to try my hand at ADC design, and decided to make it into a key-chain while I was at it.

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What? I wanted to learn more about ADCs, so I decided to design and build one from scratch. The architecture I decided to go with is a residue amplifier pipeline ADC from discrete components (mostly OpAmps and resistors). Hopefully anyone reading this project will see that ADCs aren't necessarily as complicated as everyone makes them seem! At least that is what I learned. Why? I have also been looking for some sort of "promotional" circuit to hand out with my resume at career fairs and interviews, so I decided to make a key-chain. Similar to the popular PCB business cards so many people are making these days, hopefully it will catch the eye of a future employer and land me an interview :)

Theory:

The residue amplifier pipeline ADC consists of one key block: the residue amplifier. These residue amplifier blocks are then connected in series, forming a pipeline, hence the name. The figure below shows a residue amplifier. As you can see, it takes an analog input and a "threshold" input, and then has a digital output D[n] and an analog residue output R(n) that represents the "remainder" of the input voltage - the 1st bit voltage or Vin - Vref.

Residue amplifier block diagram:

Residue amplifier block diagram

Residue amplifier output vs input plots:

If we cascade multiples of these together, passing "R(n)" output of one stage into "Vin" of the next stage, we will have an ADC with each stage's "D[n]" representing the ADC data bits. Notice that D[0] and D[1] in the plots below represent the digital level of the analog input voltage!

Two bit ADC block diagram:

Two bit ADC output voltage vs input voltage plots:

Now how to build it:

To design this kind of ADC, I just used some opamp logic to make a residue amplifier and then cascaded four of them together to make a 4 bit ADC. Below are the schematics for my design. I then just drew up these schematics in eaglecad, laid out a key-chain shaped PCB and ordered them from OSHPark.

Residue amplifier schematic:

4 bit ADC schematic:

EagleCAD schematic:

EagleCAD layout: